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Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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3

Independent Assessment of Formaldehyde

The second part of the committee’s task was to conduct an independent assessment of formaldehyde. The committee started with its peer review in Chapter 2 and the background document that supports the formaldehyde profile in the 12th RoC. It searched for additional peer-reviewed literature that had been published by November 8, 2013,1 and incorporated relevant human, experimental animal, and mechanistic studies into the independent assessment. The committee focused its attention on literature that contained primary data, but it also examined published review articles and reviews by other authoritative bodies to ensure that relevant literature was not missed and to ensure that all plausible interpretations of primary data were considered. The committee considered comments and arguments presented to it during its first meeting, comments and documents received from other sources during the study process, and independent literature searches carried out by National Research Council staff (see Appendix D). The goals of the literature searches were to identify relevant literature published around the time of the publication of the background document and later that may have missed inclusion in the 12th RoC and to identify any relevant literature that was published after the release of the 12th RoC. Each search covered the period from January 1, 2009 (the year in which the draft background document for formaldehyde was initially released; Bucher 2013), to November 8, 2013. Databases searched were PubMed, MEDLINE (Ovid), Embase (Ovid), Scopus, and Web of Science. The search strategy for each database is described in Appendix D. After identifying the relevant body of literature up to November 8, 2013, the committee reviewed the primary data and applied the RoC listing criteria to human, experimental animal, and mechanistic studies.

This chapter begins with a section on cancer studies in humans, which is followed by a section on cancer studies in experimental animals. The chapter then reviews toxicokinetic and metabolism literature and studies of mechanisms

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1The cutoff date for the literature search was chosen to allow the committee time to review the literature within the constraints of the project schedule.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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of carcinogenesis. It ends with a section that summarizes human, experimental animal, and mechanistic data and provides a conclusion and a listing recommendation for formaldehyde that is based on the listing criteria in the 12th RoC.

The committee’s assessment of formaldehyde was guided by the RoC listing criteria, which were first introduced in the present report in Box 1-2. A substance can be classified in the RoC as “reasonably anticipated to be a human carcinogen” if at least one of the following criteria is fulfilled (NTP 2010, p. iv):

• “There is limited evidence of carcinogenicity from studies in humans, which indicates that causal interpretation is credible, but that alternative explanations, such as chance, bias, or confounding factors, could not adequately be excluded.”

• “There is sufficient evidence of carcinogenicity from studies in experimental animals, which indicates there is an increased incidence of malignant and/or a combination of malignant and benign tumors (1) in multiple species or at multiple tissue sites, or (2) by multiple routes of exposure, or (3) to an unusual degree with regard to incidence, site, or type of tumor, or age at onset.”

• “There is less than sufficient evidence of carcinogenicity in humans or laboratory animals; however, the agent, substance, or mixture belongs to a well-defined, structurally related class of substances whose members are listed in a previous Report on Carcinogens as either known to be a human carcinogen or reasonably anticipated to be a human carcinogen, or there is convincing relevant information that the agent acts through mechanisms indicating it would likely cause cancer in humans.”

A substance can be listed as “known to be a human carcinogen” if “there is sufficient evidence of carcinogenicity from studies in humans, which indicates a causal relationship between exposure to the agent, substance, or mixture, and human cancer.” The RoC listing criteria are clear about the information needed to fulfill the criteria of sufficient evidence in experimental animals (see the section “Cancer Studies in Experimental Animals”). The type of information needed to meet the RoC listing criteria for limited or sufficient evidence in humans required more interpretation and expert judgment by the committee. To make the committee’s methods clear and transparent, the section “Cancer Studies in Humans” begins by describing the committee’s methodology for identifying and evaluating epidemiologic evidence and the committee’s interpretation and application of the listing criteria.

CANCER STUDIES IN HUMANS

Identification of Informative Epidemiologic Studies

In its independent analysis of formaldehyde exposure and cancers, the committee first considered each of the epidemiologic studies cited in the background document for formaldehyde. As discussed in Chapter 2, the National

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

Toxicology Program (NTP) did a thorough job of searching the literature for relevant human studies, so the committee used the background document as a starting point for its independent review. Second, the committee examined the results of the independent literature search described in Appendix D (see Box D-1 and Figure D-1). One additional study (Coggon et al. 2014)—an update of Coggon et al. (2003)—was identified after the literature-search cutoff date and was included as part of the committee’s independent assessment. Third, the committee examined review articles, meta-analyses, and materials presented during its first meeting and during the study process.

As part of its exclusion criteria (Box D-1), the committee based its assessment on the primary literature. It recognized that quantitative meta-analyses can be informative, but the heterogeneity of exposures in the primary literature on formaldehyde makes it challenging to base any conclusions of causality on resulting summary estimates. The committee agrees with a previous National Research Council report that “meta-analysis can be a valuable method for summarizing evidence but can also be subject to variable interpretations depending on how literature is selected and reviewed and data analyzed” (NRC 2011, p. 112).

Evaluation of Epidemiologic Studies

Several factors were considered in the evaluation of the strength of the epidemiologic literature. The principles of causal association, elaborated by Bradford Hill (Hill 1965), were used as a starting point for the evaluation of informative epidemiologic studies. Of Bradford Hill’s original nine criteria, the committee focused on six: strength, consistency, specificity, temporality, biologic gradient, and coherence. On the basis of the RoC listing criteria, plausibility was more relevant to supporting evidence from experimental animal studies and mechanistic data than to the evaluation of the epidemiologic evidence, and analogy was not deemed to be a useful criterion for this topic. Coherence emerged as a particularly important criterion for similarity of findings among multiple study designs and populations (and is also related to consistency). The committee recognizes that the Bradford Hill criteria can be useful guidelines for assessing causal association but agrees with NRC (2014, p. 91) that they “are by no means rigid guides to reaching ‘the truth’.”

The committee also developed criteria for rating the quality and utility of epidemiologic studies and their exposure assessments, shown in Table 3-1. The development of the exposure-assessment evaluation is presented in detail in Appendix C and summarized in Tables C-1 and C-2. In general, the committee judged a cohort or case–control study to be informative if it was large, had high and varied exposures that were systematically estimated, had reliably assessed cancer end points, and included credible comparison groups. Table 3-2 provides information about all the epidemiologic studies that the committee considered, including a description of the studies, a description of the exposure assessments used in each study, comments on strengths and limitations of the studies, and the committee’s determination of study quality (strong, moderately strong, or weak).

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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TABLE 3-1 Criteria Used to Assess Epidemiologic Studies for Hazard Assessmenta

Study Quality and Utility Classification Study Population, Design, Quality of Data, and Analysis Exposure Assessmentb
Weak study: limited utility for hazard assessment; inconclusive; uninformative Modest or small population with few cases. Design limitations, including broad case definition, no duration of exposure, short followup, limited data analysis Low discrimination between exposed and control categories, qualitative or semiquantitative evaluation, limited evidence of substantial formaldehyde exposure
Moderately strong study: somewhat useful for hazard assessment Modest-sized population with few cases or a broad case definition; sufficient followup for latency; standard data analysis Moderate discrimination between high and low exposure categories; substantial fraction of population probably highly exposed; qualitative, semiquantitative, or quantitative evaluation; use of duration of work as a proxy for exposure
Strong study: highly useful for hazard assessment Large population with many cases, precise case definition, including subcategories; large number of subjects with long-duration exposures; sufficient followup for latency; limited switching among exposure categories; sophisticated data analysis accounting for important potential confounders High discrimination between high and low exposure categories, substantial fraction of population probably highly exposed, detailed quantitative or highly selective semiquantitative evaluation
aThe epidemiologic elements in the second column are not required to match with the exposure elements in the third column to define the study quality.
bExposure-assessment levels are based on the data presented in Appendix C and Table C-2. Source: Committee generated.

The committee’s judgment of the strength of a study depended on both the epidemiologic design elements (the second column in Table 3-1) and the exposure-assessment dimensions (the third column in Table 3-1), which are somewhat independent. A strong study might not have a highly developed exposure assessment. For example, several strong case–control studies of licensed embalmers had no exposure assessments, but because the case definition required work as a licensed embalmer and that occupation has well-defined rules for practice (which define the exposure situation), the resulting studies were considered to be strong or moderately strong. A well-designed study with a high-discrimination exposure assessment could be judged to be weak because few of the subjects were exposed to formaldehyde, as was the case, for example, in the textile studies. The overall strength of each study was assessed by considering all of the variables described in Table 3-1.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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TABLE 3-2 Description of Epidemiologic Studies Reviewed by the Committee

Reference and Study Population Study Informationa Exposure Assessmentb Critique and Conclusionsc Study Qualityd
Andjelkovich et al. 1995

Iron foundry workers from Michigan, USA
Cohort = 8,147 men; outcome: mortality; nasopharyngeal cancer = 1 case, sinonasal cancer = 0 cases, lymphohematopoietic cancer = 15, leukemia = 5 cases; 3,929 workers with potential exposure to formaldehyde for ≥6 months during 1960–1987; 83,064 person-years for exposed and 40,719 person-years for controls; a smoking-history survey was administered via mail. High-discrimination quantitative exposure assessment; detailed work history available for each study subject; extensive data from industrial-hygienist sampling, technical data from plant, walk-through surveys, and job and task descriptions; information assessed by an industrial hygienist and assigned to high (median 1.5 ppm), medium (median 0.55 ppm), low (median 0.05 ppm), or no formaldehyde-exposure categories; formaldehyde used in core-making operations in 1960–1987; all workers exposed to silica Followup since first exposure was short (≤19 years), total duration of exposure was short (≤17 years) Although the study had a high-discrimination quantitative exposure assessment and the cohort was of a moderate size, it was probably not large enough to detect risk of rare tumors, such as nasopharyngeal cancer, sinonasal cancer Moderately strong
Armstrong et al. 2000

General population of Maylasia
Population case–control; outcome: prevalent and incident cases; 282 cases with histologically confirmed nasopharyngeal cancers, ≥5 years of residence in study area, and diagnosis in 1987–1992; 282 cases and matched controls identified from health-center records in Kuala Lumpur and Selangor among Malaysian Chinese Low-discrimination qualitative exposure assessment; exposure information gathered by structured interview to obtain complete dietary, residential, occupational history; exposures classified by broad Malaysian occupational codes, industrial-hygienist professional judgment Formaldehyde exposure was limited (formaldehyde exposure in only 9.0% of the sample, only eight had accumulated ≥10 years of exposure outside a 10-year latency period); short latency period Weak
Beane Freeman et al. 2009

NCI study of US chemical industry and plastics workers in 10 plants
Cohort = 25,619; outcome: mortality from lymphohematopoietic malignancy; all lymphohematopoietic types = 319 cases, leukemia = 123 cases, myeloid leukemia = 88 cases; followup period: 1966–2004 High-discrimination exposure assessment; quantitative estimation and job–exposure matrix used, but no measurements after 1980; median exposure intensity was 0.3 ppm (range 0.01–4.3 ppm); median peak exposure was about 2 ppm; about 25% were exposed at >4ppm Large, well-designed study No evidence of confounding by other exposures Study was able to assess peak exposures Strong
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×
Beane Freeman et al. 2013

NCI study of US chemical industry and plastics workers in 10 plants
Cohort = 25,619; outcome: mortality; nasopharyngeal cancer = 10 deaths; sinonasal cancer = 5 deaths; followup period: 1966–2004; update of Hauptman et al. (2004) High-discrimination exposure assessment; extensive background data and samples; quantitative estimation and job–exposure matrix used on the basis of extensive data, but no measurements after 1980; Beane Freeman et al. (2009) reported the median exposure intensity of 0.3 ppm (range 0.01–4.3 ppm); median peak exposure was about 2 ppm; about 25% were exposed at >4 ppm Large, well-designed study No evidence of confounding by other exposures Study was able to assess peak exposures Strong
Bertazzi et al. 1989

Italian resin workers
Cohort = 1,332 men; outcome: mortality; hematologic neoplasms = 7 deaths, lung cancer = 24 deaths, larynx tumors = 6 deaths; followup period: 1959–1986 Moderate-discrimination qualitative exposure assessment; cohort members worked in a department that used formaldehyde; exposure intensity in many locations peaked at >3.0 ppm Evidence of increasing mortality from hematologic neoplasms with longer latency; highest increase in mortality was in those who were employed during 1965–1969, an early period of high exposure Moderately strong
Blair et al. 2001

General population in Iowa and Minnesota
Population-based leukemia case–control; outcome: incidence; 513 incident cases; ascertainment period: Iowa 1981–1983, Minnesota 1980–1982 Low-discrimination semiquantitative exposure assessment for formaldehyde; broad job categories and industries; potential formaldehyde exposure was categorized on a 4-point scale; likely high misclassification There were 513 incident cases, but the study was judged to be weak for assessing formaldehyde because the number of cases with high exposure (n = 3) was small, misclassification likely Weak
Checkoway et al. 2011

Female textile workers in Shanghai, China
Case–cohort nested within cohort of 267,400 women textile workers; outcome: lung cancer incidence; 628 cases diagnosed in 1989–1998 Low-discrimination qualitative exposure assessment (yes/no) for formaldehyde; detailed job histories and job–exposure matrix used to assign detailed textile-dust and related exposures for all workers for all years; exposure to formaldehyde was uncommon in these workers Few workers exposed to formaldehyde (2 lung-cancer cases were exposed to formaldehyde) Weak
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×
Reference and Study Population Study Informationa Exposure Assessmentb Critique and Conclusionsc Study Qualityd
Coggon et al. 2014

Chemical workers in 6 British factories where formaldehyde was produced or used
Cohort = 14,008; outcome: mortality; nasopharyngeal cancer = 1 death, nose and nasal sinus cancer = 2 deaths leukemia = 54 deaths, myeloid leukemia = 36 deaths; followup period: 1941–2012; update of Acheson et al. (1984) and Coggon et al. (2003) Moderate-discrimination semiquantitative exposure assessment; work histories abstracted from company employment records; jobs were classified into five exposure categories (background, low, moderate, high, or unknown) by industrial-hygiene professional judgment; limited quantitative measurements available after 1970 covering many jobs, quantitative exposure assumed to be the same before 1970 (although anecdotal, the reported exposures were much higher earlier in followup period); "high" exposure category was estimated to be over 2 ppm; no peak exposures identified; authors noted that there was some exposure to paraformaldehyde Cohort was small and satisfactory for cancers that were more common, but probably too small to detect nasopharyngeal and sinonasal cancers and only had moderate power to detect myeloid leukemia effects Authors reported a concern about the quality of data when they made exposure assignments Moderately strong
Dell and Teta 1995

Workers employed in a Union Caribide plastics manufacturing plant in New Jersey
Cohort = 5,932; outcome: mortality; nasopharyngeal cancer = 0 deaths, sinonasal cancer = 0 deaths, lymphohematopoietic cancer = 28 deaths, leukemia and aleukemia = 12 deaths; workers employed in 1946–1967; followup through 1988; 5,932 males in the cohort (111 exposed to formaldehyde) Low-discrimination qualitative exposure assessment; company job histories collected; duration of employment used as a surrogate for cumulative exposure; some analysis of work department made but limited by missing work data Small study size had little power to detect risk of rare tumors Few workers exposed to formaldehyde Limited exposure information Multiple concomitant exposures (raw materials used in the manufacturing process included asbestos [usually chrysotile], carbon black, epichlorohydrin, polyvinyl chloride, acrylonitrile, styrene, chemical additives [such as plasticizers, emulsifiers, and antioxidants]) Weak
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×
Edling et al. 1987

Workers in abrasive manufacturing in Sweden
Cohort = 521 men; outcome: mortality; nasopharyngeal cancer = 0 deaths; sinonasal cancer = 0 deaths, leukemia = 1 death; men with ≥5 years of employment in 1955–1983; followup period: 1958–1983 Low-discrimination semiquantitative exposure assessment; very limited formaldehyde exposure data from 1970s; two work areas had exposures; blue-collar workers assigned exposures; no data on how many were exposed Small study size had little power to detect risk of rare tumors Few workers exposed to formaldehyde Limited exposure information Weak
Hall et al. 1991

UK pathologists
Cohort = 4,512 men; outcome: mortality; nasopharyngeal cancer = 0 cases, sinonasal cancer = 0 cases, leukemia = 4 cases; men identified in 1973 Royal College of Pathologists membership list; followup period: 1974–1987 Low-discrimination qualitative exposure assessment on the basis of job title (formaldehyde exposure was assumed from cadavers); no discussion of exposure conditions was presented Small study size had little power to detect risk of rare tumor High likelihood of misclassification on exposure to formaldehyde; pathologists have less likelihood of exposures than embalmers Weak
Hansen and Olsen 1995, 1996

Danish data-linkage study identifying incident cancers in companies in which formaldehyde was used
Cohort = 91,182 men with cancer, 2,041 men with longest work experience of ≥10 years before the date of diagnosis of cancer, 265 companies where formaldehyde was used; outcome: incidence; nasopharyngeal cancer = 4 cases, cancer of the nasal cavity = 13 cases, leukemia = 39 cases; cancer diagnosed in 1970–1984; cases obtained from national cancer registry, linked to national employment data and industry reporting on chemical use Moderate-discrimination semiquantitative exposure assessment; potentially exposed cases were identified as those with ≥10 years of blue-collar work experience in formaldehyde-using companies; formaldehyde exposures were ranked as low (white-collar jobs) and high (blue-collar jobs) with no wood-dust or high wood-dust exposure; no workplace assessment of exposure conditions or plant size were made, so high potential for misclassification by exposure intensity (for example, a large plant may only have a few workers out of a large workforce who are exposed) Study limited by lack of data on intensity of exposures and internal plant operations Cohort had no or few cases of some types of cancers, and this limited its utility Weak
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×
Reference and Study Population Study Informationa Exposure Assessmentb Critique and Conclusionsc Study Qualityd
Hauptmann et al. 2009

US funeral directors, embalmers
Nested case–control; outcome: mortality; nasopharyngeal cancer = 4 cases, lymphohematopoitic cancers = 168 cases, myeloid leukemia = 34 cases, brain cancer = 48 cases; those who died in 1960–1986; update of Hayes et al. (1990) High-discrimination exposure assessment; methods included quantitative reconstruction with statistical modeling, sensitivity analyses; average exposure intensity while embalming was 1.5–1.8 ppm and average peak exposures was 8.1–10.5 ppm depending on case group No confounding by smoking Strong trend with years in embalming; trends with average and peak exposure Strong
Hayes et al. 1986

General population in the Netherlands
Case–control; outcome: incidence; histological types of sinonasal cancer = 116 cases; cancer diagnosed in 1978–1981; cases drawn from all six major hospitals for treatment of head and neck tumors Moderate-discrimination qualitative exposure assessment; work history collected by interview included all jobs held for 6 months or more; all jobs were classified by industrial hygienists according to level and probability of formaldehyde exposure on 10-point scale; agreement between two raters was poor for adjacent scores, and this resulted in high potential for misclassification in adjacent categories, which was rare for high to low or low to high Study limited by disagreement between exposure assignments of 2 independent raters, but the association of formaldehyde exposure and nasal cancer was similar for each rater For sinonasal cancer, the study suggests an association between formaldehyde and squamous-cell carcinoma, not adenocarcinoma Moderately strong
Hildesheim et al. 2001

General population in Taiwan
Population case–control; outcome: incidence; nasopharyngeal cancer = 375 cases; newly diagnosed, histologically confirmed nasopharyngeal cancer in people younger than 75 years old who were residents of Taipei City or County for ≥6 months; cases identified at 2 tertiary-care hospitals; population-based controls drawn from national housing registry Moderate-discrimination semiquantitative exposure assessment; occupational history data obtained by interview; exposures were assigned to broad occupation codes on basis of professional judgment of study industrial hygienist; exposures were classified from 0 (not exposed) to 9 (strong) according to probability, intensity, and duration of formaldehyde exposure; 74 cases exposed to formaldehyde; dietary factors and coexposure to cigarette smoking, wood dust, and solvents were assessed Considerable overlap in wood dust, formaldehyde exposures; authors were concerned about greater misclassification for formaldehyde than wood-dust assignments >95% of cases were positive for Epstein Barr virus Moderately strong
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×
Levine et al. 1984

Licensed embalmers in Ontario, Canada
Cohort = 1,4777; outcome: mortality; nasopharyngeal cancer = 0 deaths, sinonasal cancer = 0 deaths, larynx = 1 death, lymphohematopoietic cancer = 8 deaths, leukemia = 4 deaths; 34,774 person–years of observation during 1950–1977, 17,589 of which occurred ≥20 years since first licensure Embalmers have well-defined, high exposures to formaldehyde; embalmer exposure can be sharply discriminated from that of other job groups; job and formaldehyde sources defined by regulations and training Cohort was small and the study probably had little power to detect risk of rare nasopharyngeal and sinonasal cancers Moderately strong
Li et al. 2006

Chinese female textile workers in 526 factories in Shanghai
Cohort = 267,400; outcome: incidence; nasopharyngeal cancer = 67 cases, sinonasal cancer = 10 cases; cases identified in 1989–1998; 267,400 female textile workers drawn in 1925–1958 Low-discrimination qualitative exposure assessment for formaldehyde, which was secondary to a primary evaluation of textile production exposures; complete occupational history in textile industry was collected; factory profile form was used by industrial hygienists in Shanghai to record for each factory production processes, types of workshops, and historical measurements of hazardous exposures since establishment of factory Limited use of formaldehyde in textile operations; very few workers exposed (only 10 cases exposed to formaldehyde and none of the NPC cases were classified as exposed) Weak
Luce et al. 1993

General population in France
Case–control; outcome: incidence; sinonasal cancer = 207 cases; cases with primary malignancies of the nasal cavity and paranasal sinuses diagnosed in 1986–1988; cases obtained from 27 hospitals, hospital and community controls; analyses performed separately for squamous-cell carcinoma and adenocarcinoma, the two major histologic types Moderate-discrimination semiquantitative exposure assessment; work history collected by interview; industrial hygienist classified all jobs for probability of exposure (unexposed, possible, probable, definite); 107 cases with exposure to formaldehyde; formaldehyde concentrations in exposed jobs estimated as low (<0.1 ppm), medium (0.1–1.0 ppm), high (>1.0 ppm); authors evaluated coexposures to wood dust High correlation between wood dust, formaldehyde exposure limited ability to estimate formaldehyde effect separately Moderately strong
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×
Reference and Study Population Study Informationa Exposure Assessmentb Critique and Conclusionsc Study Qualityd
Luce et al. 2002

General populations in 7 countries
Case–control; outcome: incidence; type of nasopharyngeal cancer: adenocarcinoma = 195 cases, squamous-cell carcinoma = 432 cases; cancer cases diagnosed in 1968–1990; pooled data from 12 case–control studies in seven countries High-discrimination exposure assessment; uniform methods used in all studies to gather detailed job information; job titles and industries coded uniformly; quantitative exposure data used to construct job–exposure matrix; hygienists assigned probabilities and intensities of formaldehyde exposure; cumulative exposure was principal summary measure of exposure; 192 cases with medium or high exposure to formaldehyde; authors evaluated effects of coexposures to wood dust Statistical modeling used to evaluate effects of concurrent wood-dust and formaldehyde exposure. Strong
Luo et al. 2011

General population in 13 US regions covered by SEER registries
Ecologic study; outcome: SEER lung-cancer incidence rates by county; data on age-adjusted lung-cancer incidence rates in 1992–2007; county-level correlation of Toxics Release Inventory data on formaldehyde release with lung-cancer incidence rate from the SEER database Low-discrimination semiquantitative exposure assessment; county-level quantitative data on industrial release of formaldehyde as proxy for general population exposure in the county Caution needed in interpreting ecologic associations as causal; high potential for misclassification in large counties Weak
Mahboubi et al. 2013

General population in Montreal, Canada
Population-based case–control study; outcome: lung-cancer incidence; 1,595 male cases and 465 female cases; interviews conducted in two periods: 1979–1986 and 1996–2002. Moderate-discrimination semiquantitative exposure assessment; detailed job information gathered by questionnaire; job titles and industries coded uniformly; hygienists assigned confidence, relative concentration, and frequency of formaldehyde exposure; 99 cases with “substantial” exposure to formaldehyde; authors evaluated effects of confounding by smoking and other exposures Large, well-conducted study; broad job titles limit discrimination Little or no evidence of an association with lung-cancer incidence Moderately strong
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×
Meyers et al. 2013

US garment-industry workers
Cohort = 11,043; outcome: mortality; nasopharyngeal cancer = 0 deaths, sinonasal cancer = 0 deaths, lymphohematopoietic cancer = 107 deaths, leukemia = 36 deaths, myeloid leukemia = 21 deaths; workers employed for ≥3 months after introduction of formaldehyde-treated fabric into production process (1959 in facilities 1 and 2, 1955 in facility 3); followup through1998; update of Stayner et al. (1985) and Pinkerton et al. (2004) High-discrimination quantitative exposure assessment; personal exposure samples for formaldehyde from 549 randomly selected employees in five different departments from the 1980s; Pinkerton et al. (2004) reported geometric mean 8-hr TWA of 0.09 ppm–0.20 ppm, overall geometric mean concentration of 0.15 ppm; area monitoring showed that formaldehyde concentrations were essentially constant without substantial peaks or intermittent exposures Historical data on free formaldehyde in textile fabrics strongly suggest that exposures before 1970 were at least an order of magnitude higher than exposures in the 1980s and later (Elliot et al. 1987) Although the study design was judged to be strong, the cohort was probably not large enough to detect an effect for rare cancers, such as nasopharyngeal cancer, sinonasal cancer Strong
Olsen and Asnaes 1986

General population in Denmark
Case–control; outcome: incidence; nasopharyngeal cancer = 293 cases, sinonasal cancer = 466 cases; histologically confirmed cancer cases in 1970–1982; male cases and controls selected from Danish Cancer Registry Moderate-discrimination qualitative exposure assessment; employment histories obtained from national pension, population registries and exposure classified by job description, industry; each job rated by industrial hygienist as unexposed to formaldehyde, probably or certainly exposed, or unknown; wood-products industry is widespread in Denmark Only small numbers of cases ever exposed to formaldehyde (13 cases of squamous-cell carcinoma; 17 cases of adenocarcinoma ever exposed to formaldehyde); few with formaldehyde exposure and no wood-dust exposure No evidence of confounding by wood dust or smoking Moderately strong
Ott et al. 1989

Two Union Carbide facilities
Nested case–control; outcome: mortality; lymphohematopoietic cancer = 129 cases, leukemia = 59 cases; cases identified from review of causes of death among males from the Rinsky et al. (1987) cohort who died during 1940–1978; Union Carbide facilities also evaluated by Dell and Teta (1995) Low-discrimination qualitative exposure assessment for formaldehyde; broad job and plant departments with many exposures and few cases of formaldehyde exposure; formaldehyde exposure was assigned on the basis of work in a department that used formaldehyde Exposures not localized in production areas, probably resulting in likely broad misclassification Multiple concomitant exposures (raw materials used in the manufacturing process, including asbestos [usually chrysotile], carbon black, Weak
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×
Reference and Study Population Study Informationa Exposure Assessmentb Critique and Conclusionsc Study Qualityd
epichlorohydrin, polyvinyl chloride, acrylonitrile, styrene, chemical additives [such as plasticizers, emulsifiers, and antioxidants])
Partanen et al. 1993

Finnish wood-industry workers
Nested case–control; outcome: incidence; Hodgkin disease = 4, non-Hodgkin lymphoma = 8, leukemia = 12; cancer cases diagnosed in 1957–1982 Moderate-discrimination qualitative exposure assessment; methodology assigned exposure based on personal work histories and a job–exposure matrix that identified formaldehyde exposure; no average exposure intensity was provided Medium formaldehyde exposures likely, but study limited by small number of cases Moderately strong
Pesch et al. 2008

German wood industry
Industry-based case–control; outcome: incidence; histologically confirmed sinonasal cancer = 86 cases; recognized occupational disease diagnosed in 1994–2003; cases identified from workers insured by Holz-BG insurance company Low-discrimination qualitative exposure assessment of formaldehyde; questionnaire collection of occupational history with additional data on wood-related exposures and chemical treatments, including formaldehyde; personal sampling for wood-dust exposure in 1992–2002; expert industrial hygienists estimated wood-dust exposure to identify missing information and trends; crude assessment of formaldehyde exposures (yes/no) with no measurements; 47 cases exposed to formaldehyde (54.6%), an equal fraction of controls Strong study of wood-dust association with sinonasal cancer, but weak assessment of formaldehyde exposure Substantial exposure misclassification was likely Weak
Richardson et al. 2008

General population in Germany
Population-based case–control study of non-Hodgkin lymphoma and chronic lymphocytic leukemia; outcome: incidence; non-Hodgkin lymphoma = 858 cases; newly diagnosed cases that occurred in 1986–1998 Low-discrimination semiquantitative exposure assessment; yes/no estimates of formaldehyde exposure derived from job-history data and a job–exposure matrix that used broad job and industry groups Broad job categories; likely high misclassification of exposure Weak
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×
Roush et al. 1987

General population in Connecticut
Case–control; outcome: incidence; nasopharyngeal cancer = 173 cases, sinonasal cancer = 198 cases; histologically confirmed cases were from Connecticut Tumor Registry among males who died from any cause in 1935–1975, controls from death certificates Low-discrimination semiquantitative exposure assessment; occupational histories obtained from death certificates, city directories; exposures were assigned to broad occupation codes on basis of industrial-hygienist professional judgment; high exposure ≥1 ppm Broad job categories; likely misclassification Risk estimates adjusted for smoking, race, and other risk factors Weak
Siew et al. 2012

Finnish general population
Cohort = 1.2 million working Finnish men; outcome: incidence; nose = 292 cases, nasal squamous-cell carcinoma = 167 cases, nasopharyngeal cancer = 149 cases; followup period: 1971–1995; data linkage for all men born in 1906–1945 who were employed in 1970 Moderate-discrimination quantitative exposure assessment; occupation in 1970 linked to job–exposure matrix to estimate wood-dust exposure, formaldehyde exposure, coexposures to asbestos and silica; exposure assessment completed by professional industrial hygienists Few cases with formaldehyde exposure for three of the four types of cancer investigated (17 cases of cancer of the nose, 9 cases of nasal squamous-cell carcinoma, 5 cases of nasopharyngeal cancer, and 1,831 cases of lung cancer with any exposure to formaldehyde) Significant lung cancer–formaldehyde association may have resulted from residual confounding by smoking, wood dust, asbestos, or crystalline silica Moderately strong
Stellman et al. 1998

American Cancer Society Prevention Study II
Cohort = 362,823 men enrolled in the Cancer Prevention Study-II, 45,399 men employed in a wood-related occupation, reported exposure to wood dust, or both; outcome: cancer mortality; sinonasal cancer = 1 death, nasopharyngeal cancer = 2 deaths, lymphohematopoietic cancer = 122 deaths, non-Hodgkin lymphoma = 51 Low-discrimination qualitative exposure assessment; questionnaire given to self-identified wood workers and others with wood-dust exposure or people who reported exposure to formaldehyde (yes/no), asbestos High potential for misclassification in self-reporting exposure to formaldehyde Weak
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×
Reference and Study Population Study Informationa Exposure Assessmentb Critique and Conclusionsc Study Qualityd
deaths, Hodgkin lymphoma = 5 deaths, multiple myeloma = 20 deaths, leukemia = 46 deaths; followup period 1982–1988
Stern 2003

US tannery workers
Cohort = 9,352 men; outcome: all mortality; nasal = 1 death, leukemia and aleukemia = 16 deaths; included all production workers employed for any length of time at tannery A in 1940–1979 or at tannery B during 1940–1980; followup through 1993; study is an extension of Stern et al. (1987) Low-discrimination exposure assessment; personnel records were reviewed, subjects were grouped into five departments; semiquantitative potential exposure depended on departments; Stern et al. (1987) reported that ambient formaldehyde was measured in finishing department at time of study and was 0.5–7.0 ppm (mean 2.45 ppm) Few cases with formaldehyde exposure; standardized mortality ratio for workers in finishing department potentially exposed to formaldehyde Weak
Stroup et al. 1986

Anatomists living in the United States
Cohort = 2,317 men; outcome: all mortality; buccal cavity and pharyngeal cancer = 1 death, nasal cavity and sinuses = 0 deaths, lymphohematopoietic cancer = 18 deaths, leukemia = 10 deaths, myeloid leukemia = 5 total deaths; men who joined American Association of Anatomists and lived in United States during 1888–1969 Moderate-discrimination exposure assessment; job structure strongly related to exposure; details available for duration of association membership and time period in which anatomists joined the association, which were divided into thirds to provide a crude surrogate of cumulative exposure to formaldehyde; information on research and teaching interests, department affiliations, and membership in other professional associations used to categorize each anatomist as specialist in gross anatomy, microanatomy, both, or neither; on basis of a review of reference materials and on discussions with anatomists who were Exposure was defined aspect of job and varied according to type of anatomist Moderately strong
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×
familiar with laboratory techniques used in past, gross anatomists may have been exposed to formaldehyde more frequently than microanatomists
Vaughan et al. 1986a

General population in western Washington state
Population–based case–control; outcome: incidence reported to cancer registry; all incident cases of pharyngeal cancer (27 cases diagnosed during 1980–1983) and sinonasal cancer (53 cases diagnosed during 1979–1983) in persons between 20–74 years old who resided in the study area Moderate discrimination semiquantitative exposure assessment; jobs obtained from interview histories were assigned to broad occupation codes; likelihood and intensity of exposure were assigned on basis of industrial-hygienist professional judgment in a 4-category variable; formaldehyde exposure associated with making wood products Occupational-exposure prevalence was much lower than in West et al. (1993) Only 3.5% of jobs had any formaldehyde exposure (11 cases of nasopharyngeal cancer and 12 cases of sinonasal cancer exposed to formaldehyde above background levels) Moderately Strong
Vaughan et al. 1986b

General population in western Washington state
Population–based case–control; outcome: incidence reported to cancer registry; all incident cases of nasopharyngeal cancer (27 cases diagnosed in 1980–1983) and sinonasal cancer (53 cases diagnosed in 1979–1983) in persons between the ages of 20–74 who resided in the study area Moderate-discrimination semiquantitative exposure assessment; subjects’ residential histories, including types of dwelling, were determined from structured telephone interview, which also collected smoking, alcohol, and demographic information; residential history since 1950 included type of dwelling, use of urea-formaldehyde foam insulation, and occurrence of home renovation or new construction with particle board or plywood; information collected on lifetime occupational history to adjust for potential confounding Although questionnaire data have limited discrimination of past exposures, living in a mobile home has been associated with high formaldehyde exposure in period of about 1950 to middle1980s Moderately strong
Vaughan et al. 2000

General population in catchment of 5 US cancer registries
Population-based case–control; outcome: incidence; 196 newly diagnosed nasopharyngeal cancer cases in 1987–1993; cases were identified prospectively High-discrimination quantitative exposure assessment; detailed job, industry data from structured interviews; each job assessed on basis of industrial- Large, well-conducted study with high-discrimination exposure assessment; no assessment of peak exposures performed Strong
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×
Reference and Study Population Study Informationa Exposure Assessmentb Critique and Conclusionsc Study Qualityd
in five population-based cancer registries in United States; controls identified by random-digit dialing; expanded exposure evaluation relative to Vaughan et al. (1986a) hygienist professional judgment for probability of exposure and, if exposed, the 8-hr TWA; estimated 8-hr TWA (low <0.10 pm; moderate ≥0.10,<0.50 ppm; and high ≥ 0.50 ppm); 13.2% of jobs had ≥10% probability of exposure; coexposure to wood dust was also assessed for each job
Walrath and Fraumeni 1983

New York state embalmers and funeral directors
Cohort = 1,132 men; outcome: mortality; nasopharyngeal cancer = 0 deaths, sinonasal cancer = 0 deaths, lymphohematopoitic cancer = 25 deaths, leukemia = 12 deaths, myeloid leukemia = 6 deaths, nonwhites had 3 deaths from lymphohematopoitic cancer; persons who died in 1925–1980; 1,132 white, male embalmers and funeral directors licensed in 1902–1980; no duration of employment or length of licensure available; persons who held only funeral director’s license were not included Embalmers make up group that has well-defined high exposures to formaldehyde; tasks and formaldehyde sources are defined by regulations, training; double licensure—embalmer and funeral director—has fewer exposure opportunities Although the cohort was small, exposures likely to have been substantial with good discrimination and qualitative distinctions between exposed and not exposed Cohort probably not large enough to detect risk of rare cancers, such as sinonasal cancer, nasopharyngeal cancer Moderately strong
Walrath and Fraumeni 1984

California state licensed embalmers
Cohort = 1,007 men; cohort: mortality; sinonasal cancer = 0 deaths, lymphohematopoietic cancer = 19 deaths, leukemia = 12 deaths, myeloid leukemia = 6 deaths; men who died in 1925–1980; white male embalmers licensed in 1916–1976; 1,109 deaths; duration of licensure was available but not employment Embalmers make up group that has well-defined, high exposures to formaldehyde; tasks and formaldehyde sources are defined by regulations, training; length of licensure used as surrogate of length of employment Although the cohort was small, exposures likely to have been substantial with good discrimination and qualitative distinctions between exposed and not exposed Cohort probably not large enough to detect risk of rare cancers, such as nasal cancer, nasopharyngeal cancers Moderately strong
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×
West et al. 1993

General population in the Philippines
Population case–control; outcome: incidence; nasopharyngeal cancer = 104 cases; followup period: unknown; cases identified at Philippines General Hospital; two types of controls selected: hospital (n = 104) and community controls (n = 101) Moderate-discrimination semiquantitative exposure assessment; exposure (yes/no) assigned to specific job groups on basis of industrial-hygienist professional judgment; for those exposed, several duration variables were calculated Association with formaldehyde was stronger for participants who were positive for Epstein Barr virus No evidence of confounding or effect modification by wood dust or other exposures; estimates adjusted for age, sex, education, ethnicity Moderately strong

aThe study information includes the study type, size of cohort, outcome type, followup period or source of cases and ascertainment period, and prior studies of the same population. The study information also includes the total number of cases by cancer type, which may differ from the number of cases in other tables in Chapter 3 (Tables 3-3–3-7 give the number of cases exposed to formaldehyde).
bThe exposure-assessment information includes the overall discrimination strength of the study, key data (such as work histories, exposure data and data on jobs, tasks, operations, and key history dates), professional industrial-hygienist data analysis, classification of exposures and metrics used, and data on coexposures. See Table 3-1 and discussion of exposure assessment in Appendix C for descriptions and definitions of terms used in this column
cThe committee’s critique and conclusions include information on critical study strengths and limitations.
dThe committee’s judgment of the study quality according to the criteria that it developed and presented in Table 3-1.
Abbreviations: ICD, International Classification of Diseases; NCI, National Cancer Institute; ppm, parts per million; SEER, Surveillance, Epidemiology, and End Results program of the National Cancer Institute; TWA, time-weighted average. Source: Committee generated.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

Tables 3-33-7 present the number of exposed cases for strong and moderately strong studies as a particularly useful indicator of study power. When both disease and exposure are rare, the number of exposed cases will be an important determinant of power (Thomas 2009). The number of exposed cases also has merit because it allows a comparison of size (in the common sense that bigger studies are more powerful) of both case–control and cohort studies. The definition and ascertainment of exposed differs among studies and within some studies, so it was sometimes necessary for the committee to make a judgment about which definition to use when choosing the data to present in Tables 3-33-7. The reader is referred to the primary literature to view all data and summary measures of exposure reported by specific studies.

As discussed in Chapter 2, particular attention was paid to the choice of summary measures of exposure. Ideally, an epidemiologist chooses the appropriate measure to summarize exposure data on the basis of an understanding or hypothesis about the pharmacokinetics and pharmacodynamics of the exposure-to-dose and dose-to-response processes (Checkoway et al. 2004; Smith and Kriebel 2010). The investigators studying the association between formaldehyde and cancer have little information on which to base that choice. In practice, therefore, it is common and appropriate to test the associations by using several different summary measures, including cumulative exposure, average exposure, duration of exposure, and peak exposure. It is expected that, on average, choosing the wrong metric will result in an underestimation of an association if one exists (Checkoway et al. 2004)—that is, it is not expected that choosing the wrong summary measure of exposure will create evidence of an association where one does not exist except by chance.

Another factor that complicates the assessment of risks by alternative metrics is the imprecision and other limitations of the exposure-intensity data on which the summary measures are based. As discussed above, those data are often only approximations and are likely to have substantial uncertainty. That makes it even more difficult to assert with confidence that one summary measure is more likely than another to be “correct”. For those reasons, the committee looked at the measures of association between cancer risk and all the available summary measures presented in each study rather than choosing or preferring one a priori. Furthermore, patterns in disease associations and associated confidence intervals from smaller studies that did not reach traditional significance—that is, a p value less than 0.05 and the exclusion of 1.0 from the 95% confidence interval (CI)—were not discarded in the committee’s evaluation of the literature; they were weighed as weaker but still relevant evidence of consistency in the results.

The committee reviewed the available literature on the topic of which exposure metrics are more appropriate for environmental and occupational cancer studies. There is a long history of using cumulative exposure (the product of average intensity and exposure duration) as the summary measure of exposure

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

(Checkoway et al. 2004). Cumulative exposure tends to be proportional to disease risk and loss of function due to nonmalignant respiratory diseases caused by dusts, such as coal dust, silica, and asbestos. Possibly because of that consistency, cumulative exposure has often been used as the summary measure of exposure for other exposures and other diseases, including cancer. But in the few cases in which data are adequate for examining the relative performance of different exposure metrics, it has been found that cumulative exposure is generally not proportional to cancer risk and should not necessarily be assumed to be the correct summary measure of exposure for cancer risk. Evidence for this finding first came from the studies of Doll and Peto (1978) on smoking and lung cancer, which found that lung cancer risk was not directly proportional to cumulative tobacco exposure (packs/day smoked multiplied by the years of smoking). Cumulative exposure also does not appear to be an appropriate measure for evaluating asbestos exposure and risk of mesothelioma (Peto et al. 1982) and for both asbestos and silica and risk of lung cancer (Zeka et al. 2011). More recently, Richardson (2009) showed that leukemia risk was not proportional to cumulative benzene exposure. In the absence of knowledge about which outcome measure is applicable, the committee concluded that there was no compelling reason to prefer findings for one of the standard exposure metrics mentioned above over another. And, as noted above, the pattern of findings on all available metrics should be evaluated, data permitting.

Consistent with the RoC listing criteria, the committee used its expert scientific judgment to interpret and apply the listing criteria. Limited evidence was defined by the committee as evidence from two or more strong or moderately strong studies with varied study designs and populations that suggested an association between exposure to formaldehyde and a specific cancer type, but whose limitations led the committee to conclude that alternative explanations—such as chance, bias, and confounding factors—could not be adequately excluded and that therefore a causal interpretation could not be accepted with confidence. Sufficient evidence was defined by the committee as consistent evidence from two or more strong or moderately strong studies with varied study designs and populations that suggested an association between exposure to formaldehyde and a specific cancer type and for which chance, bias, and confounding factors could be ruled out with reasonable confidence because of the study methodologies and the strength of the findings. Consistent with those definitions, the presence of negative findings in other studies, especially weak studies, did not necessarily negate positive findings.

Nasopharyngeal Cancer

The committee reviewed the literature on epidemiologic studies of formaldehyde and nasopharyngeal cancer (see Table 3-3). Vaughan et al. (2000) was a large multicenter case–control study that was conducted in a general

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

TABLE 3-3 Studies of Nasopharyngeal Cancer and Formaldehyde Exposure

Reference and Study Population No. NPC Cancer Cases in Exposed Findings (95% CI)
Beane Freeman et al. 2013

NCI study of US chemical industry and plastics workers in 10 plants

NPC defined by ICD-8 147; number of cases identified from Tables 2–4 in the publication

n = 8

OR for highest average intensity of exposure (≥1 ppm) = 11.54 (1.38–96.81)

OR for highest peak exposure category (≥4 ppm) = 7.66 (0.94–62.34) and test for trend with increasing peak categories p < 0.005

OR for highest cumulative exposure category (≥5.5 ppm–years) = 2.94 (0.65–13.28)

Hildesheim et al. 2001

General population in Taiwan

Histologically confirmed NPC; number cases identified from Table 2 in the publication

Ever exposed to formaldehyde: n = 74

>20 years since first exposure: n = 55

OR for >10 years of exposure = 1.60 (0.91–2.90)

OR among formaldehyde-exposed subjects who were positive for Epstein Barr virus = 2.6 (0.87–7.70)

Siew et al. 2012

Finnish general population

Histologically confirmed NPC; number of cases identified from Table 3 in the publication

Any exposure to formaldehyde: n = 5

RR (adjusted for wood-dust exposure) for any formaldehyde exposure compared with no formaldehyde exposure = 0.87 (0.34–2.20)
Vaughan et al. 1986a,b

General population of western Washington state

NPC defined by ICD code 146-149: number of cases identified from Tables 3 and 5 in Vaughan et al. (1986a) and Table 2 in Vaughan et al. (1986b)

n = 11

OR (adjusted for smoking and race) for highest exposure score = 2.1 (0.6–7.8)

OR for ≥10 years occupational exposure = 1.6 (0.4–5.8)

OR for ≥10 years of residence in mobile home = 5.5 (1.6–19.4)

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×
Vaughan et al. 2000

General population in catchment of 5 US cancer registries

ICD-O codes used to classify according to three histologic groups of NPC; number of cases identified from Table 2 of the publication

Ever exposed: n = 79

Duration >5 years: n = 55

OR for highest cumulative exposure category (>1.10 ppm–years) = 3.0 (1.3–6.6)

Positive trend in disease frequency over categories of cumulative exposure (p = 0.033)

Wood-dust exposure and smoking had little effect on the relationship with formaldehyde

West et al. 1993

General population in the Philippines

Histologically confirmed NPC; number of cases identified from Table 2 of the publication

n = 26

(In some calculations in Table 2 of the publication, n = 27)

OR for ≥25 years since first exposure = 4.0 (1.3–12.3)

OR derived from the final model that was adjusted for concurrent effects of education, diesel and dust, smoking, processed meats, fresh fish, mosquito coils, and herbal medicines

Abbreviations: ICD, International Classification of Diseases; NCI, National Cancer Institute; NPC, nasopharyngeal cancer; OR, odds ratio; ppm, parts per million. Source: Committee generated.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

population. Incidence data were collected from the National Cancer Institute (NCI) Surveillance, Epidemiology, and End Results Program registries. The study was identified as a strong study (Table 3-1). There were 24 nasopharyngeal-cancer cases in the highest category of cumulative exposure, so this study was one of the largest that the committee reviewed for nasopharyngeal cancer. Its methods included a quantitative exposure assessment with moderate discrimination of who was exposed and the intensity of exposure, and the study was conducted with a well-described expert assessment of formaldehyde exposures classified by self-reported jobs of cases and controls. The estimation of the probability of exposure level or intensity of exposure in each job enabled the investigators to estimate lifetime cumulative exposure of each participant. There was evidence of increasing disease frequency with increasing exposure. The odds ratio (OR) was 3.0 (95% CI 1.3–6.6) for the highest cumulative exposure category (>1.10 ppm-year) compared with nonexposed, and there was a significant trend (p < 0.001) in the association between nasopharyngeal cancer and an increasing probability of exposure and duration. Controlling for wood-dust exposure and smoking had little effect on the association. The association appeared to be restricted to squamous-cell carcinoma rather than undifferentiated and nonkeratinizing carcinoma, although this finding is limited by small numbers.

The evidence from the Vaughan et al. (2000) study is supported by several other studies. The National Cancer Institute (NCI) industrial cohort study of mortality is one of the important additional sources of evidence. The committee judged the study to be strong. Since the completion of NTP’s assessment of formaldehyde in 2011, the NCI cohort has been updated with 10 additional years of followup: NTP’s substance profile for formaldehyde cited Hauptmann et al. (2004), and the update of that study is Beane Freeman et al. (2013). The evidence from the cohort continues to suggest that formaldehyde exposure is associated with an increase in the frequency of nasopharyngeal cancer, although even with the additional followup the numbers of exposed cases are small. There were 10 total deaths from nasopharyngeal cancer (and five total deaths from sinonasal cancer, as discussed below). Although small numbers of cases for rare cancers can be a limitation, even for strong studies, because of the high quality of the quantitative, high-discrimination exposure assessment and the design and conduct of the study, the overall results were considered strong, informative, and continue to be persuasive. In the Beane Freeman et al. (2013) study, there was evidence of increasing mortality with increasing exposure for all three exposure metrics evaluated: average, cumulative, and peak exposure (see Appendix C for discussion of exposure metrics). Compared with low exposure, those in the highest categories of each of those metrics had rate ratios of 11.54 (95% CI 1.38–96.81), 2.94 (95% CI 0.65–13.28), and 7.66 (95% CI 0.94–62.34), respectively. A strength of this study is that there was very little wood-dust exposure (only one case was thought to have had such exposure), so there is little concern that the results were confounded by wood dust (a well-known risk factor for nasopharyngeal cancer).

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

Several studies were judged to be moderately strong and provided support for the finding of increased nasopharyngeal-cancer risk (Vaughan et al. 1986a,b; West et al. 1993; Hildesheim et al. 2001; Siew et al. 2012). Vaughan et al. (1986a,b) conducted a small population-based case–control study of nasopharyngeal cancer incident cases (n = 27 total cases) that were drawn from 13 counties in western Washington state. Interviews with cases (or next of kin if cases were deceased) and controls provided information on occupation (Vaughan et al. 1986a) and residence (Vaughan et al. 1986b) from which estimates of formaldehyde exposure were developed. There was a weak association between working in a job with formaldehyde exposure and incidence of nasopharyngeal cancer (OR for 10 years or more of exposure compared with none was 1.6, 95% CI 0.4–5.8). There was somewhat stronger evidence of an association between living in a mobile home (a well-documented source of formaldehyde exposure) and incidence of nasopharyngeal cancer (OR for 10 years or more of residence compared with none was 5.5, 95% CI 1.6–19.4) (Vaughan et al. 1986b).

West et al. (1993) conducted a moderately large population-based case–control study of incident cases of nasopharyngeal cancer in the Philippines. The exposure assessment appeared to be a well conducted, semiquantitative assessment with moderate discriminations of exposure and was based on blind expert evaluation of the reported job histories. Several metrics of formaldehyde exposure, particularly in the distant past, were positively associated with nasopharyngeal-cancer incidence. The authors gathered data on several potential confounders, including wood dust, smoking, and dietary factors. In a final model that controlled for confounders, the authors reported that subjects first exposed to formaldehyde 25 years or more prior to diagnosis had an OR of 4.0 compared with never exposed (95% CI 1.3–12.3). Control for smoking and “dust” exposure did not weaken the association.

A somewhat larger population-based case–control study of incident cases with a semiquantitative exposure that had moderate discrimination was conducted in Taiwan by Hildesheim et al. (2001). The exposure assessment was similar to that of West et al. (1993) in that an industrial hygienist reconstructed each subject’s occupational history. There was an increased incidence of nasopharyngeal cancer in the longest duration-of-exposure category (OR = 1.60, 95% CI 0.91–2.90), and there was some evidence that the association was stronger in subjects who were seropositive for Epstein Barr virus (OR = 2.6, 95% CI 0.87–7.7).

Siew et al. (2012) used several Finnish national databases to evaluate associations between incidence of sinonasal, nasopharyngeal, and lung cancers and exposures to wood dust and formaldehyde. Cases of those cancers were diagnosed among Finnish men during 1971–1995, and were linked to census data on occupations. A job-exposure matrix was used to estimate wood-dust and formaldehyde exposures for subjects based on their occupations. There were only five nasopharyngeal cancer cases with any formaldehyde exposure and the relative risk (RR) for any formaldehyde exposure compared to no formaldehyde exposure was 0.87. There was a wide confidence interval (95% CI 0.34–2.20).

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

An industrial cohort study of mortality by Meyers et al. (2013) was judged to be a strong study because it was well-designed with a high-discrimination, quantitative exposure assessment and it included Poisson regression modeling to control for confounding; however, it contributed little information to the evaluation of formaldehyde exposure and nasopharyngeal cancer in that it was not sufficiently large to detect an effect for rare cancers such as nasopharyngeal cancer. There was only a little more than one death expected from nasopharyngeal cancer (n = 1.33), and none were observed.

Several studies that were judged to be moderately strong also contributed little information to the evaluation of nasopharyngeal cancer in that they had a small number of subjects who had nasopharyngeal cancer and were exposed to formaldehyde: Walrath and Fraumeni (1983, 1984), Levine et al. (1984), Stroup et al. (1986), Andjelkovich et al. (1995), and Coggon et al. (2014). Walrath and Fraumeni (1983) reported on proportionate mortality in 1,132 deaths of embalmers in New York. The authors reported that there were no deaths from cancer of the nasopharynx. The authors conducted a similar study of licensed embalmers in California (Walrath and Fraumeni 1984) and again observed no deaths from nasal or nasopharyngeal cancer. The study by Levine et al. (1984) of 1,477 Ontario undertakers with 319 deaths from all causes found one death from cancer of the buccal cavity and pharynx (2.1 expected, standardized mortality ratio [SMR] and CIs not given). The authors did not report whether that death was from nasopharyngeal cancer or a different neoplasm. Stroup et al. (1986) reported a retrospective cohort study of mortality in 2,317 male American anatomists. All or nearly all worked with embalming fluid, which contains formaldehyde and other volatile chemicals. One death from buccal cavity and pharyngeal cancer was observed (6.8 deaths expected, SMR = 0.2, 95% CI 0.0–0.8). The authors did not report whether that death was from nasopharyngeal cancer or a different neoplasm. Andjelkovich et al. (1995) evaluated mortality in a subset of automotive iron-foundry workers in Michigan. The original cohort was 8,147 men, and the subcohort exposed to formaldehyde, 3,929 men. There was one death from nasopharyngeal cancer in the exposed group (no SMR or 95% CI reported). Coggon et al. (2014), an update of the industrial cohort study of mortality by Coggon et al. (2003), reported only one death from nasopharyngeal cancer.

Several studies did not contribute to the committee’s assessment of formaldehyde exposure and nasopharyngeal cancer, because the committee judged the studies to be weak and inconclusive (see Tables 3-1 and 3-2). Roush et al. (1987) conducted a population-based case–control study of incidence in 173 men drawn from the Connecticut Cancer Registry who had a history of nasopharyngeal cancer and had died. Occupation was determined from death certificates and city directories. The probable level of formaldehyde exposure was determined from job title, industry, specific employment, and year of employment. For the seven deaths in the highest exposure category—probably exposed to some level of formaldehyde for most of their working life and probably exposed at a high level for 20 years or more prior to death—the OR was 2.3 (95% CI

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

0.9–6.0; two-sided, p = 0.100), adjusted for age at death, year of death, and availability of occupational information. ORs were given for 14 specific industry categories; none was statistically significant, although numbers were small. Coexposures and residential exposures to formaldehyde were not addressed. Dell and Teta (1995) reported a long-term mortality study of an industrial cohort of workers in a single plastics manufacturing and research and development (R&D) plant in the United States. Of 5,932 male employees, 111 had job assignments that involved formaldehyde. The number of deaths in this small group was not stated, but none was from nasopharyngeal cancer. Hansen and Olsen (1995) investigated cancer incidence in an industrial cohort of men who were employed at 265 companies in Denmark in which formaldehyde exposure was identified. The authors reported standardized proportionate incidence ratios (SPIRs) adjusted for age and calendar period; the comparison group was the Danish population as reported to the Danish Cancer Register. Four cancers of the nasopharynx were reported (3.2 expected, SPIR = 1.3, 05% CI 0.3–3.2). Other coexposures were not reported or adjusted for. Stellman et al. (1998), in an update of the industrial cohort mortality study of the American Cancer Society (ACS) Cancer Prevention Study–II, found one cancer of the nasopharynx in study participants who had an occupational history of exposure to wood dust (OR = 0.44, 95% CI 0.06–3.29) and one in men who had worked in a wood-related occupation (OR 1.44, 95% CI 0.19–10.9). Coexposures were not reported. Armstrong et al. (2000) conducted a large population-based case–control study of nasopharyngeal-cancer incidence (282 cases, all cases were squamous-cell carcinomas) in predominantly Chinese Malaysians. The exposure assessment was qualitative, and the study found no evidence of an association with formaldehyde exposure. Limitations in exposure assessment may contribute to an explanation of the low reported prevalence of formaldehyde exposure (for example, only eight cases reported more than 10 years of exposure and more than 10 years of latency), or formaldehyde exposure may simply have been rare and at low in concentration in the population. In either case, the uninformative finding of this limited study does not weaken the apparent association between formaldehyde exposure and nasopharyngeal cancer. Li et al. (2006) conducted a large industrial cohort study of nasopharyngeal cancer incidence in female textile workers in China that included a low-discrimination, qualitative exposure assessment for formaldehyde (years for ever exposed vs never exposed). The authors noted that there was a potential for formaldehyde exposure to be misclassified. The study had some potential to be informative, but the investigators found few workers who had formaldehyde exposures—10 noncases and no cases were identified as having formaldehyde exposure.

In summary, the committee found that epidemiologic studies provided evidence of a causal association between formaldehyde exposure and nasopharyngeal cancer in humans. Evidence of an association was derived from a strong population-based case–control study (Vaughan et al. 2000), a strong industrial cohort study (Beane Freeman et al. 2013), and several moderately strong population-based case–control studies (Vaughan et al. 1986a,b; West et al. 1993;

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

Hildesheim et al. 2001; Siew et al. 2012). See Table 3-3 for important key measures of association. The conclusion was based on the strength, consistency, temporality, dose–response relationship, and coherence of the evidence and on the considerations presented in Table 3-1. The most informative epidemiologic studies were ones that were large, that estimated exposure systematically, that had credible comparison groups, and that assessed cancer end points reliably. Not all studies that were judged as strong or moderately strong were informative in the evaluation of the evidence on nasopharyngeal cancer, because of the rarity of tumors at this site and because the studies reported only a few or no deaths from nasopharyngeal cancer. Other studies had sufficient cases but had weak exposure evaluations. The weakest and least informative studies had limited exposure assessments and few or no cases of nasopharyngeal cancer.

Sinonasal Cancer

The committee reviewed the literature on epidemiologic studies of formaldehyde and sinonasal cancer (see Table 3-4). The strongest study was the pooled population-based case–control study by Luce et al. (2002) that assessed incidence data. It provided evidence of an association between formaldehyde exposure and sinonasal cancer. As mentioned in Chapter 2, a pooled study differs from a meta-analysis in that the data from the studies are combined into a single dataset by using the same or similar case definitions and exposure assessments; this is analogous to what is done in a multisite cohort study. The Luce et al. study was particularly valuable because a new exposure assessment was conducted to inform each of the 12 studies that were assembled for the pooled analysis. The exposure assessment was quantitative and had high discriminatory ability; it estimated the level of exposure (average air concentration) and probability of exposure. The exposure data permitted the investigators to analyze risks among categories of cumulative exposure. There was strong evidence of an association between adenocarcinoma and formaldehyde exposure. For example, the OR for sinonasal-cancer incidence was 3.0 (95% CI 1.5–5.7) in men who were in the highest tertile of cumulative formaldehyde exposure compared with no exposure. The comparable OR in women was 6.2 (95% CI 2.0–19.7). The association between formaldehyde and squamous-cell carcinoma was weaker and showed little evidence of a trend. The association between formaldehyde and adenocarcinoma was investigated for possible confounding or effect modification by wood-dust exposure. The researchers used multiple logistic regressions, including analysis of the level of wood-dust exposure as a covariate and stratification on wood-dust exposure, to examine the association between formaldehyde exposure and adenocarcinoma in those who had no wood-dust (or leather-dust) exposure. The results showed only a modest weakening of the formaldehyde risk. In women, the OR for high cumulative exposure fell from 6.2 to

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

TABLE 3-4 Studies of Sinonasal Cancer and Formaldehyde Exposure

Reference and Study Population No. SNC Cases in Exposed Findings (95% CI)
Hayes et al. 1986

General population in the Netherlands

Histologically confirmed ICD-9 160, 160.2–160.5; two raters (A and B) for exposure; number of cases identified from Tables 3 and 4 of the publication

Any formaldehyde exposure, low wood-dust exposure: rater A, n = 15; rater B, n = 24

Squamous-cell carcinoma with any formaldehyde exposure, low wood-dust exposure: rater A, n = 12; rater B, n = 19

OR for squamous-cell carcinoma cases comparing any vs no formaldehyde exposure = 3.0 (90% CI 1.3–6.4) for rater A, 1.9 (90% CI 1.0–3.6) for rater B

OR for squamous-cell carcinoma cases comparing high vs no formaldehyde exposure (with low wood-dust exposure) = 3.1 (90% CI 0.9–10.0) for rater A, 2.4 (90% CI 1.1–5.1) for rater B

Rater B assigned proportionally more controls to formaldehyde exposure compared with rater A; rating from both raters showed an increase in OR with increasing formaldehyde assignments

Luce et al. 1993

General population in France

Cancer of nasal cavity and paranasal sinuses ICD-9 160.0, 160.2–160.9; number of cases identified from Table 2 of the publication

Adenocarcinoma with probable or definite exposure (male and female): n = 70

Squamous-cell carcinoma with probable or definite exposure (male and female): n = 18

OR for adenocarcinoma from possible, probable, or definite formaldehyde exposure and no or low wood-dust exposure = 8.1 (0.9–72.9)
Luce et al. 2002

General populations of 7 countries

Number of cases identified from Table 3 of the publication

Adenocarcinoma cases with medium or high exposure: n = 122 male; 5 female

Squamous-cell carcinoma cases with medium or high exposure: n = 70 male; 13 female

OR for adenocarcinoma (adjusted for age and wood- and leather-dust exposure) from high formaldehyde exposure, male = 3.0 (1.5–5.7); female = 6.2 (2.0–19.7)

OR for adenocarcinoma from high formaldehyde exposure and no wood- or leather-dust exposure, male = 1.9 (0.5–6.7); female = 11.1 (3.2–38.0)

OR for squamous carcinoma from high formaldehyde exposure, male = 1.2 (0.8–1.8)

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×
Reference and Study Population No. SNC Cases in Exposed Findings (95% CI)
Olsen and Asnaes 1986

General population in Denmark

Histologically confirmed ICD-7 160.0, 160.2–160.9; number of cases identified from Table 4 of the publication; most formaldehyde exposures occurred in Danish wood-working industry and few formaldehyde cases not exposed to wood dust

Ever vs never exposed to formaldehyde:

    - Squamous-cell carcinoma: n = 13

    - Adenocarcinoma: n = 17

Ever vs never exposed to formaldehyde, pooled estimate for formaldehyde exposure adjusted for wood-dust exposure:

    - Squamous-cell carcinoma of the nasal cavity and sinuses: OR = 2.3 (95% CI 0.9–5.8)

    - Adenocarcinoma of nasal cavity and sinuses: OR = 2.2 (95% CI 0.7–7.2)

≥10 years since first exposure, pooled estimate for formaldehyde exposure adjusted for wood-dust exposure:

    - Squamous-cell carcinoma of the nasal cavity and sinuses: OR = 2.4 (0.8–7.4)

    - Adenocarcinoma of nasal cavity and sinuses: OR = 1.8 (0.5–6.0)

Siew et al. 2012

Finnish general population

Nasal cancer; number of cases identified from Table 3 in the publication

Any exposure to formaldehyde: n = 17

RR (adjusted for wood dust) for any formaldehyde exposure compared with no formaldehyde exposure = 1.11 (0.66–1.87)
Vaughan et al. 1986a,b

General population in western Washington state

SNC defined by ICD 160: number of cases identified from Tables 3 and 5 in Vaughan (1986a) and Table 2 in Vaughan (1986b)

Exposed to formaldehyde above background, n = 12

OR (adjusted for age, sex, smoking, and alcohol) for number of years exposed: 1–9 years = 0.7 (0.3–1.4); ≥10 years = 0.4 (0.1–1.9)

OR (adjusted for age, sex, smoking, and alcohol) for cumulative exposure score (all years): 5-19 = 0.5 (0.1–1.6); ≥20 years = 0.3 (0.0–2.3)

OR (adjusted for age, sex, smoking, and alcohol) for cumulative exposure score (15-year lag period): 5-19 = 1.0 (0.3–2.9)

Abbreviations: ICD, International Classification of Diseases; OR, odds ratio; ppm, parts per million; RR, relative risk; SNC, sinonasal cancer. Source: Committee generated.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

5.8 (95% CI 1.7–19.4), and males showed a similar reduction. A number of other studies that were judged to be moderately strong contributed to the conclusion that this study was not anomalous. The two key strengths of the Luce et al. (2002) study are the great size and the high-quality exposure assessment; the other studies were smaller and had less adequate exposure assessments. All of the studies have their own limitations, but taken as a whole they provide corroborating evidence.

The moderately strong studies identified by the committee that supported an association between exposure to formaldehyde and sinonasal cancer were Hayes et al. (1986), Olsen and Asnaes (1986), Vaughan et al. (1986 a,b), Luce et al. (1993), and Siew et al. (2012). Hayes et al. (1986) conducted a population-based case–control study of the incidence of histologically confirmed cases of sinonasal cancer in the Netherlands from 1978 to 1981. A low-discrimination, qualitative exposure assessment was conducted independently by two trained hygienists (rater A and rater B) who classified all jobs as to the level (intensity) and probability of formaldehyde (and wood-dust) exposure. The study was large enough to permit separate assessment of risks specifically for cases of squamous-cell carcinoma (there were at least 12 cases with formaldehyde exposure). For all sinonasal cancer combined, the OR was approximately doubled when the exposed were compared with the nonexposed; the CIs excluded 1.0. The authors stratified their analysis by wood-dust exposure (none and low vs high) and found that there were trends of increasing incidence with increasing level of formaldehyde exposure in the no or low wood-dust stratum. That pattern was more evident for squamous-cell carcinomas (there were not enough adenocarcinomas in the group with low wood-dust exposure to permit this analysis). The OR was 3.1 (90% CI 0.9–10.0) for high formaldehyde exposure and low or no wood-dust exposure vs no formaldehyde exposure for rater A and 2.4 (90% CI 1.1–5.1) in the same category for rater B. Rater B assigned proportionally more controls to formaldehyde exposure compared with rater A. The rating from both raters showed an increase in OR with increasing formaldehyde exposure.

Olsen and Asnaes (1986) was an update of Olsen et al. (1984). In the 1986 study, the authors conducted a population-based case–control study of incidence nested in the Danish cancer registry, and they included cancer controls. Denmark has a large wood-working industry, which also includes some formaldehyde exposures. As a result, few cases have formaldehyde exposure without wood-dust exposure. The study had a limited exposure assessment that was based on expert evaluation of job information. The exposure assessment was qualitative and was of moderate discrimination in its assessment in determining whether each subject had certainly or probably been exposed to formaldehyde. The authors investigated separately the association between formaldehyde exposure and incidence of the two main histologic types of nasal and paranasal sinus cancer—squamous-cell carcinoma and adenocarcinoma. When the ever exposed to formaldehyde were compared with the never exposed to formaldehyde, the ORs were very similar for the two subtypes; 2.3 (95% CI 0.9–5.8) for squamous-cell carcinoma and 2.2 (95% CI 0.7–7.2) for adenocarcinoma. Although limited by small

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

numbers, there was evidence of increased incidence of adenocarcinoma from formaldehyde exposure in subjects who were not exposed to wood dust (OR = 7.0, 95% CI 1.1–43.9). When the data were examined for 10 or more years since first exposure, the OR for squamous-cell carcinoma was 2.4 (95% CI 0.8–7.4) and the OR for adenocarcinoma was 1.8 (95% CI 0.5–6.0).

Vaughan et al (1986a) undertook a population-based case–control study in Washington state of 53 incident cases of sinonasal cancer, including 12 in people thought to have had occupational exposure to formaldehyde. The authors found no evidence of increased risk with maximum exposure, number of years exposed, a cumulative exposure score, or the cumulative exposure score with a 15-year lag period. Vaughan et al. (1986b) used the same study group as Vaughan et al. (1986a) to examine the role of residential exposures and sinonasal cancer. Evaluations were reported for people exposed in mobile homes (5 cases, OR = 0.6, 95% CI 0.2–1.7), people living for not more than 10 years in new or renovated housing with particle board or plywood (13 cases, OR = 1.8, 95% CI 0.9–3.8), and people living for 10 years or more in new or renovated housing with particle board or plywood (12 cases, OR = 1.5, 95% CI 0.7–3.2). The authors did not investigate coexposures except for lifetime smoking history and recent consumption of alcoholic beverages.

Luce et al. (1993) conducted a large population-based case–control study (207 cases and 409 controls) of the incidence of sinonasal cancer in France. Histologic data allowed separate investigations of adenocarcinoma and squamous-cell carcinoma. The exposure assessment was semiquantiative with moderate discrimination in that it was based on expert judgment without measurement data for assessment of jobs (which were classified by probability of exposure) and expert assessment of exposure frequency and intensity. The investigators started with a large case series: there were 38 adenocarcinoma cases that had more than 30 years of exposure to formaldehyde. The squamous-cell carcinoma series was somewhat smaller—five in the longest duration category. The study was limited in its ability to discriminate risks associated with potentially confounded wood-dust and formaldehyde exposure, and nearly all cases that had formaldehyde exposure also had probable or definite wood-dust exposure; only four adenocarcinoma cases that had possible, probable, or definite formaldehyde exposure were believed to have had no or low wood-dust exposure (OR = 8.1, 95% CI 0.9–72.9). The authors also reported that the combination of wood dust plus formaldehyde exposure was associated with a higher risk of adenocarcinoma than wood dust alone, although confidence intervals were wide because of the small number of cases.

Siew et al. (2012), the cohort of Finnish men from a national database, was summarized above in the section on nasopharyngeal cancers. There were 17 cases of cancer of the nose and paranasal sinuses in Finnish men identified as having any occupational exposure to formaldehyde. There was a weak association of cancer in those who had any exposure to formaldehyde compared to no exposure to formaldehyde (RR = 1.11, 95% CI 0.66–1.87).

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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The recently updated NCI industrial cohort study of mortality was judged to be strong, but the number of sinonasal-cancer cases was small (Beane Freeman et al. 2013). There were five deaths from sinonasal cancer in this large cohort (three deaths in the exposed population compared to 3.3 expected deaths). There was no evidence of increased mortality from this cancer, but because of the small numbers of expected deaths from sinonasal cancer, little weight was given to these findings.

Meyers et al. (2013), an update of Pinkerton et al. (2004), was also judged to be a strong industrial cohort study of mortality, but it contributed little information because of its size; there were only 0.95 cases of sinonasal cancer expected and none were observed. The authors investigated mortality in 11,043 workers in three garment plants (Meyers et al. 2013). There were no deaths from sinonasal cancer among in 3,915 deaths reported. Additional details were not provided.

Several studies were judged to be moderately strong, but they contributed little information to the evaluation of sinonasal cancer because few subjects who had sinonasal cancer had been exposed to formaldehyde: Walrath and Fraumeni (1983, 1984), Levine et al. (1984), Stroup et al. (1986), and Coggon et al. (2014). The studies by Walrath and Fraumeni (1983, 1984) were described in the nasopharyngeal-cancer section above; the results of the two studies were not informative for evaluating sinonasal cancer, because no cases were reported. The study by Levine et al. (1984) of a cohort of 1,477 Ontario undertakers with 319 deaths from all causes found no deaths from cancer of the nose, middle ear, or sinuses (0.2 deaths expected, SMR and CIs not given). Stroup et al. (1986) reported a retrospective cohort study of mortality in 2,317 male American anatomists. All or nearly all worked with embalming fluid, which contains formaldehyde and other volatile chemicals. None of the 738 deaths was from cancer of the nasal cavity or sinuses (0.5 deaths expected, SMR = 0, 95% CI 0.0–7.2). Coggon et al. (2014) completed a long-term study of mortality in a cohort of 14,014 men in six British plants where formaldehyde was produced or used. In the group of workers whose jobs that were classified as having potential formaldehyde exposure, there were two deaths from cancer of the nose and nasal sinuses (2.8 deaths expected from US national rates, SMR = 0.71, 95% CI 0.09–2.55). Coexposures were not discussed.

Several studies did not contribute to the committee’s assessment of formaldehyde exposure and sinonasal cancer, because the committee judged the studies to be weak and inconclusive (see Tables 3-1 and 3-2). Roush et al. (1987) conducted a population-based case–control study of incident cases in 198 men in the Connecticut Cancer Registry who had a history of sinonasal cancer and died. Occupation was determined from death certificates and city directories. Probable level of formaldehyde exposure was determined from job title, industry, specific employment, and year of employment. The OR for the seven deaths in the highest exposure category was 1.5 (95% CI 0.6–3.9) (adjusted for age at death, year of death, and availability of occupational information). ORs were given for 14 specific industry categories, and none was statistically signifi-

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

cant, but the numbers were small. Coexposures and residential exposures to formaldehyde were not addressed.

Dell and Teta (1995) reported a long-term study of mortality in a cohort of industrial workers in a single plastics manufacturing and R&D plant in the United States. Of 5,932 male employees, 111 had job assignments that involved formaldehyde. The number of deaths in this small group was not stated, but none was from sinonasal cancer.

Hansen and Olsen (1995, 1996) conducted a study in a large national cancer cohort of industrial workers and reported SPIRs. The authors obtained government employment data on blue-collar workers employed in Danish industries who were identified as having used formaldehyde and linked those data with cancer-registry data. A national product register was used to identify workers in broad industries in which formaldehyde was used and formaldehyde exposure was likely. The records were used to determine a moderate-discrimination, semiquantitative metric of formaldehyde exposure: duration of work with potential formaldehyde exposure. A similar approach was used to determine wood-dust exposure at the industry level by identifying industrial classification codes that corresponded with jobs that used wood products. Only 13 cases of cancer of the nasal cavity were reported to the national cancer registry (compared with 5.2 deaths expected on the basis of the proportionate distribution of all cancers combined) in men whose longest job was in a company that used formaldehyde. The investigators calculated an SPIR as an estimate of the rate ratio; for nasal cancer, the SPIR was 2.3 (95% CI 1.3–4.0). When the data were limited to blue-collar workers in formaldehyde-using industries in which wood products were not used, the SPIR increased to 3.0 (95% CI 1.4–5.7).

Stellman et al. (1998), in an update of the industrial cohort mortality study of the ACS Cancer Prevention Study-II, found one death from sinonasal cancer in men who had wood-dust exposure and found no evidence of an association with formaldehyde. Stern (2003) completed a study of mortality in an industrial cohort of 9,352 tannery workers in jobs that often included formaldehyde exposure; one death from cancer of the nasal cavity was reported (SMR not given). Pesch et al. (2008) conducted an industry-based case–control study of incident cases of adenocarcinoma of the nasal cavity and paranasal sinuses in the German wood industry (86 male cases, 204 controls). In the group of workers who were exposed to formaldehyde and wood products, eight cases were exposed to formaldehyde before 1985 (OR = 0.46, 95% CI 0.14–1.54), and 39 cases were exposed to formaldehyde in 1985 or later (OR = 0.94, 95% CI 0.47–1.90). Because both cases and controls were exposed to wood dust, a recognized cause of sinonasal cancer, extension to the general population is uncertain.

The committee found that epidemiologic studies provided evidence of a causal association between formaldehyde and sinonasal cancer in humans. Evidence of an association was derived from the strong pooled case–control studies of sinonasal cancer (Luce et al. 2002) and several moderately strong population-based case–control studies (Hayes et al. 1986; Olsen and Asnaes 1986; Vaughan et al. 1986a.b; Luce et al. 1993; Siew et al. 2012). See Table 3-4 for important

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

key measures of association. The conclusion was based on the strength, consistency, temporality, dose–response relationship, and coherence of the evidence and on the considerations presented in Table 3-1. The most informative epidemiologic studies were the ones that were large, that estimated exposure systematically, that had credible comparison groups, and that assessed cancer end points reliably. The studies that did not find associations were usually too small to detect an effect for these rare cancers or used methods of exposure assessment that had little ability to discriminate exposures, and they did not provide convincing evidence that there were sufficient numbers of highly exposed subjects.

Lymphohematopoietic Cancers

The committee reviewed the literature on a potential association between formaldehyde exposure and lymphohematopoietic cancers. This section begins with a discussion of methodologic considerations in exposure assessment in studies of lymphohematopoietic cancers and then discusses in greater detail studies in industrial cohorts and studies in embalmers and others in the funeral trade, anatomists, and pathologists. Data from studies that the committee judged to be strong and moderately strong and informative are presented in Tables 3-5 (industrial workers), 3-6 (funeral workers, embalmers, pathologists, and anatomists), and 3-7 (general population).

Methodologic Considerations in Exposure Assessment in Studies of Lymphohematopoietic Cancers

In the substance profile for formaldehyde, NTP considered the most informative primary studies for the evaluation of lymphohematopoietic cancers to be the study of mortality in the large NCI cohort of formaldehyde-industry workers (Beane Freeman et al. 2009) and the NCI nested case–control mortality study of embalmers and funeral directors, which was based on a cohort of funeral-industry workers (Hauptmann et al. 2009). Those were judged to be the strongest studies because of the high quality of the quantitative exposure assessments, which included assignments of participants into exposure categories with high discrimination.

When large occupational cohorts are used to study relatively rare cancer, subpopulations are drawn from several worksites of varying size to obtain sufficient cases. Although the worksites have exposure to formaldehyde as a common feature, they can have large differences in exposure conditions even if the job titles and types of operations are the same (see Appendix C for a more detailed discussion). Beane Freeman et al. (2009) conducted a comprehensive exposure assessment, which increases confidence that valid exposure–response trends can be derived from the diverse industries and exposure conditions.

Both the formaldehyde-industry (Beane Freeman et al. 2009) and funeral-industry (Hauptmann et al. 2009) cohorts included extensive separate evalua-

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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tions of occupational exposures, their determinants, and modeling approaches to reconstructing unmeasured historical exposures.2 The exposure studies of the formaldehyde-industry cohort were reported by Blair et al. (1986, 1990) and Hauptmann et al. (2004). The exposure studies of the funeral-industry cohort were reported by Stewart et al. (1992). The committee recognized that those additional exposure studies were keys to the strength of the epidemiologic studies. Because Beane Freeman et al. (2009) and Hauptmann et al. (2009) were critical for the formaldehyde assessment of lymphohematopoietic cancers, this section elaborates on their approaches.

The exposure assessments for the formaldehyde-industry and funeral-industry cohorts were designed to determine exposures associated with job titles and worksites listed in the work histories of the study subjects so that exposures and subjects could be linked. Historical changes in job activities and in the formaldehyde industry produced substantial differences in temporal profiles of exposure. Industrial exposures have declined considerably since the early 1970s as a result of process changes and engineering controls of process emissions. The exposures in the Beane Freeman et al. (2009) study changed (more in some jobs than in others), and the data suggest that exposures in the 1960s were much higher than those after 1970 (Blair et al. 1986, 1990). Embalming-fluid emissions of formaldehyde have probably changed little, but local exhaust ventilation was added in some funeral homes and was estimated to have reduced exposure by 50–90% (Stewart et al. 1992).

Exposures in the industrial and embalming settings were described by time-weighted averages (TWAs) and short-term measurements. The short-term measurements were used to capture brief (15 minutes) intense exposures called peaks. Although peaks are part of the distribution of short-duration concentrations that contribute to the longer TWA measurements, they might not correlate well with the overall average (Blair and Stewart 1990), as was seen in the Beane Freeman et al. study (2009). Blair and Stewart (1990) also noted that exposure metrics can differ among manufacturing plants because in some plants everyone is exposed but in others only half the workforce is in areas with exposure or because similar work areas had lower exposures.

As explained in Appendix C, the summary measures of exposure (which are also called exposure metrics or dose metrics) used in epidemiologic studies are weighting schemes applied to summarize the complex temporal profiles of personal exposure histories. In that application, they are analogous to the concept of dose applied in toxicologic studies, but there is no universal dose metric that applies to all toxic responses, including carcinogenesis. Some dose metrics are not appropriate for the underlying biology, and when an inappropriate metric is used, a weaker or no dose–response relationship will usually be observed

__________________

2Appendix C provides a general summary of exposure assessments, the rationale for estimating exposures on the basis of physical principles, and a description of methods for measuring airborne formaldehyde exposures.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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(Blair and Stewart 1990; Smith and Kriebel 2010). Although cumulative exposure is the most common dose metric for chronic, minimally reversible disease processes, it is probably not the optimal dose measure for studying cancer (Smith and Kriebel 2010), as noted above. A fundamental feature of cumulative exposure is that it gives equal weight to long, low-intensity exposures and short, high-intensity exposures, which may not be biologically appropriate for cancer biology. A lag time until effects are observed may also be included in the exposure metric to account for an induction period between the first exposure to formaldehyde and the diagnosis of cancer. That period includes any delay from first exposure to the exposure that initiated the cancer, the time from initiation through the biologic events that led to malignant change, and the time required for that change to produce signs or symptoms that result in diagnosis. Those steps are commonly thought to require at least 10 years for solid cancers in adults, perhaps less for leukemia and lymphomas.

Epidemiologic models that use exposure metrics for peak exposures hypothesize an underlying nonlinear damage process in which exposures at low concentrations have little or no effect and exposures at high concentrations produce disproportionate effects. That might indicate a threshold process, or some protective process might be overwhelmed or a damaging secondary process might occur. When the mode of action is unknown, it is common for epidemiologists to try several exposure metrics, such as cumulative exposure and peak exposure that have different biologic implications (Blair and Stewart 1990).

The mechanistic process associated with the cumulative exposure and peak exposure metrics appear to be different, and conceptually the metrics should be useful for obtaining insight about the possible mechanism of the effects. Unfortunately, the precision of estimated metric values is often limited by sparse historic data and the cost of making measurements, variation of exposure between subjects, process and material variation in the industrial operations, and business and economic variations in the demand for a product. If the precision is too limited, it may not be possible to determine which metric is the strongest. Data quality and extrapolation approaches may favor one dose metric over another. Thus, as discussed above, it is common for epidemiologists to calculate several different exposure metrics, such as cumulative exposure, average exposure, and the occurrence or frequency of peaks. When data and resources are limited, epidemiologists often use simpler metrics, such as years of work in a job, categories of ever exposed vs never exposed on the basis of job title or work location, or sometimes even ‘ever having worked in an exposed industry’.

In addition to the NCI formaldehyde-industry study (Beane Freeman et al. 2009) and the NCI nested case–control study (Hauptmann et al. 2009), Meyers et al. (2013), an update of Pinkerton et al. (2004), was considered to have strong methods (Table 3-2). The study investigated mortality in an industrial cohort of garment workers. The authors relied on earlier studies of the same sites by Stayner et al. (1985, 1988), Acheson et al. (1984), and Gardner et al. (1993).

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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Semiquantitative exposure estimates were developed on the basis of small numbers of measurements, job activities, and reports of sensory irritation in jobs or work locations.

There were also several moderately strong studies of limited utility in industrial workers (Bertazzi et al. 1989; Partanen et al. 1993; Andjelkovich et al. 1995; Coggon et al. 2014) and embalmers, anatomists, or pathologists (Walrath and Fraumeni 1983, 1984; Levine et al. 1984; Stroup et al. 1986). Those had smaller populations and less discriminating exposure assessments and as a result contributed less to the evidence of an association between formaldehyde and lymphohematopoietic cancers than did the strong studies. Most of the smaller studies used job information alone to define those who were “exposed”—an approach that has little ability to discriminate among people with varied levels of exposures. Duration of exposure obtained from occupational histories was used as a semiquantitative exposure metric, but again, duration alone does not discriminate among exposures that have different intensities.

Population-based case–control studies have the most serious problem of exposure misclassification because they draw from the broad mixture of personal and industrial activities throughout the population in a wide area. For example, the broad job categories of “mortician” and “undertaker” include embalmers (the most highly exposed) but also include a number of less exposed occupations. People in some of those other occupations may occasionally do embalming, but less frequently, and embalming is not one of their main job activities. The categories also include funeral directors, who usually do not embalm. And differences are related to the size of funeral homes’ businesses. Use of narrow, well defined, specific job titles, such as a focus on embalmers, can greatly reduce misclassification even without specific measurements.

Studies of Industrial Cohorts Exposed to Formaldehyde

Table 3-5 provides the studies of industrial cohorts exposed to formaldehyde that the committee judged to be strong or moderately strong. As already stated, the NCI industrial-worker cohort mortality study is large, well conducted, and informed by a quantitative, high-discrimination exposure assessment (Beane Freeman et al. 2009). The investigators collected mortality data on workers employed in US chemical factories that used formaldehyde during 1966–2004. The study was the largest in terms of numbers of exposed cancer cases—there were 286 hematologic-malignancy cases, including 116 leukemia cases, and 44 of the leukemia cases were classified as myeloid leukemia. Exposure levels varied widely over time and among plants; the estimated overall median daily exposure was 0.3 ppm. The manufacturing plants produced a various of products, including formaldehyde (plants 2, 7, and 10), formaldehyde resins and molding compounds (plants 1, 2, and 7–10), molded plastic products (plants 8 and 9), photographic film (plants 4 and 5), decorative laminates (plant 6), and plywood (plant 3) (Blair et al. 1990).

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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TABLE 3-5 Lymphohematopoietic Cancers: Industrial Workers

Reference and Study Population No. Cancer Cases in Exposed Findings (95% CI)
All Lymphohematopoi etic Cancer Leukemia Myeloid Leukemia All Lymphohematopoietic Cancer Leukemia Myeloid Leukemias
Andjelkovich et al. 1995

US iron-foundry workers

(Number of cases from Table 3 of the publication)
7 2 SMR = 0.59 (0.23–1.21) SMR = 0.43 (0.05–1.57)
Beane Freeman et al. 2009

NCI study in US chemical workers

(Number of cases from Table 1 of the publication)
286 116 44 peak >4 ppm: RR = 1.37 (1.03–1.81), trend with increasing peak exposure peak >4 ppm: RR = 1.42 (0.92–2.18), trend with increasing peak exposure peak >4 ppm: RR = 1.78 (0.87–3.64)

highest peak category before 1994: RR = 2.79 (1.08–7.21), p trend = 0.02
Bertazzi et al. 1989

Italian resin workers

(Number of cases from Table 3 of the publication)
7 SMR = 7/3.9 = 1.8 (0.72–3.70)
Coggon et al. 2014

UK chemical workers

(Number of cases from Table 6 of the publication)
18 9 high exposure ≥1 year: OR = 0.59 (0.23–1.50) high exposure: OR = 1.26 (0.39–4.08)
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×
Reference and Study Population No. Cancer Cases in Exposed Findings (95% CI)
All Lymphohematopoi etic Cancer Leukemia Myeloid Leukemia All Lymphohematopoietic Cancer Leukemia Myeloid Leukemias
Meyers et al. 2013

Update of Pinkerton et al. (2004)

US garment workers

(Number of cases from Table 2 of the publication)
107 36 21 SMR = 1.11 (0.91–1.34) ≥10 years of exposure and ≥20 years since first exposure: SMR = 1.74 (1.10–2.60) ≥10 years of exposure and ≥20 years since first exposure: SMR = 1.90 (0.91–3.50)

16–19 years exposure vs none: SRR = 6.42 (1.40–32.30); test for trend with increasing duration: p = 0.01
Partanen et al. 1993

Finnish wood-industry workers

(Number of cases from Tables 1 and 3 of the publication)
7 2 OR = 2.49 (0.81–7.59) OR = 1.40 (0.25–7.91)

Abbreviations: CI, confidence interval; OR, odds ratio; RR, relative risk; SMR, standardized mortality ratio; SRR, standardized rate ratio. Source: Committee generated.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

That complexity might have introduced problems of noncomparability among the plants, but a thorough reconstruction of historical formaldehyde average and peak exposures was conducted consistently for all sites until 1980. Good-quality historical data on potential confounders were also assembled from plant records and interviews of long-term employees. Because it pooled data from many plants, the study was powerful enough to detect effects that would not be measurable in plant-by-plant analyses. The formaldehyde exposure assessment was conducted only for jobs held until 1980. Thus, there is likely to have been more error in the exposure assignments in the later time period; in the primary analyses, exposure after 1980 was assumed to be zero. Two sensitivity analyses were conducted to evaluate the effect of that assumption on the results.

About one-fourth of the NCI industrial-worker cohort was estimated to have experienced peak exposures of at least 4.0 ppm (Beane Freeman et al. 2009). A 1999 Agency for Toxic Substances and Disease Registry literature review found that the threshold for mild to moderate human eye, nose, and throat irritation by formaldehyde ranged from 0.4 to 3 ppm in 17 laboratory studies (ATSDR 1999). Thus, the highest peak exposure category (greater than 4 ppm) was above the irritation threshold, and at this level about 50–100% of subjects would have experienced an irritation response.

There was evidence of increased risk of myeloid leukemia with increasing formaldehyde exposure (Beane Freeman et al. 2009). The evidence was strongest when the peak-exposure metric was used, weaker when average exposure was used, and very weak when the effect of cumulative exposure was assessed. In the primary analysis (which assumed zero exposure for all jobs after 1980), the RR of myeloid leukemia increased with increasing exposure. Compared with those who had peak exposures less than 2.0 ppm, the RR in those who had peak exposures from 2.0–4.0 ppm was 1.30 (95% CI 0.58–2.92) and in those who had peak exposures of at least 4.0 ppm, 1.78 (95% CI 0.87–3.64). The data also show the expected pattern wherein the RRs for the highest peak category compared with the lowest peak category increased as the tumor category was narrowed—the RR of all lymphohematopoietic cancers was less than that of all leukemias grouped, and the RR of all leukemias grouped was less than that of myeloid leukemias grouped. The associations were weaker when average exposure was used as the summary measure of exposure than when peak exposure was used, but the trends were similar. A modest increase in RRs was observed among categories of increasing average exposure. The RR increased from the group of all lymphohematopoietic cancers to the grouping of all leukemias, and the RR increased further from the grouping of all leukemias to the grouping of myeloid leukemia.

Beane Freeman et al. (2009) investigated the sensitivity of their results to the assumption of zero exposure after 1980 by censoring all persons who were still exposed in 1979 (this resulted in a loss of about 5% of the person–time of followup). The resulting effect estimates were stronger for both peak and average exposure metrics. For example, the RR for the highest peak exposure category increased from 1.79 (cited above) to 2.64 (95% CI 1.12–6.20), and the

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

trend among categories was also stronger (p = 0.03). The authors reported that there were stronger associations with exposures in the distant past, which may be explained either by higher air concentrations or by a relatively short latency for formaldehyde-induced leukemia. There was evidence to support the former explanation; exposures in the plants were much higher before 1970 than in later years when exposure controls were instituted (Stewart et al. 1986). The possibility of a relatively short latency (compared with that of solid tumors) is supported by two studies of the association between benzene and leukemia (Silver et al. 2002; Glass et al. 2004). In both cohorts, the RR of leukemia after benzene exposure decreased with increasing follow up, and the authors proposed that this is likely due to a relatively short latency for the effects caused by benzene.

Beane Freeman et al. (2009) reported that for the period up to 1994, the RR for the highest peak-exposure category compared with the lowest was 2.79 (95% CI 1.08–7.21), and there was evidence of an increasing trend among categories (p = 0.02). It is not clear why Beane Freeman et al. (2009) found an association with peak exposure and not with cumulative exposure. The committee noted that there were only 10 cases of myeloid leukemia in the highest cumulative exposure category, which was defined as at least 5.5 ppm-years. That is not very many cases and not a very high level of exposure. As a result, this finding is not strong evidence against an association between formaldehyde and myeloid leukemia.

As noted earlier in this chapter, the alternative exposure metrics of peak, average, and cumulative exposure are expected to be proportional to the incidence of a disease as related to different biologic mechanisms or pathways. A complicating factor that must also be considered is the effect of exposure assessment errors on the resulting summary measures. However, it cannot be predicted with any confidence which exposure metric would be expected to be closer to the “truth” in the investigation of formaldehyde and cancer. Therefore, the committee assessed peak, average, and cumulative exposure with equal weight on its overall evaluation. More precise studies in the future may be able to resolve this issue.

Hodgkin lymphoma was strongly associated with peak exposure (RR = 3.96, 95% CI 1.31–12.02) when the subgroups with the highest and lowest peak exposure were compared. A positive association with multiple myeloma was also observed when the highest and lowest peak-exposure subgroups were compared (RR = 2.04, 95% 1.01–4.12). For both outcomes, there was evidence of a trend of increasing mortality with increasing peak exposure. The findings on Hodgkin lymphoma and multiple myeloma are potentially important for further investigation, but the committee did not find additional evidence of these associations in other studies.

An important strength of the NCI industrial-cohort study was its ability to investigate possible confounding by other chemical exposures (antioxidants, asbestos, benzene, carbon black, dyes and pigments, hexamethylenetetramine, melamine, phenol, plasticizers, urea, and wood dust); none was found. Beane Freeman et al. (2009) specifically investigated a potential confounding effect of

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

benzene by excluding all workers who were known to have been exposed to benzene, and the results were not changed. Plant heterogeneity was investigated and found not to be an important factor in the results. There were some limitations. Despite the size of the study, the numbers of deaths in some categories of rare neoplasms were still small, and this limited the power to detect associations in the smallest subgroups. The magnitude of the exposure–response associations changed over time, and it is not possible without strong a priori assumptions to distinguish alternative explanations, such as disease latency, changes in exposures associated with changes in industrial operations and engineering controls, or time-dependent measurement uncertainties.

The committee concluded that although those limitations exist, the study was of high quality. The careful and clearly documented design and analysis reduced the likelihood that the results could be explained by bias. As noted, the authors investigated important sources of confounding and found no important evidence of confounding that might seriously undermine their results. Chance is an unlikely explanation given the consistent patterns of increased RR among exposure categories and tumor categories noted above. Thus, the committee determined that the findings are relevant to evaluating an association between formaldehyde exposure and myeloid leukemia.

Additional evidence of an association between formaldehyde exposure and lymphohematopoietic cancers in workers who were exposed during industrial operations was found in the National Institute for Occupational Safety and Health (NIOSH) study of garment workers. Meyers et al. (2013) updated earlier reports by Stayner et al. (1988) and Pinkerton et al. (2004) on mortality in a cohort of 11,043 industrial workers who were exposed to formaldehyde in three garment-manufacturing plants. The cohort was considerably smaller than the NCI formaldehyde-industry cohort (21 myeloid-leukemia deaths compared with 44 in the NCI cohort). The study methods included a high-discrimination, quantitative exposure assessment for current exposures that was performed during the early 1980s, which was an important strength of the study, but it did not cover the full period of exposures. The investigators did not attempt to estimate earlier exposures. The only known source of formaldehyde exposure was off-gassing from treated fabrics (which were produced elsewhere), so the amount of free formaldehyde in the fabric was a primary determinant of the workroom exposure (Elliot et al. 1987). Before 1970, the free-formaldehyde content of the fabric was estimated to be over 4,000 ppm; by 1980, the fabric concentrations had been reduced to 100–200 ppm. The air concentration measured in the workrooms in 1984 (geometric mean exposure, 0.15 ppm) was a result of off-gassing of the 100–200 ppm in the fabric. The ratio of fabric content to air content was about 1,000:1. Assuming that the ratio is fairly constant, fabric that contained 4,000 ppm probably produced an air concentration of about 4 ppm before 1970. However, the investigators did not make use of that simple estimate of earlier exposure; they merely noted that air exposure was likely to have been higher before 1970. Goldstein (1973) reported that industry efforts to reduce formaldehyde levels in work rooms by reducing the amount of resin in the fabric resulted

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

in a decreased from 10 ppm in 1968 to 2 ppm in 1973. Formaldehyde air concentrations were found to be similar between plants and across departments within the same plant. TWA concentrations were reported in a fairly narrow range (0.09–0.20 ppm), and there was little evidence that short-term peaks exceeded the mean. Given the relatively homogenous exposure scenario, it was reasonable to use all employed workers as the exposed group and to compare their mortality with that in the general population. They used years of work from the workers’ company job histories to approximate cumulative exposure and implicitly assumed that each year had roughly the same intensity of exposure, so the cumulative exposures of the workers who entered the cohort before 1970 were substantially underestimated.

The committee considered Meyers et al. (2013) to be a strong study for the evaluation of formaldehyde and myeloid leukemia. The study found evidence of an association with myeloid leukemia. The committee reviewed the evidence from both Meyers et al. (2013) and Pinkerton et al. (2004) together because the only important difference between them was that the former had 10 more years of followup (through 2008 instead of 1998). As noted earlier, some evidence in the literature on benzene and leukemia suggests risks decrease with increasing followup (Silver et al. 2002; Glass et al. 2004), and this pattern was observed in the two analyses of the NIOSH garment workers cohort. With followup through 1998, the SMR for all leukemia in those who had an exposure duration of 10 years or more and whose time since first exposure was 20 years or more was 1.92 (95% CI 1.08–3.17); with 10 additional years of followup, the SMR decreased to 1.74 (95% CI 1.10–2.60). For myeloid leukemia, the SMR for the same exposure definition as above with followup through 1998 was 2.55 (95% 1.10–5.03); with followup through 2008, it was 1.90 (95% CI 0.91–3.50). There was little evidence of increased mortality from lymphocytic leukemia in either reports of the NIOSH garment-workers cohort (Pinkerton et al. 2004; Meyers et al. 2013).

The Meyers et al. (2013) report included additional Poisson regression modeling of the data on all leukemia and myeloid leukemia. Those analyses enabled better control of confounding and a more thorough investigation of alternative exposure metrics than were available in Pinkerton et al. (2004). There was a strong positive trend in mortality with increasing duration of formaldehyde exposure (p = 0.01). The standardized rate ratio for 16–19 years of exposure was 6.42 (95% CI 1.40–32.20), although the rate ratio dropped in the longest duration category, at least 19 years. Again, that decrease may reflect the pattern of decreasing risk with extended followup.

The garment workers’ coexposures were generally different (lint particles and cleaning-solvent vapors) from those of the NCI formaldehyde-industry cohort, and this reduced the likelihood that an unmeasured confounder would explain both associations. No other potentially carcinogenic exposures were identified in the plants. As noted above, the exposure assessment had some important limitations. However, the committee agreed with the authors that it is reasonable to assume relatively constant exposure intensity throughout the period of em-

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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ployment. On balance, the committee concluded that the finding of an association between formaldehyde exposure and an association with myeloid leukemia was unlikely to have been explained by an unknown bias or confounder, and chance was an unlikely explanation given the pattern of statistically significant findings.

Coggon et al. (2014), an industrial cohort study of mortality in UK chemical workers, was judged to be moderately strong. The publication was an update of Coggon et al. (2003) and included 12 additional years of followup and more than 2,000 additional deaths. The earlier study included very few leukemia deaths and did not provide data specifically on myeloid leukemia. In some respects, Coggon et al. (2014) is similar to the NCI formaldehyde-industry study, but it is smaller and provides less information on its exposure assessment. The 2014 update included substantially fewer exposed myeloid-leukemia deaths; for example, there were nine deaths with “high” exposure in Coggon et al. (2014) and 19 deaths in Beane Freeman et al. (2009) with peaks greater than or equal to 4.0 ppm. Coggon et al. (2014) benefited from a semiquantitative exposure assessment that provided moderate discrimination among jobs with varied exposure intensities. Work histories were abstracted from employment records. Each job was classified into one of five exposure categories—background, low, moderate, high, or unknown—by an industrial hygienist who used professional judgment. Quantitative environmental measurements were available after 1970 that covered many jobs, but the authors judged the data insufficient to estimate cumulative exposure or other formal metrics. Exposures were assumed to be the same before 1970 (although anecdotally reported exposures were much higher earlier in the followup period). Peak exposures were not evaluated, nor were temporal trends evaluated or estimated. The authors reported that “each job title [within a factory] was assigned to the same exposure category across all time periods” (Coggon et al. 2014). More than 95% of subjects were exposed before the middle 1980s, and less than 5% of the cohort was still working after the middle1980s. The authors extended the followup of a previously reported cohort of 14,014 men (Acheson et al. 1984; Gardner et al. 1993) who had worked in six plants where formaldehyde was made or used. Mortality was compared with national rates in England and Wales and, in some cases, local rates. Coggon et al. (2014) mention several coexposures, but they do not provide details or report adjusted rates. In the most detailed exposure–response analysis, a nested case–control study, ORs for myeloid leukemia were estimated for four categories of exposure intensity and for a duration 5 years before disease onset. No analysis by duration, cumulative exposure, or other standard continuous exposure metric was presented. CIs for the effect estimates were wide and included the null value. An effect of the size observed in the NCI cohort would probably not have been detectable, so although the results were not inconsistent with those of Beane Freeman et al. (2009), Hauptmann et al. (2009), and Pinkerton et al. (2004), the committee determined that, on balance, the study was generally inconclusive.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

The committee judged three additional studies of small industrial cohorts that evaluated formaldehyde and lymphohematopoietic cancers to be moderately strong (Bertazzi et al. 1989; Partanen et al. 1993; Andjelkovich et al. 1995). Each was based on only a handful of cases. Two of the three yielded some evidence of an association with lymphohematopoietic cancers (Bertazzi et al. 1989 and Partanen et al. 1993). Bertazzi et al. (1989) reported on cancer mortality in an industrial cohort of 1,330 male workers who produced formaldehyde resins, including 219 for whom specific work histories could not be determined. Among the 179 deaths, there were seven from lymphohematopoietic cancer; 3.9 deaths were expected from national rates and 4.9 deaths expected from local rates, but regardless of which standard was used, the observed excess could have been due to chance. For the entire category of lymphohematopoietic cancers, the authors reported an SMR of 5.35 (95% CI 1.56–14.63) in plastic-resin workers who had formaldehyde exposures during 1965–1969, a period that had no exposure controls and therefore likely high exposure. Formaldehyde exposures before 1975 were often greater than 2.4 ppm (3.0 mg/m3). Duration of work in the plant was often short. There was no discussion of possible coexposures. The seven cases of lymphohematopoietic cancer were not further categorized, so no analyses for leukemia was possible. Partanen et al. (1993) conducted a small industrial nested case–control study of the incidence of lymphoma and leukemia in Finnish wood-industry workers who were exposed to formaldehyde. There were only two exposed leukemia cases (type unspecified) with an adjusted OR for formaldehyde exposure of 1.40 (95% CI 0.25-7.91). The Andjelkovich et al. (1995) industrial cohort study of foundry workers examined mortality in 3,929 men who had potential exposure to formaldehyde for at least 6 months during their work in a single automotive iron foundry. Comparisons were with the US population and with workers in the plant who were not exposed to formaldehyde. There were two deaths from leukemia (type not specified) in exposed workers and three deaths from leukemia in unexposed workers. The study was too small to be informative.

Studies of Embalmers and Others in the Funeral Trade, Anatomists, and Pathologists

Table 3-6 summarizes the studies that the committee judged to be strong or moderately strong that investigated embalmers and others in the funeral trade, anatomists, and pathologists. NCI assembled and followed a cohort of inactive or deceased embalmers and funeral directors (Hauptmann et al. 2009). The study is particularly useful for evaluating the association between formaldehyde exposure and cancer because of the likelihood of high exposures and a high-quality exposure assessment that was conducted by Stewart et al. (1992) and extended by Hauptmann et al. (2009). The authors conducted a nested case–control analysis of data on the cohort, using mortality as the outcome measure. The case

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

TABLE 3-6 Lymphohematopoietic Cancers: Funeral Workers, Embalmers, Pathologists, and Anatomists

Reference and Study Population No. Cancer Cases in Exposed Findings (95% CI)
All Lymphohematopoi etic Cancer Leukemia Myeloid Leukemia All Lymphohematopoietic Cancer Leukemia Myeloid Leukemias
Hauptmann et al. 2009

US funeral directors, embalmers

(Number of cases identified from Tables 1 and 2 of the publication)
168 44 (lymphohematopoietic malignancy of nonlymphoid origin) 33 Ever embalm: OR = 1.4 (0.8–2.6) Ever embalm: OR = 3.0 (1.0–9.5) Ever embalm: OR = 11.2 (1.3–95.6)

Highest level of all exposure metrics had p<0.05
Levine et al. 1984

ON provincial licensed embalmers

(Number of cases identified from Table 1 of the publication)
8 4 O/E = 1.2 (0.53–2.43) O/E = 1.6 (0.44–4.10)
Stroup et al. 1986

US anatomists

(Number of cases identified from Table 3 of the publication)
18 10 3 SMR = 1.2 (0.7–2.0) SMR = 1.5 (0.7–2.7) SMR = 8.8 (1.8–25.5)
Walrath and Fraumeni 1983

NY state-licensed embalmers
25 12 6 PMR = 1.2 (0.79–1.79) PMR = 1.4 (0.73–2.47) PMR = 1.5 (0.54–3.19)
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×
Reference and Study Population No. Cancer Cases in Exposed Findings (95% CI)
All Lymphohematopoi etic Cancer Leukemia Myeloid Leukemia All Lymphohematopoietic Cancer Leukemia Myeloid Leukemias
(Number of lymphohemtopoietic and leukemia cases identified from Table 3 of the publication; number of cases of myeloid leukemia noted on page 408 of the publication)
Walrath and Fraumeni 1984

CA state-licensed embalmers

(Number of lymphohemtopoietic and leukemia cases identified from Table 3 of the publication; number of cases of myeloid leukemia noted on page 4640 of the publication)
19 12 6 PMR = 1.2 (0.73–1.90) PMR = 1.8 (0.90–3.04)

PMR for ≥20 years of licensure = 2.2
PMR = 1.5 (0.55–3.26)

Abbreviations: CI, confidence interval; O/E, observed/expected; OR, odds ratio; PMR, proportionate mortality ratio; SMR, standardized mortality ratio. Source: Committee generated.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

subjects were 6,808 embalmers and funeral directors who died during January 1, 1960–January 1, 1986, and deaths were included if they had an underlying or contributory cause identified as lymphohematopoietic cancers of lymphoid origin (99 cases) or nonlymphoid origin (48 cases). Myeloid leukemia (34 cases) was analyzed as a separate subgroup. The control subjects were identified randomly from people in the funeral industry who died of other causes, excluding cancers of the buccal cavity and pharynx, of the respiratory system, and of the eye, brain, or other parts of the nervous system. A quantitative exposure assessment was conducted by using information on workplaces and job tasks drawn from interviews with former co-workers and next of kin (Hauptmann et al. 2009) and a NIOSH air-monitoring study (Stewart et al. 1992). All subjects had interview job histories that indicated funeral home or not, embalming or not, and funeral-home ventilation characteristics, which were the predominant factors that affected exposures. The authors found that the average exposure intensity during embalming was 1.7 ppm.

The study group was relatively large: there were 34 myeloid-leukemia deaths in the latest followup (33 had “ever embalmed”) (Hauptmann et al. 2009), nearly as many as the 44 in the NCI formaldehyde-industry cohort (Beane Freeman et al. 2009). The findings of Hauptmann et al. (2009) point strongly toward an association between formaldehyde exposure and myeloid leukemia, although measures of associations were stronger in the broad category of all lymphohematopoietic cancers and all leukemias. The simplest exposure metric—distinguishing ever vs never embalming—was moderately associated with increased mortality from all lymphohematopoietic cancers (OR = 1.4, 95% CI 0.8–2.6), more strongly associated with mortality from all leukemias (OR = 3.0, 95% CI 1.0–9.5), and strongly associated with increased myeloid leukemia mortality (OR = 11.2, 95% CI 1.3–95.6). There was a trend of increasing mortality with increasing duration of embalming (p = 0. 02), rising to OR = 13.6 (95% CI 1.6–119.7) when the group that had more than 34 years of embalming was compared with the group that had never embalmed. There was also a clear trend (p = 0.04) with increasing peak exposure, which is a metric similar to the one that Beane Freeman et al. (2009) found to be associated with myeloid leukemia in the different setting of the NCI industrial-cohort workers. In the highest peak-exposure category (greater than 9.3 ppm), the OR was 13.0 (95% CI 1.4–116.9) compared with no exposure. Another similarity to the findings of Beane Freeman et al. (2009) was that there was not a clear trend of increasing mortality with increasing cumulative exposure (p = 0.19).

Hauptmann et al. (2009) found no evidence of an association between formaldehyde exposure and leukemia of lymphoid origin. The specificity within the broader grouping increased the committee’s confidence that the results were not likely to be due to an unknown bias. A striking finding of the study was that of the 34 myeloid-leukemia cases, only one did not ever embalm. The ratio of 33:1 contrasts with the ever: never embalming ratio of roughly 4:1 in controls (the exact numbers were 210:55). The 4:1 ratio is a simple way to see the associations noted above by using different exposure metrics, but it created a methodologic limitation for the authors in that the unexposed reference group only

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

had one case. That limitation reduced the precision of the OR reported above. To investigate the effect, the authors repeated the analyses with an enlarged “unexposed” group, which included those who reported fewer than 500 embalming procedures in their career. As expected, the measures of association in the redefined reference group were lower than those reported above, but the patterns were very similar. For example, the OR for those who reported more than 34 years of embalming was 3.9 (95% CI 1.2–12.5) compared with the OR of 13.6 reported above.

Strengths of Hauptmann et al. (2009) were that high exposures were readily identified and there were good supporting data on the range for exposure assignments (Stewart et al. 1992). The model used by the authors explained a high percentage of variability of exposure measurements (74%) (Hauptmann et al. 2009). Errors in quantification would probably not affect the relative ranking of individual exposure histories, especially in the high-exposure category. There was no evidence of confounding by smoking, and few additional chemicals that might confound the association with formaldehyde were involved. In addition, the authors did not adjust for possible changes in work or employer; this could lead to overestimates or underestimates of exposure. The total duration of embalming work was estimated for all subjects, but some exposure information was missing. Exposures from large spills were important for peaks but infrequent and generally not recorded. The authors also noted that “there was a considerable amount of missing data that required imputation for analyses” (Hauptmann et al. 2009, p. 1697). However, sensitivity analyses suggested that the key findings were unaffected by the absence of some data points.

On balance, the committee concluded that Hauptmann et al. (2009) was a strong study. The committee did not identify any important biases that might have explained the key finding of an association between formaldehyde and myeloid leukemia. The authors persuasively demonstrated that confounding was an unlikely explanation. In addition, the clear pattern of associations with multiple increasing exposure metrics and after several sensitivity analyses makes it unlikely that chance could have explained the findings.

Several small studies of embalmers (Walrath and Fraumeni 1983, 1984; Levine et al. 1984) and anatomists (Stroup et al. 1986) in the 1980s provided supporting evidence and were judged to be moderately strong. Each study had only a handful of leukemia deaths and inadequate exposure assessment that was based on the high likelihood of job exposure to formaldehyde and documentation of years of work. Three of the four studies found a pattern of increasing mortality from leukemia in general and from myeloid leukemia specifically, although few were statistically significant; Walrath and Fraumeni (1983, 1984) and Stroup et al. (1986) provided data on myeloid leukemia as the cause of death.

Walrath and Fraumeni reported proportionate mortality ratios (PMRs) and proportionate cancer mortality ratios (PMCRs) in a cohort of embalmers in New York State (1983) and California (1984). The PMRs for all leukemias combined were 1.2 (based on 12 deaths) and 1.8 (based on 12 deaths) in New York and California, respectively. Confidence intervals were not given in the publication, but

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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they were calculated by the committee (see Table 3-6). There was a small excess in PMRs among workers who had less than 20 years of experience and a statistically significant excess in those who had more than 20 years. The authors noted that embalming fluid contains potentially carcinogenic substances other than formaldehyde.

Levine et al. (1984) studied mortality in a cohort of 1,477 licensed undertakers in Ontario and found four deaths from leukemia, not further specified (2.5 deaths expected, SMR not given).The authors also presented a brief analysis of mortality in formaldehyde-exposed men in eight plants and cohorts of pathologists and anatomists; when the results were combined with their own study of undertakers, 53 leukemia deaths were observed and 44 deaths expected. The publication does not provide additional details.

Stroup et al. (1986) reported a retrospective cohort mortality study of 2,317 anatomists, who are exposed to a wide array of solvents, stains, and preservatives, including formaldehyde. The authors found 10 deaths from leukemia (6.8 deaths expected, SMR = 1.5, 95% CI 0.7–2.7). Information on potential confounders and biases was not presented, but the authors suggested that low SMRs for smoking-related cancers and cirrhosis of the liver suggested that cohort members used cigarettes and alcohol less than the general population.

Other Studies Potentially Relevant to Formaldehyde and Lymphohematopoietic Hematologic Cancers

The committee reviewed all other studies in the background document for formaldehyde for evidence bearing on the question of the carcinogenicity of formaldehyde. Studies that were reviewed were judged to be weak and contributed no informative evidence to this review of lymphohematopoietic cancers were those by Edling et al. (1987), Ott et al. (1989), Hall et al. (1991), Dell and Teta (1995), and Stern (2003). Each was small with a low-discrimination exposure assessment that did not permit reliable estimation of an association between formaldehyde exposure and any of the types of cancers of interest. The study by Edling et al. (1987) was a cohort study of mortality that focused on abrasives and leather tanneries, respectively, and formaldehyde constituted a secondary exposure. Hall et al. (1991) updated a study of mortality in a cohort of 4,512 British pathologists (Harrington and Oakes 1984) and found four deaths from leukemia (2.63 deaths expected, SMR = 1.52, 95% CI 0.41–3.89). Followup was nearly complete. Coexposures were not discussed. Dell and Teta (1995) and Ott et al. (1989) studied the same large chemical plants that manufacture a variety of chemicals; few people were exposed to formaldehyde, and the broad job titles limited the specificity of exposure assignments. Dell and Teta (1995) reported on mortality in a cohort of 5,932 male employees in a plastics manufacturing and R&D facility in New Jersey. SMRs for leukemia and aleukemia were 0.98 in hourly employees (12 deaths observed, 12.31 deaths expected, 95% CI 0.50–1.70) and 1.98 in salaried employees (11 deaths observed, 5.56 expected, 95%

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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CI 0.99–3.54) in salary employees. Numerous possible coexposures were mentioned by the authors. The text reports eight leukemia deaths (three expected) in the R&D workers, but does not include details. Dell and Teta (1995) provided no data on lymphohematopoietic cancers and formaldehyde. Ott et al. (1989), building on a cohort mortality study by Rinsky et al. (1987), conducted a nested case–control study of mortality in male workers in two chemical-manufacturing facilities and an R&D center in New Jersey. The four causes of death that they studied included nonlymphocytic leukemia. Controls were group-matched on decade of first employment and survival. Exposure was assessed on the basis of departmental usage; coexposures were numerous. There were two cases of nonlymphocytic leukemia (2.6 expected, SMR not given). The Stern (2003) study followed mortality in a cohort of workers in two leather tanneries. It had no formal assessment of formaldehyde exposure, and workers were exposed to many toxic agents, including possible carcinogens. Comparisons were with both US and state rates. There were 16 deaths from leukemia and aleukemia (22 deaths expected according to US rates, SMR = 0.72, 95% CI 0.41–1.18). Results in the two tanneries were similar, as were SMRs based on state rates. There was little evidence of a trend with years of employment. The study did not break down leukemia mortality to permit assessment of the myeloid subgroup.

The committee also identified several studies based on general-population registries or surveys that it judged to be weak and that contributed little or no evidence to this review of lymphohematopoietic cancers. Blair et al. (2001) was a population-based case–control study of 513 incident cases and 1,087 matched controls. It focused on agricultural risk factors in leukemia cases drawn from cancer registries in Iowa and Minnesota. The authors investigated workers who had job-related chemical exposures. In those whose work histories suggested low or high formaldehyde exposure, the ORs for chronic myeloid leukemia were 1.3 in the low-exposure category (7 cases, 95% CI 0.6–3.1) and 2.9 in the high-exposure category (1 case, 95% CI 0.3–24.5). Coexposures were numerous. Richardson et al. (2008) conducted a population-based case–control study of non-Hodgkin lymphoma and chronic lymphocytic leukemia incidence in Germany. Semiquantitative estimates of formaldehyde exposure derived from job-history data, and a job–exposure matrix were weakly positively associated with non-Hodgkin lymphoma and chronic lymphocytic leukemia, but confidence intervals were wide and included the null. The study did not address myeloid leukemia.

Hansen and Olsen (1995), which was a Danish cancer incidence study, was described earlier because it found an increased incidence of sinonasal cancer in formaldehyde-exposed workers. The authors reported an SPIR for leukemia in men who worked in 265 factories that imported or manufactured formaldehyde. They found 39 leukemia deaths (47.0 deaths expected, SPIR = 0.8, 95% CI 0.6–1.6). Coexposures were not investigated. The exposure definition used in the study (being a blue-collar worker in a company that was registered with the government as a user of formaldehyde) probably led to substantial misclassification with the likely consequence of underestimation of true risks. Another limitation of the study was that it did not report results separately for leukemia

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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types. For all leukemia types combined, the study did not find evidence of an increased incidence in formaldehyde-exposed workers, although the confidence interval was wide (SPIR = 1.0, 95% CI = 0.6–1.4).

Stellman et al. (1998) analyzed cancer mortality in members of the ACS Cancer Prevention Study II, a very large prospective industrial cohort study. Mortality was examined after 6 years in 45,399 men who had reported being employed in wood industries or occupationally exposed to wood dust and 362,823 who did not report such exposures. Thirty-two leukemia cases were observed in those who reported wood-dust exposure (SMR = 0.90, 95% CI 0.63–1.30), and 14 were observed in the partially overlapping group in wood-related occupations (SMR 1.08, 95% CI 0.6–1.85). The exposure assessment for formaldehyde was by self-report alone, which is likely to be of poorer quality than an expert review and job–exposure matrix. Furthermore, the authors did not report results for subtypes of leukemia. As a result, this study was judged to be of little utility for the committee’s assessment.

Summary of Evidence on Lymphohematopoietic Cancers

In summary, the committee concluded that the epidemiologic studies provided evidence of a causal association between formaldehyde and myeloid leukemia in humans. Evidence of an association was derived from two strong industrial cohorts (Beane Freeman et al. 2009; Myers et al. 2013), one strong cohort of embalmers (Hauptmann et al. 2009), and several moderately strong cohorts from the chemical industry (Coggon et al. 2014) and the funeral trade (Walrath and Fraumeni 1983, 1984; Stroup et al. 1986). See Tables 3-5 and 3-6 and Figures 3-1 and 3-2 for key measures of association supporting this conclusion. The conclusion was based on the strength, consistency, temporality, dose–response relationships, and coherence of the evidence according to the quality criteria presented in Table 3-1.

To present data from the studies, it was necessary to choose a particular exposure definition; however, it is important to note that, in its evaluation of the body of evidence, the committee did not choose a single exposure metric a priori for analysis. Instead, it looked at the full set of exposure metrics and their associations with disease.

Figure 3-1 emphasizes a pattern noted earlier—that is, in the studies that were large enough and detailed enough to present associations between formaldehyde and the “nested” case definitions of all types of lymphohematopoietic cancers, all leukemias, and myeloid leukemia, the measures of association tended to increase as the definition was narrowed (the data points for the nested sets of case definitions are linked by a solid line in Figure 3-1).The figure also illustrates that the stronger and larger studies generally reported stronger associations with formaldehyde and were more likely to present confidence bounds for their

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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images

FIGURE 3-1 Summary of strong and moderately strong studies of formaldehyde and lymphohematopoietic cancers. Note: Data points connected by a line indicate results from the same study according to the same exposure metrics but for different tumor sites.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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effect estimates that excluded the null. Measures of association between formaldehyde exposure and myeloid leukemia are represented in Figure 3-2 for all studies that reported this association. There is a pattern of positive findings from studies that were judged to be large and strong studies.

Low-precision studies, such as those with a small cohort, only a few cases, or limited exposure assessments, may provide some useful data on risk estimates if several studies were performed. When several small populations are studied using a good design, the measures of association would not be expected to be the same. They would have a distribution that would cluster around the overall risk value for the population; some estimates would be above that value and some would be below that value. If the risk estimates for formaldehyde exposure and myeloid leukemia showed a distribution that was shifted above 1.0 so that few studies showed RRs below 1.0, that pattern of results suggests that there may be a causal relationship between exposure and disease risk. The closer the risk values cluster around 1.0 (some above and some below), the less likely it is that a relationship exists. In Figures 3-1 and 3-2, nearly all RRs are above 1.0, which suggests that a relationship exists. That argument does not imply that all studies are equal. Strong studies make more precise estimates of the RR and are more useful in assessing factors that may affect the RR compared with weaker studies. Strong studies should not produce large RRs when the relationship is weak or absent unless there is a bias in the data.

images

FIGURE 3-2 Summary of key findings from all studies that reported associations between formaldehyde and myeloid leukemia.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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As noted above, the informative epidemiologic studies were the ones that were large, that estimated exposure systematically, that had credible comparison groups, and that assessed cancer end points reliably. Studies that did not find associations between exposure and myeloid leukemia were usually too small to detect an effect, did not break out results for myeloid leukemia, or used methods of exposure assessment that resulted in exposure misclassification. A single, large, high-quality study (Beane Freeman et al. 2009) found evidence of increased risk of Hodgkin lymphoma and multiple myeloma in those who had a history of high peak exposures. Those findings do not appear to be supported by other epidemiologic evidence and, in the committee’s view, constitute insufficient evidence of effects.

Cancer at Other Sites

The committee conducted a literature search (see Appendix D) to identify studies that examined associations between formaldehyde and cancers at other sites (Table 3-7). Four studies were identified that reported measures of association between formaldehyde and lung cancer. Two of the studies were judged to be moderately strong (Siew et al. 2012; Mahboubi et al. 2013) and two studies were judged to be weak (Checkoway et al. 2011; Luo et al. 2011).

TABLE 3-7 Other Cancer Sites

Reference and Study Population No. Lung Cancer Cases in Exposed Findings (95% CI)
Checkoway et al. 2011

Female textile workers in Shanghai, China

Number of cases identified from Table 3 of the publication

Cases with ≥10 years of formaldehyde exposure: n = 2

Hazard ratio for ≥10 years formaldehyde exposure = 2.1 (0.4–11.0)
Luo et al. 2011

General population in 13 US regions covered by SEER registries

Not relevant; unit of analysis was county RR for counties with any formaldehyde release vs none = 1.14 (1.05–1.24)
Mahboubi et al. 2013

General population in Montreal, Canada

Number of cases identified from Table 3 of the publication

Cases with “substantial” exposure: n = 99

OR for pooled population comparing substantial with no exposure = 0.88 (0.63–1.24)

No evidence of trend with duration, time since first exposure

Siew et al. 2012

Finnish general population

Number of cases identified from Table 3 of the publication

Cases with any formaldehyde exposure: n = 1,831

RR for any formaldehyde exposure = 1.18 (1.12–1.25)

Abbreviations: CI, confidence interval; OR, odds ratio; RR, relative risk; SEER, Surveillance, Epidemiology, and End Results program of the National Cancer Institute. Source: Committee generated.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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Mahboubi et al. (2013) published a large case–control study of lung cancer and formaldehyde exposure. The authors used a long-running study of lung cancer in Montreal that was based on incident cases gathered during two time periods: 1979–1986 and 1996–2002. The well-described exposure assessment methods were based on a detailed questionnaire on jobs and duties performed. Trained occupational hygienists evaluated each questionnaire, blinded to case and control status, on three dimensions of formaldehyde exposure: confidence (possible, probably, definite); relative concentration (low, medium, high); and frequency of use in a normal week (low, medium, high). The study was relatively large; there were 99 cases with exposure to formaldehyde that were judged by the occupational hygienists to be “substantial” exposures. The study found little to no evidence of incidence of lung cancer associated with any of the formaldehyde exposure measures. The study investigated potential confounding by smoking, and none was found. The study was able to evaluate effects separately in men and women, and no effect was observed in either gender. It was also able to stratify on the three primary histologic types of lung tumors (squamous cell, small cell, and adenocarcinoma) and, again, there was no evidence of an association with formaldehyde exposure for any type.

Siew et al. (2012) established a population-based cohort of all Finnish men who were born during 1906–1945 and followed the cohort for cancer incidence by linking to data in the Finnish Cancer Registry. They used the men’s occupations reported to the 1970 national census to estimate occupational exposures to a wide array of chemicals, including formaldehyde, and found that men who developed lung cancer were 18% more likely to have jobs that involved exposure to formaldehyde than men who did not develop lung cancer (RR = 1.18, 95% CI 1.12–1.25). That finding was positive, and the size of the study (more than 30,000 lung-cancer cases) resulted in tight confidence limits, but the authors were doubtful of the finding because of the likelihood that they were unable to control fully for confounding by smoking and by concurrent exposures to other strong lung carcinogens, particularly asbestos. The committee concurred with those concerns.

Checkoway et al. (2011) had a strong study design, but the committee judged it to be weak for the purposes of this assessment because few cases were exposed to formaldehyde. The study was a large industrial case-cohort study (628 incidence lung-cancer cases) of Chinese female textile workers and it had detailed exposure assessment. However, the prevalence of formaldehyde exposure was low, and only two cases had 10 years or more of formaldehyde exposure. The resulting measure of association was imprecise: the hazard ratio for 10 or more years of formaldehyde exposure was 2.1 (95% CI 0.4–11).

Luo et al. (2011) conducted a population-based ecologic study of incident cases in US counties. They linked lung-cancer incidence from the Surveillance, Epidemiology, and End Results Program cancer registries to US Environmental Protection Agency Toxics Release Inventory data on formaldehyde emissions from industries. They found that a county’s lung-cancer rate was positively associated with releases of formaldehyde (and chromium and nickel). For exam-

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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ple, the RR was 1.18 (95% CI 1.05–1.33) when nonmetropolitan counties that had any formaldehyde release were compared with counties that had no formaldehyde release. The results are intriguing, but, as the authors note, evidence from individual-level studies is needed to support the finding.

The committee concluded that the newly identified studies do not provide enough evidence to indicate a causal association between formaldehyde and lung cancer. There remains a good possibility that confounding factors explain the increase in lung cancer reported in some formaldehyde studies. In addition, the studies yielded no epidemiologic evidence that indicated an association between formaldehyde exposure and cancer at other sites.

CANCER STUDIES IN EXPERIMENTAL ANIMALS

This section reviews the evidence of carcinogenicity in experimental animal studies and applies the NTP criteria to produce the committee’s independent evaluation. In reviewing the evidence, the committee looked at primary literature and considered analyses in other reviews, including those by the International Agency for Research on Cancer (IARC 1982, 1995, 2006a) and NTP (2010, 2011). To capture studies that may have been published concurrently with the completion of the background document for formaldehyde up to 2013, the committee undertook an independent literature search. See Appendix D (Box D-2 and Figure D-2) for more information.

Studies of Low Power for Detecting Malignancies

Some bioassays discussed in the section “Studies of Cancer in Experimental Animals” of NTP’s background document for formaldehyde are of limited adequacy to evaluate the carcinogenicity of formaldehyde (Table 3-8). Some of the studies were designed to follow up on studies that found carcinogenicity, for example, to explore hypotheses related to etiology or to look for differences in activity in different species. Those studies have findings of interest in considering progression to carcinogenesis, but they had low power to detect malignancy, mostly because they were not of sufficient duration. In addition, some studies have small groups, particularly the studies that used monkeys (Rusch et al. 1983; Monticello et al. 1989).

All the studies that were of low power to detect malignancies were inhalation studies except that by Tobe et al. (1989), which exposed animals to formaldehyde via drinking water. Tobe et al. had a relatively small group (20 male and 20 female) at the start of the study; all the animals in the high-dose group receiving 5,000 ppm of formaldehyde in drinking water and a substantial fraction in the low-dose groups receiving 200 ppm of formaldehyde in drinking water (46.9% of males and 33.7% of females) died before the end of the study, although survival in the group receiving 1,000 ppm of formaldehyde in drinking water was relatively good. Mortality began within the first month of the study. With the small initial group and substantial noncancer mortality in the high- and

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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low-dose groups, the study has little overall power for evaluating the oral carcinogenicity of formaldehyde. Additional studies published decades ago that were identified from bioassay tabulations (for example, the US Public Health Service 149 series Survey of Compounds Which Have Been Tested for Carcinogenicity) were also of short duration and had other deficiencies (Garschin and Schabad 1936; Watanabe et al. 1954; Muller et al. 1978), as discussed in more detail in Chapter 2.

TABLE 3-8 Studiesa of Low Power for Detecting Malignancies

Species Limitations Findings of Interest in Formaldehyde-Treated Animals Reference
C3H mice • Examined only lung; no examination of nose
• Study terminated for most groups at 35 weeks
• Small group in single animal group allowed to live longer
Basal-cell hyperplasia, epithelial stratification, squamous-cell metaplasia, and atypical metaplasia in trachea and major bronchi Horton et al. 1963
Wistar rats • Short duration (13 weeks)
• Small group (10 male and 10 female)
Proliferative lesions in nasal and olfactory epithelium Woutersen et al. 1987
Wistar rats • Short duration (13 weeks)
• Histopathology only of nasal cavity
Disarrangement, hyperplasia, squamous metaplasia with keratinization of epithelium Wilmer et al. 1989
Wistar rats • Short duration (1 year)
• Small group (10 male)
• Only nasal cavity examined
Increased basal-cell hyperplasia and squamous-cell metaplasia Appelman et al. 1988
Wistar rats • Relatively small initial group (20 male and 20 female) and high mortality Forestomach hyperkeratosis, basal and squamous-cell hyperplasia; glandular stomach hyperplasia Tobe et al. 1989
Wistar rats • Short duration (32 weeks)
• Small group (10 male)
8 of 10 treated rats with forestomach papilloma, none in controls Takahashi et al. 1986
Fischer rats • Short duration (26 weeks)
• Relatively small group (20 male and 20 female)
Increased squamous-cell metaplasia and hyperplasia, basal-cell hyperplasia at high doses Rusch et al. 1983
Syrian golden hamsters • Short duration (26 weeks)
• Small group (10 male and 10 female)
No significant findings Rusch et al. 1983
Cynomolgus monkeys • Short duration (26 weeks)
• Small group (6 male)
• Age unknown
Squamous-cell metaplasia and hyperplasia of nasal turbinates Rusch et al. 1983
Rhesus monkeys • Short duration (1–6 weeks)
• Small group (9 male)
Mild degeneration and squamous-cell metaplasia of nasal epithelium; increased cell proliferation rate Monticello et al. 1989

aAll studies conducted by inhalation except studies by Tobe et al. (1989) and Takahashi et al. (1986), which were via drinking water.
Source: Committee generated.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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The study by Takahashi et al. (1986), which exposed male Wistar rats to formaldehyde in water at 5,000 ppm for 32 weeks is notable. Although it was of short duration, eight of 10 exposed rats and no control animals developed forestomach papilloma. The formaldehyde group was serving as a reference group in a study of the effect of formaldehyde on N-methyl-N′-nitro-N-nitrosoguanidine carcinogenicity. Because of the very short study duration, the finding of tumors is particularly notable.

The two studies conducted in nonhuman primates are also noteworthy. They were of short duration and used small numbers of animals, but both studies demonstrated clear cellular and proliferative lesions of the nasal turbinates. Rusch et al. (1983) reported squamous-cell metaplasia and hyperplasia in the high-dose exposure group of six cynomolgus monkeys exposed to formaldehyde at 2.95 ppm 22 hours/day, 7 days/week for 26 weeks. Monticello et al. (1989) exposed rhesus monkeys to formaldehyde at 6 ppm 6 hours/day, 5 days/week for 1 week (n=3) or 6 weeks (n=3). The authors reported increased rates of nasal epithelial cell-proliferation with squamous-cell metaplasia of the transitional and respiratory epithelia of the nasal passages and squamous-cell metaplasia of the respiratory epithelia of the trachea and large airways of the bronchial tree. Even though those findings do not reflect overt carcinogenesis, they are highly reminiscent of the preneoplastic epithelial lesions of the nasal cavity that were observed to precede nasal malignancies in chronic rat studies.

Evidence from Informative Studies

Chapter 2 discusses whether the committee found NTP’s evaluation of the evidence and application of its criteria scientifically sound. The committee’s independent application of the NTP criteria emphasizes studies that are designed with greater sensitivity to detect an effect. Table 3-9 shows the highest-quality inhalation studies in boldface. They all had relatively large groups (90 animals or more), handled test material adequately, and included well-defined comparison groups (Kerns et al. 1983; Sellakumar et al. 1985; Monticello et al. 1996). The studies were all conducted in rats. In each, formaldehyde caused high incidences of rare malignant nasal tumors (squamous-cell carcinomas) at air-chamber concentrations of 10–15 ppm; these tumors are rarely seen in carcinogenesis bioassays and can be characterized as occurring “to an unusual degree” with respect to incidence. It is noteworthy that none of the animals in control groups in any of the long-term exposure studies had a tumor of this type. The Kerns et al. (1983) study was among the group of highest-quality studies. That experiment had a robust finding of squamous-cell carcinoma in both male and female rats, and the incidences were also increased to an unusual degree. The initial report of this study (Battelle 1981) stated there was a significant increase in bone marrow hyperplasia in rats following exposure to formaldehyde. The short-term exposure study by Feron et al. (1988) did not achieve statistical significance (p = 0.1 by Fisher exact comparison between the top dose group and controls).

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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TABLE 3-9 Nasal Squamous-Cell Carcinoma in Long-Term Inhalation Studies of Formaldehyde1

Species and Strain Study Duration (week)2 Sex Concentrations in Air (Incidences) Other Findings Reference
No SCC Effect SCC
Mouse B6C3F1 104 M 0 ppm (0/109) 2 ppm (0/100) 5.6 ppm (0/106) 14.3 ppm (2/104) Epithelial dysplasia and squamous metaplasia in high- and middle-dose groups; epithelial hyperplasia at high doses Kerns et al. 1983; Battelle 1981
F 0 ppm (0/114) 2 ppm (0/114) 5.6 ppm (0/112) 14.3 ppm (0/119) Dysplasia in high- and middle-dose groups; squamous metaplasia in the high-dose group
Rat Wistar 130 (13 weeks of exposure)3 M 0 ppm (0/45) 10 ppm (1/44) 20 ppm (3/44) One carcinoma in situ and two polypoid adenomas at 20 ppm Feron et al. 1988
120 M 0 ppm (0/26) 0.1 ppm (1/26) 1 ppm (1/28) 10 ppm (1/26) Woutersen et al. 1989
Rat F344 104 M 0 ppm (0/118) 2 ppm (0/118) 5.6 ppm (1/119) 14.3 ppm (51/117*) Four high-dose animals with other nasal malignancies Kerns et al. 1983
F 0 ppm (0/114) 2 ppm (0/118) 5.6 ppm (1/116) 14.3 ppm (52/115*) One high-dose female with other nasal malignancy
104 M 0 ppm (0/90) 0.7 ppm (0/90) 2 ppm (0/96) 6 ppm (1/90) 10 ppm (20/90*) 15 ppm (69/147*) Nasal malignancies in one animal at 10 ppm and one animal at 15 ppm; polypoid adenomas in 14 animals at 15 ppm Monticello et al. 1996
120 M 0 ppm (0/32) 0.3ppm (0/32) 2 ppm (0/32) 15 ppm (13/32*) An additional 3 rats at 15 ppm with squamous-cell papilloma Kamata et al. 1997
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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Species and Strain Study Duration (week)2 Sex Concentrations in Air (Incidences) Other Findings Reference
No SCC Effect SCC
Rats Sprague Dawley Life M 0 ppm (0/99) 15 ppm (38/100*) Two treated rats with other nasal malignancies; 10 with squamous-cell papillomas Sellakumar et al. 1985
104 F 0 ppm (0/15) 12.4 ppm (1/16) Squamous-cell metaplasia or dysplasia in 10 exposed rats Holmström et al. 1989
Hamster Syrian Golden Life M 0 ppm (0/132) 10 ppm (0/88) 30 ppm (0/50) Minimal hyperplastic and metaplastic response Dalbey 1982

*Statistically significant, p < 0.0001 by pairwise Fisher exact comparison.
1Well-conducted studies with relatively large groups are in boldface.
2All exposures were for 6 hours/day, 5 days/week except the Dalbey (1982) study in hamsters, which had one group at 5 hours/day, 5 days/week and one group at 5 hours/day, 1 day/week.
313 weeks of exposure followed by a long period of no exposure. Results of experiments with shorter exposure times not tabulated.
Abbreviation: ppm, parts per million; SCC, squamous-cell carcinoma. Source: committee generated.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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In addition to the findings of the robust rat studies, Kerns et al. (1983) carried out a study in mice. Nearly all 17 high-dose mice that survived 24 months had nasal lesions (dysplasia and metaplasia), and two also had squamous-cell carcinoma. As noted by the authors and in the background document for formaldehyde, that finding is sufficient to demonstrate the potential for these tumors in the mice exposed by inhalation when put into the context of evidence for this site in the rat and when the rarity of the tumor is considered. The findings of squamous-cell carcinoma in long-term studies that exposed mice and rats via inhalation are supported by the preneoplastic lesions (for example, squamous metaplasia with keratinization of epithelium) and other nasal lesions found in the shorter-term studies. The study using hamsters found no effect (Dalbey 1982).

The Kerns et al. (1983), Kamata et al. (1997), and Sellakumar et al. (1985) inhalation studies included histopathologic examinations of non–respiratory tract tissues; the other inhalation studies did not. Kerns et al. (1983) was reported in full in the Battelle (1981) report to the Chemical Industry Institute of Toxicology. The Battelle report discusses findings of leukemia and lymphoma that were not found to be exposure-related. However, diffuse multifocal bone marrow hyperplasia in formaldehyde-exposed animals was increased in both treated males (six of 114 controls vs 26 of 111 treated, p = 0.0001) and females (seven of 113 controls vs 28 of 115 treated, p = 0.0001). Kamata et al. (1997) and Sellakumar et al. (1985) reported no statistically significant nonrespiratory tumor findings but provided no detail regarding other non–respiratory tract histopathology.

The database for evaluating oral exposure to formaldehyde is less robust than for inhalation exposure. Three studies exposed rats to formaldehyde via drinking water over long periods (Til et al. 1989; Soffritti et al. 1989, 2002). The studies are described at length by IARC (2006a) and NTP (2010).

The study by Til et al. (1989) exposed Wistar rats to formaldehyde that was generated with 95% pure paraformaldehyde and 5% water. The administered drinking-water concentrations were 0, 20, 260, and 1,900 mg/L; the initial groups were 70 animals per sex at each dose; and the interim sacrifices occurred at 53 and 79 weeks. The intestines were not examined histologically in the middle- and low-dose groups but were in the high-dose group. The authors found no increases in cancer incidence in the gastrointestinal tract. A male in the low-dose group and a female in the control group had gastric papilloma. Nearly all male (seven out of 10) and female (five out of nine) animals in the highest-dose group had epithelial hyperplasia of the forestomach, and substantial fractions had focal hyperkeratosis of the forestomach and hyperplasia of the glandular stomach. In contrast, in the 32-week study by Takahashi et al. (1986), noted above in the discussion of the low-power studies, eight of 10 male Wistar rats exposed via drinking water to formaldehyde at 5,000 mg/L had stomach papilloma. The exposure level in the Takahashi et al. (1986) study was higher than in the Til et al. (1989) study.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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In a series of experiments in Sprague Dawley rats, Soffritti et al. (1989, 2002) administered formaldehyde via drinking water. The studies included full histologic examination of all tissues. In the first study (Soffritti et al. 1989), formaldehyde of unspecified purity was administered to 25-week-old breeders (20 controls and 18 treated) at 2,500 mg/L in water. The offspring were exposed in utero via the dam and then postnatally via water for 104 weeks. In the breeders, no stomach or intestinal tumors were observed in the controls, whereas stomach tumors were observed in one treated female (benign) and one treated male (malignant). In the offspring, similarly, there were no stomach or intestinal tumors in the control animals (59 males and 49 females). However, in treated offspring (36 males and 37 females), a variety of benign and malignant gastrointestinal tumors were observed at a low incidence, including malignant leiomyosarcoma, which is exceedingly rare in these animals. Leiomyosarcoma was observed in stomach tissues in one treated female and one treated male and in intestinal tissue of five treated females (statistically significant at p = 0.01) (IARC 2006a, NTP 2010). In addition, nonleiomyosarcoma gastrointestinal tumors were observed in two males (one benign and one malignant) and one female (malignant).

Soffritti et al. (2002) later followed up with a long-term drinking-water study with multiple exposure groups and groups with lower exposures than in the earlier (Soffritti et al. 1989) study: 0, 10, 50, 100, 500, 1,000, and 1,500 mg/L; 50 animals of each sex per group, except for the controls, which had a group size of 100. Four treated males developed leiomyosarcoma at 10 mg/L (forestomach, one animal), 1,000 mg/L (glandular stomach, one animal), and 1,500 mg/L (intestine, two animals), and seven treated females developed leiomyoma at 10 mg/L (two animals), 50 mg/L (one animal), and 1,500 mg/L (three animals) or leiomyosarcoma at 50 mg/L (one animal). None of the 200 untreated control animals (100 male and 100 female) had these tumors.

Soffritti et al. (2002) also reported an increased incidence of hemolymphoreticular tumors in some groups. The finding is of interest, but there is uncertainty about it because of the changing counts of the tumors in earlier study reports (as noted by IARC 2006a), the pooling of tumors of different cellular origins, and recent questions raised about the evaluation of this class of tumors by this laboratory (Malarkey and Bucher 2011; Gift et al. 2013). Total mammary tumors also increased with increasing dose in the females; this, too, involved pooling of tumors of different origins (for example, adenocarcinoma and liposarcoma). Although noteworthy, the findings of hemolymphoreticular and mammary tumors are not used in the committee’s independent evaluation.

Committee Evaluation in the Context of the Report on Carcinogens Listing Criteria

Applying the NTP criteria to the bioassay data for formaldehyde, the committee draws the following conclusions about exposure to formaldehyde in experimental animals:

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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1. Multiple species and multiple tissue types affected by the exposure:

  • Multiple species: Increase in malignant tumors in rats (F344 rats [Kerns et al. 1983; Monticello et al. 1996; Kamata et al. 1997], Sprague Dawley rats [Sellakumar et al. 1985; Soffritti et al. 1989], and Wistar rats [Feron et al. 1988; Woutersen et al. 1989]) and mice (B6C3F1 mice [Kerns et al. 1983]).
  • Multiple tissue types: Malignancies of nasal epithelium (mostly squamous-cell carcinoma) (Kerns et al. 1983; Sellakumar et al. 1985; Feron et al. 1988; Woutersen et al. 1989; Monticello et al. 1996; Kamata et al. 1997) and gastrointestinal tract (leiomyosarcoma) (Soffritti et al. 1989 [offspring]; Soffritti et al. 2002 [adults]).

2. Carcinogenicity by multiple routes of exposure: Inhalation (Kerns et al. 1983; Sellakumar et al. 1985; Feron et al. 1988; Woutersen et al. 1989; Monticello et al. 1996; Kamata et al. 1997) and oral (Soffritti et al. 1989 [offspring]; Soffritti et al. 2002 [adults]).

3. Carcinogenicity to an unusual degree with respect to incidence, site, type of tumor, or age at onset: Nasal tumors are rare in untreated rats and in multiple studies occurred in treated rats at relatively high incidence (Kerns et al. 1983; Monticello et al. 1996).

The committee concludes that there is sufficient evidence that formaldehyde is carcinogenic in experimental animals.

TOXICOKINETICS

This section outlines multiple aspects of the toxicokinetics of gas-phase formaldehyde. The most likely route of exposure in humans is inhalation, and the committee has focused on this route. Information on the reactivity and metabolism of formaldehyde is followed by specific information on endogenous vs exogenous formaldehyde levels and on the inhalation dosimetry of this gas, particularly as related to the potential for absorption into the bloodstream and systemic distribution. The current report focuses on formaldehyde gas; however, it is worth noting that paraformaldehyde powder is used in some embalming and chemical applications. These uses may produce exposures to airborne particles of paraformaldehyde in addition to gas-phase formaldehyde. There is currently a dearth of information on human health effects associated with exposure to paraformaldehyde particles.

Reactivity and Metabolism

Formaldehyde is a volatile, organic, one-carbon aldehyde that exists as a gas at room temperature. It is water-soluble and reacts reversibly with water to form methanediol, which is the principle aqueous form in tissues after exposure to formaldehyde (Fox 1985). It can self-polymerize to form paraformaldehyde,

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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which is a solid at room temperature that has the ability to break down when heated to release the monomer. It also reacts reversibly with amine and sulfhydryl groups, and this may ultimately result in cross-links between macromolecules. The inherent chemical reactivity of gas-phase formaldehyde is important to note because it plays a key role in its interaction with many macromolecules and cellular processes. The innate chemical reactivity of formaldehyde allows it to act as a cross-linking agent to fix tissue for pathological analysis and as a reactant in the synthesis of numerous industrial products. Those same chemical properties can, in part, explain its numerous toxic properties. Formaldehyde is reactive because its carbonyl atom acts as an electrophile, which reacts reversibly with nucleophilic sites on cell membranes, amino groups on proteins and DNA, and thiol groups on such biochemicals as glutathione (Bolt 1987).

The native reactivity of formaldehyde contributes to the well-established irritant properties of formaldehyde. Studies have found formaldehyde to cause dermatitis on dermal exposure and both eye and nasal irritation on inhalation exposure (Paustenbach et al. 1997). The nasal sensitization does not appear to be related to concentrations of glutathione–formaldehyde dehydrogenase; this indicates that formaldehyde itself, not metabolic products, is the irritant (Zeller et al. 2011b). Formaldehyde also reacts with macromolecules—a feature that has been used extensively to detect exogenous exposure to formaldehyde through measurement of formaldehyde–DNA adducts (ATSDR 1999; Lu et al. 2011) and proteins (Edrissi et al. 2013a). The reaction of formaldehyde with cellular components contributes to the sensitization of people to formaldehyde, which is manifested as allergic reactions and alterations in a person’s immune system (Costa et al. 2013; Hosgood et al. 2013; Lino-dos-Santos-Franco et al. 2013). Although the mechanism is unclear, several reports associate formaldehyde with induction of an occupational asthmatic response in exposed people (Tang et al. 2009; McGwin et al. 2011) and in animal models (Wu et al. 2013).

Formaldehyde is rapidly absorbed and biotransformed extensively at the point of contact after ingestion or inhalation. It is primarily oxidatively biotransformed by glutathione-dependent formaldehyde dehydrogenase (FDH), officially named alcohol dehydrogenase 5 (ADH5), and S-formyl-glutathione dehydrogenase to formic acid (IARC 2006a). Formic acid can be ionized to formate and excreted via the kidney, further biotransformed to CO2 and exhaled, or condensed with tetrahydrofolate and enter the one-carbon pool (IARC 2006a). In one study, 70% of a 14C-labeled formaldehyde dose was found to be excreted as [14C]CO2 within 12 hours, and the remainder entered the one-carbon pool, where it was incorporated into biomolecules in the body (Buss et al. 1964). Formaldehyde dehydrogenases are ubiquitous in all tissues, including the respiratory tract, with no distinct “regional” differences in the biotransformation of formaldehyde (Casanova-Schmitz et al. 1984; Thompson et al. 2008). The biotransformation of formaldehyde is similar in all species tested. The rapid biotransformation of formaldehyde at the point of contact limits the access of formaldehyde systemically.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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Endogenous vs Exogenous Sources

Formaldehyde exposure has both exogenous and endogenous sources. It is produced intracellularly as a component of the one-carbon pool intermediary metabolism pathways. It is also the product of metabolism of drugs and other exogenous compounds (NTP 2010; NRC 2011). Because formaldehyde is normally present in tissues, the toxicokinetics of exogenous formaldehyde exposure must be evaluated in the context of the relatively large amounts of formaldehyde (near 0.1 mM) that are endogenously present. Measurement of tissue formaldehyde is somewhat difficult because of its volatility and reactivity. Many techniques rely on extraction followed by mass spectrometry (for example, Heck et al. 1982). Those methods provide a measure of free and reversibly bound formaldehyde but do not differentiate between the two. Formaldehyde, through the one-carbon pool, is metabolically incorporated into tissue macromolecules. Therefore, simple use of 14C-labeled formaldehyde does not provide a direct measure of the distribution of parent exogenously administered formaldehyde (NTP 2010; NRC 2011). As noted above, because of its reactivity, formaldehyde may form DNA–protein cross-links, DNA–DNA cross-links, and protein or DNA adducts (Lu et al. 2010a; NTP 2010; NRC 2011; Edrissi et al. 2013b). Those moieties have the advantage of being more stable and longer-lasting than formaldehyde itself and have been used as biomarkers of cellular exposure to formaldehyde. It is important to recognize that use of the moieties (for example, DNA–protein cross-links) as biomarkers of cellular formaldehyde delivery does not require a direct link to tumorigenesis.

The endogenous formaldehyde concentration in whole blood of rodents and nonhuman primates is about 0.1 mM. The concentration in tissues is probably somewhat higher (NTP 2010; NRC 2011). That value represents free plus reversibly bound formaldehyde. Information on the fraction of blood formaldehyde that is free vs bound is not available. Whether from endogenous or exogenous sources, formaldehyde is extensively metabolized to formate via formaldehyde dehydrogenase as described above.

Inhalation Dosimetry

Because inhalation is the most likely route of exposure to formaldehyde, an understanding of the fate of inhaled formaldehyde is critical for evaluation of its toxicity. As would be expected for a water-soluble highly reactive gas (Kimbell 2006), inhaled formaldehyde is effectively removed from the airstream. Thus, it is expected that formaldehyde will be efficiently removed from the airstream in the first airways with which it comes into contact, either the nose during nose breathing or the tracheobronchial airways during mouth breathing. Water-soluble reactive gases may be absorbed efficiently in the mouth and pharynx during mouth breathing (Frank et al. 1969); although this is likely to occur with formaldehyde, it has not been confirmed experimentally. Experimental studies in the dog (Egle 1972) indicate greater than 95% deposition of inhaled formal-

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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dehyde in the nose, lower respiratory tract, and total respiratory tract. A published abstract (Patterson et al. 1986) provides similar data on nasal deposition in the rat.

Numerous state-of-the-art inhalation dosimetry mathematical models have been directed toward dosimetry of inhaled formaldehyde. They have recently been extensively and appropriately reviewed (NRC 2011). The models suggest that inhaled formaldehyde is not deposited uniformly throughout the nose, but local areas, “hot spots”, receive a higher delivery of the dose than other areas. Those areas correlate closely, in the rat, with areas in which DNA–protein cross-link studies indicate high cellular delivery and with areas in which tumors are most likely to arise. Models suggest that rates of localized delivery to small regions in the human nose may be similar to those observed in rats exposed at the same concentration (Kimbell et al. 2001). The modeling prediction adds weight to the idea that formaldehyde may pose a carcinogenic hazard to the human nose. Models suggest that, despite the existence of localized hot spots within the nose, nasal deposition efficiency averaged over the entire nose is lower in humans or nonhuman primates than in rats, leading to greater penetration of inhaled formaldehyde to the lower respiratory tract. That is supported by DNA–protein cross-links studies that suggest higher cellular delivery of inhaled formaldehyde to the trachea and mainstream bronchi in nonhuman primates than in rats (Heck et al. 1989; Casanova et al. 1991). Unlike the obligate nose-breathing rodent, humans are capable of mouth breathing; this would greatly increase the delivery of inhaled formaldehyde to the lower airways.

The airway epithelium is metabolically active. Of relevance to formaldehyde disposition within nasal tissues is the presence of ADH5/FDH. The metabolic pathways offer an effective clearance mechanism for formaldehyde. Only formaldehyde that escapes metabolism is available for binding to tissue macromolecules or potentially available for absorption into the blood. Like all metabolic pathways, formaldehyde metabolism demonstrates saturation kinetics. As saturation occurs, the likelihood of reaction of formaldehyde with tissue macromolecules or of penetration of formaldehyde to deeper tissues increases. On the basis of modeling efforts and DNA–protein cross-link assessments, saturation kinetics may occur at concentrations above 2 ppm in the rodent nose. Specifically, a nonlinear relationship between inspired concentration and DNA–protein cross-links in the nose is observed at exposure concentrations of 6 ppm or higher, greatly exceeding what would be expected for a linear increase from the DNA–protein cross-links observed at concentrations of 2 ppm or lower (NTP 2010; NRC 2011).

Absorption into Blood

The disposition of formaldehyde in airway tissues and distribution throughout the body are important for understanding the potential for tissue injury in airways or distant tissues. As previously noted, formaldehyde reacts readily and reversibly with sulfhydryl and amine moieties. Formaldehyde reacts revers-

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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ibly with water to form methanediol, with the equilibrium strongly favoring methanediol. As outlined by Georgieva et al. (2003), it is not likely that the dissociation of methanediol to form formaldehyde is rate-limiting (in contrast with the reaction with macromolecules), so this process is not critical for determining formaldehyde disposition in nasal tissues (NRC 2011). Because formaldehyde reactions are reversible, it is possible that an individual formaldehyde molecule, if it is not metabolically degraded, may shuttle from one binding site to another. Therefore, an individual endogenous formaldehyde molecule could be distributed away from its site of formation, and an individual exogenous formaldehyde molecule could be distributed to tissues away from its site of first contact. That would occur only if the formaldehyde molecule escaped metabolic transformation. Because ADH5/FDH is ubiquitously expressed, including expression in red blood cells, the likelihood of metabolic transformation is high, and this lowers the likelihood of penetration to distant tissues through the bloodstream.

Anatomic features of the airways are highly relevant to the potential for absorption into the blood and systemic distribution of formaldehyde (NRC 2011). The air–blood barrier of the nose and large tracheobronchial airways consists of a mucous lining layer overlying a pseudostratified columnar mucociliary epithelium. Residing below the basement membrane, the submucosal space of the nasal airways is highly vascularized. In the nose, a superficial capillary layer is present just below the basement membrane (Figure 3-3). This relationship is important for evaluation of formaldehyde disposition in the nose. Presumably, the target cells for tumorigenesis in the nasal airways are the basal cells that reside on the basement membrane. Immediately below the basement membrane are the vessels of the superficial capillary layer of the nose. The total epithelial thickness in the nose depends on the site but is generally less than 0.05 mm in rodents and humans (Schroeter et al. 2008). A similar structure exists with respect to the nasal associated lymphoid tissue (NALT), which resides just below the basement membrane (Figure 3-3).

On the basis of mathematical modeling and estimation of the rates of reaction and metabolism, it has been estimated that formaldehyde would penetrate to some depth in nasal tissues (see Figure 3-4) (Georgieva et al. 2003). Specifically, the modeling efforts suggest that the formaldehyde concentration at the depth of 0.05 mm (below the basement membrane) is greater than 50% of the concentration at the mucus–tissue interface. Thus, the concentration–tissue depth profile appears to have a shallow slope. Formaldehyde is clearly cytotoxic to the nasal epithelium, and the nasal epithelial basal cells are probably the target for nasal tumorigenesis; this indicates that reactive formaldehyde penetrates to this depth in the nose. Given the shallow slope of the concentration–tissue depth profile, it is likely that toxicologically significant concentrations of formaldehyde penetrate somewhat deeper to the superficial capillary layer of the nose, inasmuch as these capillaries are adjacent to the basement membrane and basal

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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images

FIGURE 3-3 Schematic representation of the structure of the nasal mucosa of the respiratory epithelium and follicle-associated epithelium. For both epithelia, a concentration gradient for exogenous formaldehyde during inhalation exposure will exist with concentrations at the superfici l layer (closest to the airstream) being higher than concentration in deeper layers. As outlined in the text, this gradient is due to the reaction of formaldehyde with tissue substrates or metabolism via ADH5/FDH. It is worth noting that basal cells, a target for formaldehyde-induced carcinogenesis, lie immediately above the basement membrane and capillaries and nasal associated lymphoid tissue (NALT) lie immediately below the basement membrane. Source: NRC 2011, p. 32.

cells (see above). Thus, at sufficient airborne concentrations, biologically significant concentrations of formaldehyde may be present in the nasal submucosa and capillary bed. It should be recognized, however, that the presence of formaldehyde in the nasal submucosa and capillary bed does not itself indicate that biologically significant concentrations of formaldehyde penetrate via the bloodstream to distant tissues. A toxicokinetic approach could be formulated to estimate the exposure concentrations that would be required to raise systemic blood formaldehyde substantially above endogenous concentrations. To the committee’s knowledge, that has not been performed.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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images

FIGURE 3-4 Model-based estimates of exogenous formaldehyde concentration in nasal tissues during inhalation exposure to 6 ppm formaldehyde. Tissue concentrations increase quickly from 0.1 to 0.5 minutes after the onset of exposure as a quasi-steady state is established. Readily apparent is the prediction that the formaldehyde concentration at a depth of 50 μm, measured from the mucus:tissue interface, is fairly similar to the concentration at the interface itself. Source: Georgieva et al. 2003. Reprinted with permission; copyrig t 2003, Inhalation Toxicology.

Distribution of Inhaled Formaldehyde

The nose receives about 1% of cardiac output, and mathematical models suggest that about one-third of nasal circulation (0.33% of total cardiac output) may perfuse the superficial capillary layer (Gloede et al. 2011). Venous blood from the nose is ultimately mixed with the systemic venous blood. On the basis of relative perfusion rates, blood from the entire nose is diluted by a factor of 100 (because the nose receives 1% of the cardiac output) with systemic venous blood; blood from the superficial capillary layer is diluted by a factor of about 300 with systemic venous blood before distribution to the body. From that perspective, it can be appreciated that although the concentration of an inhaled xenobiotic in the nasal capillary blood may be high, its concentration is greatly reduced (by a factor of 100–300) as blood from the nose mixes with systemic venous blood. The underlying structure of the large tracheobronchial airways is similar to that of the nose; thus, the relationships described above are qualitatively similar for the lower airways. The entire tracheobronchial tree receives about 1% of cardiac output (Gloede et al. 2011). As for the nose, any xenobiotic absorbed into the tracheobronchial circulation of the large airways is diluted by a factor of about 100 as the venous output from the airways mixes with the systemic venous blood.

Although it is theoretically possible that an individual exogenous formaldehyde molecule could be distrib ted away from the portal of entry, mass-

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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balance and kinetic arguments and experimental data strongly suggest that this does not occur to a great extent. Specifically, multiple studies that used different conceptual approaches, from simple mass-balance estimates (Heck and Casanova 2004; Nielsen et al. 2013) to more detailed pharmacokinetic analysis (Franks 2005), universally support the conclusion that the amount of formaldehyde that is inhaled (at reasonable exposure concentrations) and absorbed into circulation is much lower than the endogenous amounts in circulation. Analytic studies did not observe a large increase in the total content of formaldehyde in blood or tissue above the endogenous concentrations during inhalation exposure (NTP 2010; NRC 2011). Published literature, relying on gas chromatographic and mass spectrometry techniques, indicates that blood formaldehyde (measured as free plus reversibly bound) is not increased in the rat, monkey, or human by inhalation exposure to formaldehyde (Heck et al. 1985; Casanova et al. 1988). Studies that use bound formaldehyde as a biomarker and that rely on dual-labeled formaldehyde also did not observe an increase in tissue formaldehyde during inhalation exposure in any tissue except the nose (Lu et al. 2011; Moeller et al. 2011; Edrissi et al. 2013b). Contrary to these findings are findings of formaldehyde adducts in the blood of exposed individuals. One study reported increases in blood albumin–formaldehyde adducts in workers exposed to formaldehyde (Pala et al. 2008); another reported increases in formaldehyde–hemoglobin adducts (Bono et al. 2006). Mass-balance arguments call the validity of those findings into question (Nielsen et al. 2013), specifically that the amount of formaldehyde that would be required to raise albumin adducts or hemoglobin adducts to the levels reported is much greater than the amount that was inhaled.

Recent well-designed studies have relied on dual labeled formaldehyde to measure formaldehyde–DNA adducts as a biomarker of delivered dose of exogenous formaldehyde for comparison with endogenous concentrations (Lu et al. 2010a,b; Moeller et al. 2011). They indicate that endogenous formaldehyde–DNA adducts are ubiquitous throughout the body. Increased exogenous formaldehyde–DNA adducts are observed in nasal tissues of rodents and nonhuman primates after inhalation exposure to formaldehyde, and this validates the sensitivity of the technique. High concentrations of exogenous formaldehyde–DNA adducts are not observed in distal tissues, including bone marrow, after formaldehyde inhalation. Those experiments provide strong evidence that formaldehyde exposure at the concentrations used (up to 15 ppm) does not result in substantial delivery of exogenous formaldehyde to nonrespiratory tissues. The results have recently been confirmed by using formaldehyde–lysine adducts as biomarkers instead of formaldehyde–DNA adducts (Edrissi et al. 2013b).

MECHANISMS OF CARCINOGENESIS

The mechanisms of carcinogenesis of formaldehyde have been the subject of intense research for decades, and a large evidence base is available from

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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which to draw inferences and conclusions. Despite the wealth of information available on a variety of test systems, from naked DNA (that is, DNA without any associated proteins) to experimental animals and exposed humans, it is still being debated what mechanistic events take place in tissues that have been suggested as targets for formaldehyde-associated carcinogenesis. Such debate is informed, in large part, by the considerations of formaldehyde toxicokinetics, inasmuch as formaldehyde is both a highly reactive molecule and an endogenously formed compound produced in the course of normal cellular metabolism. There is evidence that exogenously administered formaldehyde is responsible for noncancer and cancer effects at the portal of entry, such as nasal mucosa or other parts of the upper aerodigestive tract, depending on the mode of administration and breathing patterns. It has been more controversial whether formaldehyde itself or products of its biotransformation may reach tissues that do not come into direct contact with inhaled or ingested formaldehyde in experimental animals or humans, and a detailed discussion of the available evidence is provided under the section “Toxicokinetics” above. There is general agreement that systemic delivery of formaldehyde is unlikely (NRC 2011), but it is also true that various toxicity phenotypes (for example, genotoxicity and mutagenicity in circulating blood cells, changes in the number of circulating cells and bone marrow cells, and gene expression changes in blood) have been found in cells and tissues that are not in direct contact with exogenously administered formaldehyde. That apparent inconsistency notwithstanding, the committee concurs with the conclusions drawn by the National Research Council Committee to Review EPA’s Draft IRIS Assessment of Formaldehyde (NRC 2011) that it is important to differentiate between systemic delivery of formaldehyde and systemic effects. It is possible that the “systemic delivery of formaldehyde is not a prerequisite for some of the reported systemic effects seen after formaldehyde exposure. Those effects may result from indirect modes of action associated with local effects, especially irritation, inflammation, and stress” (NRC 2011, p. 36).

The present committee found that the most sensible characterization of the adverse health effects of formaldehyde and associated mechanisms is that proposed by NRC (2011). Specifically, a wide array of the adverse outcomes that have been associated with formaldehyde exposure are best classified into portal-of-entry and systemic categories, which are defined as follows: portal-of-entry effects are effects that arise from direct interaction of inhaled or ingested formaldehyde with the affected cells or tissues; systemic effects are effects that occur beyond tissues or cells at the portal of entry. The committee notes, however, that it is plausible that some of the systemic effects, most notably genotoxicity in circulating blood cells, may have resulted from the exposure of these cells at the portal-of-entry tissues (for example, lymphoid tissue in the nasal mucosa).

As discussed in previous sections, the committee relied on the background document for formaldehyde, published reviews, and assessments performed by other authoritative bodies to ensure that relevant literature was captured up to the publication of the 12th RoC. It also considered literature, comments, and

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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arguments provided during its open session and submitted by other sources during the duration of the study. The committee carried out its own literature search (see Appendix D) for publications that are pertinent to the major postulated modes of carcinogenic action of formaldehyde (genotoxicity, cell proliferation and apoptosis, and effects on the immune system). The committee’s exclusion criteria and detailed search strategies for studies related to genotoxicity and mutagenicity are presented in Box D-3 and for studies related to immune effects are presented in Box D-4. Literature trees were used to document identification and selection of the literature evidence (Figures D-3 and D-4). The general question that the committee addressed was, What is the evidence that the following mechanistic events—genotoxicity and mutagenicity or effects on the hematologic system—are part of the overall mode of action of formaldehyde-associated carcinogenicity? The outcomes of the searches and the evidence available in the background document for formaldehyde (NTP 2010) were evaluated together and are detailed below.

The committee notes that because of the limitations of time and resources several of the mechanisms that have been proposed by NTP (2011) to explain the carcinogenicity of formaldehyde (such as cytotoxicity followed by compensatory proliferation and oxidative stress) have not been evaluated by conducting new literature searches. In the course of the review of the substance profile for formaldehyde in the NTP 12th RoC (see Chapter 2), the committee found that the mechanism of cytotoxicity followed by compensatory cell proliferation is a well-established portal-of-entry mechanism that is not controversial. On the contrary, oxidative stress is a mechanistic event that has not been addressed in detail and on which the evidence base is too small to draw firm conclusions. The committee focused its attention on the mechanistic evidence that is related to genotoxicity and mutagenicity, hematologic effects, and data from toxicogenomic studies, which reflects broad biologic responses and is thus informative as both the overall effect and specific pathways that may be perturbed by exposure to formaldehyde.

The RoC does not present quantitative assessments of risks of cancer associated with the substances listed. Therefore, the committee did not explicitly take into consideration the issue of the dose or concentration of formaldehyde that was applied or evaluated in each study. The background document for formaldehyde contains extensive information on the doses and concentrations used in various studies, and, where it is available, the committee notes dose-dependent and time-dependent trends in the new studies that have been published since June 10, 2011.

Finally, the committee notes that although the mode of action of a chemical substance is an important component of decision-making to protect human health, the guidelines established by various national and international agencies that conduct such assessments differ in how such information is gathered, presented, and evaluated (Box 3-1). The guidance documents of IARC, the US Environmental Protection Agency (EPA), and the International Programme on Chemical Safety (IPCS) are informative, but the committee’s charge (see Ap-

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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pendix B) was to integrate the level-of-evidence conclusions and to consider all relevant information in accordance with RoC listing criteria. In that respect, for each listed substance, the RoC includes studies of genotoxicity and of biologic mechanisms. The listing criteria are used to guide the evaluation of the human, animal, and mechanistic evidence. The listing criteria specifically state that “data derived from the study of tissues or cells from humans exposed to the substance in question, which can be useful for evaluating whether a relevant cancer mechanism is operating in humans” (NTP 2010, p. iv), constitute one of the lines of evidence used to support whether there is sufficient or limited evidence of carcinogenicity from studies in humans.

BOX 3-1 Guidance from Various Agencies on the Use of Mechanistic and Other Relevant Data

The IARC Monographs Program operates under the general guidance of a preamble, which specifies that a working group is to consider mechanistic and other relevant data because they “may provide evidence of carcinogenicity and also help in assessing the relevance and importance of findings of cancer in animals and in humans” (IARC 2006b, p. 15). The preamble outlines “scientific principles, rather than a specification of working procedures” (p. 1), for the experts who participate in the development of each monograph. It notes that “the procedures through which a Working Group implements these principles are not specified in detail” (p. 1).

The EPA Guidelines for Carcinogen Risk Assessment (EPA 2005) state that the agency’s assessments should discuss the available information on the modes of action and associated key events of chemicals under evaluation. Specifically, the assessments aim to address several questions pertaining to the extent and quality of the evidence on the hypothesized mode of action. The questions include sufficiency of supporting information from test animals, relevance to humans, and any information that may suggest that particular populations or life stages can be especially susceptible to the hypothesized mode of action. It is noted, however, that “in the absence of sufficiently, scientifically justifiable mode of action information, EPA generally takes public health-protective, default positions regarding the interpretation of toxicologic and epidemiologic data” (EPA 2005, p. 1-10).

IPCS developed a mode-of-action relevance framework for the analysis of mechanistic evidence on chemical carcinogens in experimental animals and its relevance to humans (Boobis et al. 2008). The framework calls for determining whether the weight of evidence based on experimental observations is sufficient to establish a hypothesized mode of action. A series of key events causally related to the toxic effect are then identified using an approach based on the Bradford Hill criteria and compared qualitatively and quantitatively between experimental animals and humans.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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Genotoxicity and Mutagenicity

The data available to examine the potential role of genotoxicity and mutagenicity of formaldehyde are extensive. Those effects are likely to be relevant for all cancer sites that have been associated with formaldehyde exposure. Nearly all aspects of genotoxicity and mutagenicity have been studied with formaldehyde, so assertive conclusions can be drawn from the available evidence.

The committee collated the evidence on all the mechanistic events that make up the genotoxic mode of action into separate tables (see Appendix E). In each table, the committee separated studies by type of the model system, including a clear division between the portal-of-entry and systemic effects in in vivo studies. Publications that have evaluated a particular mechanistic event and found evidence supporting or refuting each were included. In addition, a summary table (Table 3-10) was constructed to present the totality of the evidence available on each mechanistic event in each experimental model system.

Overall, the evidence on genotoxicity and mutagenicity of formaldehyde resulted from studies that evaluated DNA adducts (Table E-1), DNA–DNA cross-links (Table E-2) and DNA–protein cross-links (Table E-3), DNA strand breaks (Table E-4), mutations (Table E-5), sister-chromatid exchanges (Table E-6), micronuclei (Table E-7), and chromosomal aberrations (Table E-8). Several published studies have also examined the DNA-repair responses to formaldehyde-induced DNA damage. Owing to the paucity of data, the model systems used in these studies, and the scope of the present committee’s charge, that information was not included in the evaluation. Similarly, the committee found that although some reports examined the possible role of genetic polymorphisms in the genotoxic potential of formaldehyde or ensuing adverse outcomes, the overall database was not robust and did not provide strong evidence that human variability factors (genetic polymorphisms) may be critical for drawing conclusions. All studies included in Appendix E were examined in full text (including translations, where applicable) by at least two committee members, who independently determined whether a given study observed an important effect or lack thereof with respect to the phenotype named in each table. Studies were categorized as positive if a statistically significant effect was observed. Studies were categorized as negative if the results reported an absence of a particular effect (that is, no statistically significant difference from the appropriate control group). Although the committee members exercised their scientific judgment in categorizing studies and determining their relevance to each phenotype, the committee did not perform a formal quality assessment of each individual study, whether it was categorized as positive or negative. The committee members also did not make judgments about the study design or methodology, recognizing that all the studies had been subjected to some form of peer review before publication.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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TABLE 3-10 Summary of Published Studies on the Genotoxic and Mutagenic Effects of Formaldehyde in Test Systems and Organisms1

DNA Adducts DDX DPX Strand breaks Mutations SCE MN CA
Cellfree systems + (7/0) + (3/0) + (3/0)
Nonmammalian model organisms + (6/0) +**
Mammalian in vitro Rodent + (1/0) + (14/1) + (6/2) +/- (3/2) + (9/0) + (4/0) + (5/0)
Human + (2/0) + (23/0) + (8/0) + (6/0) + (6/0) + (4/0) + (6/2)
Mammalian in vivo: portal-of-entry effects Rodent + (2/0) + (8/0) - (0/1) +/- (1/1) -/+ (1/2) + (1/0)
Primate + (1/0) + (2/0)
Human +/- (11/3)
Mammalian in vivo: systemic* effects Rodent - (0/1) +/- (2/2) +/- (2/1) + (1/0) - (0/2) -/+ (4/5) - (2/5)
Primate - (0/1) - (0/2)
Human +# (1/0) + (3/0) + (9/2) -/+ (7/9) + (18/3) +/- (11/5)

1Total numbers of studies demonstrating effect or lack thereof are indicated in parentheses. See Appendix E for data that support this summary table: DNA adducts (Table E-1), DNA–DNA cross-links (Table E-2), DNA–protein cross-links (Table E-3), DNA strand breaks (Table E-4), mutations (Table E-5), sister-chromatid exchanges (Table E-6), micronuclei (Table E-7), and chromosomal aberrations (Table E-8).
+: all or most of the studies indicate the effect.
+/-: most of the studies indicate the effect, although many show lack thereof.
-/+: most of the studies indicate lack of the effect, although many positive studies have been published.
-: all or most of the studies indicate lack of the effect.
*The committee acknowledges that although most investigators consider the effects on circulating-blood mononucleated cells as systemic because cells for the analyses were collected from the systemic circulation, it is plausible that the cells had been exposed to formaldehyde in the nose through lymphoid tissue in the mucosa.
**The results are overwhelmingly positive for point mutations and overwhelmingly negative for frame-shift mutations.
#M1G adduct has been postulated to be the result of secondary DNA damage caused by formaldehyde-associated oxidative stress.
Abbreviations: DNA, deoxyribonucleic acid; DDX, DNA–DNA cross-links; DPX, DNA–protein cross-links; SCE, sister-chromatid exchanges; MN, muconuclei; CA, chromosomal aberrations. Source: Committee generated.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

The committee’s work was informed by the Bradford Hill criteria (Hill 1965) for determining causality between exposure to formaldehyde and findings of genotoxicity and mutagenicity. Although those criteria have been proposed for determinations of causality in epidemiologic studies, they do not all apply to the evaluation of the mechanistic evidence. As noted in EPA guidelines (EPA 2005, p. 2-13), “one…cannot simply count up the numbers of studies reporting statistically significant results or statistically non-significant results for carcinogenesis and related MOAs [modes of action] and reach credible conclusions about the relative strength of the evidence and the likelihood of causality.” Thus, the committee, upon systematizing the available mechanistic evidence pertaining to the genotoxicity and mutagenicity of formaldehyde into tables, appraised the evidence by using the general guidance of the “causal criteria” (EPA 2005) to determine its overall strength for drawing conclusions about causality for each of the mechanistic events identified in the tables. Because the body of evidence on genotoxicity and mutagenicity of formaldehyde is very large, the mechanistic synthesis does not contain many citations to the individual publications; all the evidence is presented in multiple tables.

Owing to the challenge of establishing whether and how formaldehyde can exert point-of-entry and systemic effects, the committee chose to evaluate causality for each of the mechanistic events in three broad categories:

1) Effects on the naked DNA or on the DNA of nonmammalian organisms or mammalian cells in vitro.

2) Effects observed on the portal-of-entry tissues of animals or humans exposed to formaldehyde.

3) Systemic effects in animals or humans exposed to formaldehyde.

The latter two are most relevant to the determination of the cancer-hazard classification according to the RoC listing criteria, which call for conclusions to be based on the information “derived from the study of tissues or cells from humans exposed to the substance in question” (NTP 2011, p. 198). Again, the committee acknowledges that although most investigators consider the effects on circulating blood mononucleated cells to be systemic because cells for the analyses were collected from the systemic circulation, it is plausible that these cells have been exposed to formaldehyde in the nose through lymphoid tissue in the mucosa.

Effects of Formaldehyde on Naked DNA or on DNA of Nonmammalian Organisms or Mammalian Cells in Vitro

The totality of the evidence overwhelmingly shows that when formaldehyde is added to naked DNA or nonmammalian organisms or mammalian cells are incubated in the presence of formaldehyde, DNA adducts (Table E-1), crosslinks (Tables E-2, E-3), strand breaks (Table E-4), mutations (Table E-5), and

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

clastogenic damage (Tables E-6, E-7, and E-8) are found. Studies were conducted in different types of model systems and have produced consistent results.

The evidence of genotoxicity and mutagenicity of formaldehyde comes from studies where different model systems were tested and various molecular techniques were used to evaluate the effects. Because all studies evaluated in this category used formaldehyde, specificity of the effects being caused by formaldehyde has been firmly established. In addition, many studies used appropriate positive and negative controls, and this further strengthens the specificity of the association. The temporal relationship of the observed association is clear in that the studies evaluated genotoxic and mutagenic effects after DNA or cells came into contact with formaldehyde. Dose–response relationships between genotoxic and mutagenic effects and formaldehyde were observed in studies that had appropriate designs. For example, DNA–protein cross-links were formed in a concentration–response manner in human lymphoblastoid cell lines (Ren et al. 2013), epithelium-like human lung cells (Speit et al. 2010), and isolated human lymphocytes (Neuss et al. 2010a,b). Similar observations were made in whole-blood cultures for sister-chromatid exchanges, micronuclei, and chromosomal aberrations (Schmid and Speit 2007; Ren et al. 2013).

The committee concludes that the genotoxic and mutagenic mode of action of formaldehyde in studies of naked DNA, studies of DNA from nonmammalian organisms, and studies of mammalian cells in vitro is consistent, strong, and specific to the formaldehyde exposure. Both temporal and dose–response relationships have been established. This mechanistic event is relevant to human cells because all the genotoxic effects observed in studies of naked DNA, nonmammalian model organisms, or cells from rodents have been also observed in human cells, either established cell lines or primary cells.

Effects on the Portal-of-Entry Tissues of Animals or Humans Exposed to Formaldehyde

Because various studies reviewed by the committee may have used different routes of administration of formaldehyde and because of the differences in breathing patterns among rodents and humans, the committee considered the following anatomic regions as points of entry: nasal passages, oral cavity and upper aerodigestive tract, and forestomach (in gavage studies). The committee identified no studies that evaluated DNA–DNA cross-links or sister-chromatid exchanges in exposed rodents or humans at the portal of entry, so these mechanistic events were not considered in this section.

Most of the evidence of genotoxic and mutagenic effects at the portal of entry, depending on the end point studied, is from studies of laboratory rodents and exposed humans. Several reports evaluated pertinent mechanistic events in nonhuman primates. Studies of DNA adducts (Table E-1), even though the database is not large, showed that formaldehyde-induced DNA damage is consistently observed in both rodents (Lu et al. 2010a, 2011) and nonhuman primates

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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(Moeller et al. 2011). Similarly, consistent evidence from a large number of studies of rodents and nonhuman primates demonstrates formation of DNA–protein cross-links (Table E-3). Positive and negative findings, albeit from a small number of studies of formaldehyde exposure of rodents, are equally divided for strand breaks (Table E-4), mutations (Table E-5), micronuclei (Table E-7), and chromosomal aberrations (Table E-8). In humans exposed to formaldehyde, formation of micronuclei was examined in cells at the portal of entry, and 11 of 14 studies demonstrated a positive association (Table E-7). Overall, the findings are consistent with genotoxic and mutagenic effects of formaldehyde observed in naked DNA, in the DNA of nonmammalian organisms, and in mammalian cells in vitro.

Evidence of genotoxicity and mutagenicity of formaldehyde in exposed humans is strong, even though several studies reported no induction of micronuclei. The positive observations were made in studies of diverse groups of subjects that were exposed to formaldehyde. Various assays have been used to evaluate the mechanistic events, and statistical significance of the effects was established in the positive studies.

In rodent and nonhuman primate studies, formaldehyde exposures were well documented (for example, purified reagent-grade formaldehyde was used). Furthermore, several studies of DNA damage have used 13C-labeled formaldehyde (Lu et al. 2010a, 2011; Moeller et al. 2011), which shows that the genotoxic effects of formaldehyde occur at the portal of entry. In human studies, many investigators established the association between formaldehyde and these mechanistic events through exposure monitoring, albeit most of the studies were of occupational cohorts and the presence of other agents cannot be excluded. Some of the studies that found no evidence of micronuclei in portal-of-entry tissues from humans (Speit et al. 2007; Zeller et al. 2011a) is evidence that questions the association in controlled exposures of volunteers to formaldehyde.

Studies of rodents and nonhuman primates provide strong evidence for a temporal relationship of the observed association because the genotoxic and mutagenic effects were observed after exposure to formaldehyde. In many human studies, temporality was established by collecting samples before and after exposure in the workplace.

Studies of rodents and nonhuman primates provide strong evidence of concentration–response relationships in the genotoxicity of formaldehyde at the portal of entry (Lu et al. 2010a, 2011; Moeller et al. 2011). The concentrations of formaldehyde used in the studies (around 1–10 ppm) are comparable with or an order of magnitude higher than those documented in human occupational exposures. The shape of the concentration–response curve of several biomarkers of genotoxicity in the portal-of-entry tissues in rodents is nearly identical with that for tumorigenesis in the noses of rodents (Swenberg et al. 2013).

The committee concludes that the genotoxic and mutagenic mode of action of formaldehyde in the portal-of-entry tissues of animals or humans exposed to formaldehyde is supported by the experimental evidence. Several negative studies notwithstanding, the evidence is consistent, strong, and specific with

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

respect to an association following exposure to formaldehyde. Both temporal and exposure–response relationships have been established, most strongly in the studies of experimental animals (rodents and nonhuman primates). This mode of action is relevant to humans because statistically significant increases in the number or frequency of micronuclei, known biomarkers of clastogenesis, have been observed in most, but not all, of the studies of portal-of-entry tissues from humans exposed to formaldehyde.

Systemic Effects in Animals or Humans Exposed to Formaldehyde

Systemic effects are effects that occur outside cells or tissues that come into direct contact with exogenous formaldehyde. Most studies in the systemic-effects category examined genotoxic and mutagenic effects of formaldehyde in circulating blood mononucleated cells unless stated otherwise. The committee acknowledges, however, that although most investigators consider the effects on circulating blood mononucleated cells as systemic because cells for the analyses were collected from the systemic circulation, it is also plausible that these cells were exposed to formaldehyde in the nose through lymphoid tissue in the mucosa.

Most of the experimental evidence that is available for drawing conclusions about systemic genotoxic and mutagenic effects of formaldehyde comes from studies in humans exposed to formaldehyde, mostly in occupational settings. Fewer experimental-animal (for example, rodent) studies have been conducted, and only two studies of nonhuman primates examined some of the mechanistic events in question. Overall, the database pertaining to this question is most consistent in exposed humans in whom formaldehyde exposure-associated DNA–protein cross-links (Table E-3), strand breaks (Table E-4), micronuclei (Table E-7), and chromosomal aberrations (Table E-8) were detected in most of the studies. Data on sister-chromatid exchange formation in response to exposure to formaldehyde in humans are almost equally divided for and against (Table E-6). In studies in rodents, there is little positive evidence of clastogenic effects of formaldehyde on circulating blood cells but some evidence of strand breaks and mutations. Studies of nonhuman primates found no evidence of the increased formation of DNA adducts in bone marrow after exogenous administration of 13C-labeled formaldehyde (Moeller et al. 2011) or the presence of DNA–protein cross-links in the most distal regions (lung parenchyma) of the respiratory tract (Casanova et al. 1991).

Evidence of genotoxicity and mutagenicity of formaldehyde in exposed humans is strong because various assays were used to evaluate these effects, data come from a number of independent laboratories around the world, and the positive studies were conducted on humans exposed in a variety of occupational settings (for example, pathologists, embalmers, and anatomy students). The negative human studies also contribute important information in that the diversity of

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

the study designs and occupational and laboratory-based exposures is appreciable.

The studies of rodents and nonhuman primates used controlled exposures to purified reagent-grade formaldehyde, and some studies even used controlled exposures to 13C-labeled formaldehyde, which increases the specificity of the negative observations. Human studies were largely in occupational exposure scenarios in which formaldehyde was the primary—not the only—agent and other chemical (for example, solvent) or physical (for example, wood-dust) exposures were possible. Formaldehyde-associated DNA–protein cross-links were found in three human studies (Table E-3); however, most of the end points that were evaluated in the positive studies, such as strand breaks (Table E-4) and clastogenic effects (Tables E-6, E-7, and E-8), are difficult to attribute specifically to formaldehyde. Thus, the specificity of the observed positive associations is somewhat uncertain.

In many—not all—positive human studies, a temporal relationship was established by collecting samples before and after exposure in the workplace (Lin et al. 2013) or by considering the extent of employment in an occupation in which formaldehyde exposure is very likely (Viegas et al. 2010; Ladeira et al. 2011; Souza and Devi 2014). Some studies of rodents and nonhuman primates provide strong evidence of lack of a dose–response relationship in the formation of exogenous formaldehyde-induced DNA adducts (Lu et al. 2010a, 2011; Moeller et al. 2011). Recent studies that evaluated DNA–protein cross-links, however, show dose-dependent increases in this biomarker of genotoxicity in tissues (bone marrow, liver, spleen, and testes) that are not in direct contact with inhaled formaldehyde (Ye et al. 2013). Some of the positive human studies found a relationship between the clastogenic effects of formaldehyde and exposure duration (Viegas et al. 2010; Ladeira et al. 2011; Souza and Devi 2014) or dose (Jiang et al. 2010).

The committee concludes that the systemic genotoxic and mutagenic mode of action of formaldehyde is sufficiently supported by the evidence from studies of humans exposed to formaldehyde. The committee acknowledges that reporting bias against negative results could be a limitation of its approach to reviewing the mechanistic evidence (NRC 2014); however, that limitation does not detract from the conclusion that formaldehyde can induce systemic genotoxic changes. The evidence is consistent and strong, albeit it is difficult to establish unequivocal specificity of the effects following exposure to formaldehyde in the human studies. Whereas the committee recognizes some inconsistencies among data in experimental animals and humans and among genotoxicity biomarkers, this variability does not undermine the committee’s conclusion. Both temporal and exposure–response relationships have been demonstrated in studies of humans exposed to formaldehyde. This mode of action is relevant to humans because most of the positive evidence comes from studies of humans exposed to formaldehyde. The data do not exclude the possibility of other modes of action but strongly suggest a causal relationship between exposure to formaldehyde and human cancer.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

Hematologic Effects

The systemic effects of formaldehyde exposure and the association with hematopoietic malignancies have been a source of debate, and there has been much interest in the hematologic effects of formaldehyde exposure. Several recent studies have evaluated the effects of formaldehyde on circulating hematopoietic cells, and a number of them were published after the release of the NTP 12th RoC. In this section, the committee focuses on changes in hematopoietic-cell number or function—that is, “hematologic effects”. It did not consider genotoxicity studies and studies of altered gene expression because they are covered in other sections of this chapter. In addition, given that few studies have been designed to address the clinical significance of hematologic effects, to address the mechanisms by which hematologic effects may arise after exposure, or to address mechanisms that contribute to adverse health effects (including cancer), these topics were not considered by the committee. The focus of this section is on evaluation of recently available evidence related to the hematologic effects of formaldehyde in human and animal exposure studies and evidence that is available from in vitro studies.

Hematologic Effects in Humans Exposed to Formaldehyde

Hematologic effects of formaldehyde include effects on cells of the hematopoietic system that are circulating in the peripheral blood, are present in hematologic tissues (such as bone marrow, lymph nodes, and spleen), or are present in other tissues, whether at the portal of entry or not. The available data primarily reflect the hematologic consequences of exposure to inhaled formaldehyde in humans without addressing the mechanism or health consequences of the findings.

Many studies have addressed the hematologic effects of exposure to formaldehyde in humans (Tables 3-11 and 3-12). Six studies that examined inhalation exposures of formaldehyde in humans reported decreases in overall white blood cells, and three reported decreases in red cells and platelets. Studies have also reported many other hematologic effects, such as increases in monocytes, eosinophils, and some T-cell subsets and decreases in neutrophils and T-cell function. It should be noted that several studies have reported contrasting findings in the same hematologic characteristic, such as increases vs decreases in total lymphocyte concentration and T-, B-, and NK-cell subsets. Given that formaldehyde exposure concentrations, durations, and sources varied greatly among studies, it is difficult to reconcile those results. However, taken as a whole, the body of evidence demonstrates consistently that exposure of humans to inhaled formaldehyde is associated with an array of hematologic effects.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×

TABLE 3-11 Recent Studies of Hematologic Effects of Formaldehydea

Model Subjects Exposure Sample Main Hematologic Findings (Excluding Genotoxicity)b Reference
Inhalation exposure in humans Workers (43 formaldehyde-exposed, 51 age- and sex-matched controls) Factory workers exposed to formaldehyde–melamine resins compared with workers without formaldehyde exposure; mean formaldehyde exposure 1.28 (0.63–2.51) ppm vs <0.03 ppm Peripheral blood tested for lymphocyte subsets Extension of Zhang et al. (2010) using the same subjects and reporting additional assays. Total NK-cell and T-cell counts were 24% and 16% lower, respectively, in exposed workers. Decreased counts in exposed workers were observed for CD8+ T cells, CD8+ effector memory T cells, and regulatory T cells. B-cell numbers did not differ significantly. Hosgood et al. 2013
Workers (43 formaldehyde-exposed, 51 age- and sex-matched controls) Factory workers exposed to formaldehyde–melamine resins; exposures same as Hosgood et al. (2013) Peripheral blood measures (complete blood count and WBC differential) Reanalysis of Zhang et al. (2010) data. Differences in blood measures when examined in context of population averages for Chinese and general populations and when controlled for potential confounders (for example, suspected thalassemia trait) suggest that effects attributed to formaldehyde are not clinically significant. Concerns were raised regarding relevance of CFU-GM assays to AML stem-cell biology. Gentry et al. 2013
Male workers (46 formaldehyde-exposed, 46 controls) Factory workers in two medium-density fiberboard-producing plants; measured formaldehyde levels; 8-hour TWA = 0.20 ± 0.06 ppm (0.10–0.33 ppm) Blood samples measured for lymphocyte subsets, immunoglobulins, complement proteins, and TNFα concentrations Percentage of lymphocytes was increased 13% in formaldehyde-exposed workers. Absolute numbers and percentages of T cells (17% and 6%, respectively) and NK cells (48% and 34%, respectively) were higher, IgG (23%) and IgM (27%) in exposed workers were statistically lower, TNFα was significantly higher (308%). No significant differences in white blood cell, erythrocytes, hemoglobin, neutrophils, or monocytes were observed. Aydin et al. 2013
Workers (35 formaldehyde-exposed, 35 controls) Pathology anatomy workers with >1 year exposure in four hospitals in Portugal Blood sample measured for lymphocyte subsets (T, B, and NK cells) and MN, SCE, and TCR mutations Overall, 30% decrease in percentage of B cells (CD19+) found in formaldehyde-exposed workers compared with controls (p < 0.05). Decreased B-cell percentage was significant in multivariate analysis Costa et al. 2013
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×
and nonexposed administrative workers in same facilities; 8-hour TWA mean exposure = 0.36 ± 0.03 ppm (range 0.23–0.69 ppm) (including sex, smoking, and age) (p = 0.014). T cells (CD3+) and helper T cells (CD3+/CD4+) increased when analyzed by formaldehyde exposure (p = 0.002 and 0.006, respectively) and in multivariate analysis (p = 0.024 and 0.037, respectively). NK cells (CD16+/CD56+) decreased on basis of individual exposure levels (p < 0.001) and in multivariate analysis (p < 0.001).
Female workers (37 formaldehyde-exposed, 37 controls) Workers, formaldehyde-exposed women in four pathology departments in Hungary; 8-hour TWA mean exposure = 0.9 mg/m3 measured in three of four sites; 16 subjects identified as having exposure to organic solvents in addition to formaldehyde were analyzed separately Blood samples measured for apoptosis, proliferation, HPRT function, UV-induced DNA synthesis, CA, SCE, and T-cell activation marker CD71 after PHA stimulation in vitro Apoptotic cells after PHA stimulation were mean of 77% higher in formaldehyde-only exposed workers compared with controls. Lectin labeling index and variant frequency, measures of HPRT function, were significantly increased and decreased, respectively, in formaldehyde-exposed workers. CD71 expression on T cells and BrdU incorporation were not significantly changed. Jakab et al. 2010
Inhalation exposure in animals Male Balb/c mice Inhaled formaldehyde at 0, 0.5, 3 mg/m3, 8 hours/day, 5 days/week (5 days on, 2 days off), 13 days Blood measured for complete blood count (cell types and hemoglobin), BM for histology, ROS, GSH, cytochrome 1A1, GSTT1, NFkB, TNFα, and IL-1b Formaldehyde exposure led to a significant decrease (p<0.05) in white blood cells, red blood cells, and lymphocytes after exposure to 0.5 mg/m3 of formaldehyde (43%, 7%, and 39%, respectively), and 3.0 mg/m3 of formaldehyde (52%, 27%, and 43% respectively). Platelet counts were significantly increased (p<0.05) after exposure to formaldehyde at 0.5 mg/m3 (109%) and 3.0 mg/m (67%). Monocytes and granulocytes were not significantly changed. At a formaldehyde exposure of 0.5 mg/m3 and 3.0 mg/m3, ROS levels in BM increased by 31% and 102%, respectively; CYP1A1 increased by 8% and 37%, respectively; and GSTT1 decreased by 0% and13%, Zhang et al. 2013
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×
Model Subjects Exposure Sample Main Hematologic Findings (Excluding Genotoxicity)b Reference
Inhalation exposure in animals respectively. At 3.0 mg/m3 of formaldehyde, NFkB increased by 34%, and inflammatory cytokines were increased—TNFα by 42% and IL-1b by 98%.
Female C57BL/6 Inhaled formaldehyde at 0, 5, 10 ppm, 6 hours/day, 5 days/week, 14 days of exposure BM, lymph node, spleen, liver, and lung measured for cell types and NK function Formaldehyde-exposed mice showed 30% increase in percentage of T cells (CD3+), 38% increase in CD8+ T cells, and 28% decrease in B cells (B220+) in spleen at 10 ppm, but absolute numbers were not significantly different. No change in percentage of CD4+ or CD8+ T cells in BM, lymph nodes, liver, or lung. Percentage of NK cells (NK1.1+) in lung was decreased in concentration-dependent manner (decrease of 19% at 5 ppm and 58% at 10 ppm) and returned nearly to normal in 2 weeks after last formaldehyde exposure. Absolute numbers of NK cells were reduced in lung, but total leukoctye numbers were not changed at 10 ppm. Total number of cells present in BAL was increased >20-fold in formaldehyde-exposed mice, but absolute number of NK cells was decreased by over 65%, as were Ly49 receptor expression levels on NK cells. Similarly, percentage and total NK cells and Ly49 expression were decreased in spleen in a time-dependent manner, but no change in total splenocytes was observed. IFNg, perforin, and CD122 were decreased in NK cells from lung and spleen of formaldehyde-exposed mice, and LPS-mediated increase in these proteins was inhibited after formaldehyde exposure in lung. NK cytolytic activity (chromium release assay) of splenic NK cells was decreased at 2–3 weeks of formaldehyde exposure. Decrease in NK-cell numbers (approximately 30%) and function were seen in tumor-bearing mice exposed to formaldehyde. Decreases in NK viability and differentiation in vitro were also observed. Kim et al. 2013
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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Outbred female white rats Inhaled formaldehyde at 12.8 ± 0.69 mg/m3, 4 hours/day, 5 days/week, 10 weeks Blood measured for blood cell types, hemoglobin, MN, and multiple serum proteins and amino acids Of blood-cell types and hemoglobin, formaldehyde-exposed rats had statistically significant differences (p < 0.05) in percentage of lymphocytes (11% increase) and percentage of segmented neutrophils (31% decrease). Katsnelson et al. 2013
Female Wistar rats Inhaled formaldehyde, nebulized at 0.32%, 90 minutes/day for 3 consecutive days Blood and bone marrow samples measured for cell subsets; BAL fluid leukocytes Sham-control rats were part of a larger study of female sex hormone effects on formaldehyde-induced airway inflammation. Formaldehyde exposure in these control rats showed a 111% increase in WBC, including mononuclear and neutrophil subsets in BAL fluid. Sham-control rats had 197% increase in WBC, but there was >70% decrease in BM cell numbers in formaldehyde-exposed rats. >19-fold increase in degranulated mast cells was seen in lungs of formaldehyde-exposed control rats. Lino-dos-Santos-Franco et al. 2011
In vitro studies Primary expanded human erythroid progenitor cells from PBMCs 0–150 mcM formaldehyde in tissue culture Cell growth and cell cycle distribution Formaldehyde exposure suppressed in vitro human erythroid progenitor cell expansion in dose-dependent manner. Ji et al. 2013
Primary expanded human NK cells from PBMCs 0–3,200 µM formaldehyde in tissue culture examined at 10, 30, 60, and 120 minutes Morphology, viability, apoptosis, cytotoxicity (killing tumor-cell activity), cytokine and cytolytic proteins, and secretion of NK cells were evaluated NK-cell viability, cytolytic activity, and perforin secretion were decreased above 800 micromolar. Li et al. 2013
Primary mouse BM MSCs 0–200 mcM formaldehyde in tissue culture Viability (MTT assay) BM MSCs demonstrated cytotoxicity >75 micromolar. She et al. 2013
Human lymphoblastoid cell lines 0–200 mcM formaldehyde for 24 hours in tissue culture Viability (AnnexinV binding and PI staining) FANCD2-deficient lymphoblastoid cell line was statistically more sensitive to formaldehyde-induced cell death than FANCD2-expressing control. Ren et al. 2013
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
×
Model Subjects Exposure Sample Main Hematologic Findings (Excluding Genotoxicity)b Reference
Primary human lymphocytes from 30 volunteers 0–1.152 mg/mL formaldehyde after PHA stimulation for 72 hours Viability (trypan blue and MTT assay) Statistically significant decreases in viability seen at formaldehyde concentrations above 0.036 mg/mL. Pongsavee 2011

aThe studies in this table were identified through the committee’s literature search. See Appendix D for more details of the search.
bAll reported findings are significant with p <0.05.
Abbreviations: AML, acute myeloid leukemia; B, bursa-derived cells; BAL, bronchoalveolar lavage; BM, bone marrow; BrdU, bromodeoxyuridine; CA, chromosomal aberrations; CD, cluster of differentiation; CFU-GM, colony-forming unit-granulocyte-macrophage; CYP1A1, cytochrome P450, family 1, subfamily A, polypeptide 1; DNA, deoxyribonucleic acid; FANCD2, fanconi anemia group D2 protein; GSTT1, glutathione s-transferase theta 1; HPRT, hypoxanthine-guanine phosphoribosyltransferase; GSH, glutathione; IFNg, interferon gamma; IgG, immunoglobulin G; IgM, immunoglobulin M; IL-1b, interleukin-1 beta; LPS, lipopolysaccharide; Ly49 - killer cell lectin-like receptor subfamily A; mg/m3, milligram per cubic meter; mg/mL, milligrams per milliliter; MN, micronucleus test; MSC, mesenchymal stem cell; MTT, methylthiazol tetrazolium; NFkB, nuclear factor kappa-light-chain-enhancer of activated B cells; NK, natural killer cells; PBMC, peripheral blood mononucleated cell; PHA, phytohemagglutinin; ppm, parts per million; ROS, reactive oxygen species; SCE, sister-chromatid exchange; T, thymus cells; TCR, T-cell receptors; TNFa, tumor necrosis factor alpha; TWA, time-weighted average; UV, ultraviolet; WBC, white blood cell count. Source: Committee generated.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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TABLE 3-12 Studies Grouped by Hematologic Effects

Model Cell Type Hematologic Effectsa Reference
Inhalation exposure in humans WBC ↓ Total WBC Qian et al. 1988; Kuo et al.1997; Tang and Zhang 2003; Cheng et al. 2004; Tong et al. 2007; Zhang et al. 2010
↑ Percentage of lymphocytes Aydin et al. 2013
↓ Total lymphocytes Zhang et al. 2010
↓ CFU formation
T cells ↓ Total T cells and CD8+ T cells Ying et al. 1999; Ye et al. 2005; Hosgood et al. 2013
↓ CD4+ T cells Ying et al. 1999
↑ CD4/CD8 ratio Ying et al. 1999; Ye et al. 2005
↑ CD26+ activated T cells Madison et al. 1991
↑ T cells Aydin et al. 2013; Costa et al. 2013
Impaired mitogen-induced proliferation of lymphocytes Vargova et al. 1992
↑ PHA-induced apoptosis Jakab et al. 2010
NK cells ↓ NK cells Costa et al. 2013; Hosgood et al. 2013
↑ NK cells Aydin et al. 2013
B cells ↑ B cells percentage Ying et al. 1999; Ye et al. 2005
↓ B cell percentage Costa et al. 2013
↑ autoantibodies and anti-FA-albumin conjugates Madison et al. 1991
↑ IgM/IgA Qian et al. 1988
↓ IgG/IgM Aydin et al. 2013
Erythrocytes ↓ erythrocyte count and hematocrit level Lyapina et al. 2004
↓ hemoglobin level Yang 2007
↑ MCV Zhang et al. 2010
Neutrophils ↓ spontaneous respiratory burst activity Lyapina et al. 2004
↑ susceptibility to infection
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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Model Cell Type Hematologic Effectsa Reference
Monocytes ↑ monocytes in indoor FA+nitrogen dioxide exposure Erdei et al. 2003
Eosinophils ↑ eosinophils Qian et al. 1988
Platelets ↓ platelets Tong et al. 2007; Yang 2007; Zhang et al. 2010
Inhalation exposure in animals WBC ↓ WBC Brondeau et al. 1990; Zhang et al. 2013
↑ WBC Lino-dos-Santos-Franco et al. 2011
↓ lymphocytes Zhang et al. 2013
↓ lymphocyte viability Pongsavee 2011
↑percentage lymphocytes Kim et al. 2013
↓ bone marrow cell numbers Lino-dos-Santos-Franco et al. 2011
↑ bone marrow cell numbers Battelle 1981
T cells ↑ percentage of T cells and CD8+ T cells Kim et al. 2013
NK cells ↓ total and percentage of NK cells Kim et al. 2013
↓ IFNg, perforin, and CD122 in NK cells.
↓ cytolytic activity and NK differentiation ex vivo
B cells ↓ B cells Kim et al. 2013
Neutrophils ↓ segmented neutrophils Katsnelson et al. 2013
Erythrocytes ↓ erythrocytes Zhang et al. 2013
Platelets ↑platelets Zhang et al. 2013
In vitro studies T cells ↓ IFNg and IL-10 in stimulated human T cells Sasaki et al. 2009
B cells ↓ viability of human lymphoblastoid cells Ren et al. 2013
NK cells ↓ NK cell viability, cytolytic activity, and perforin secretion Li et al. 2013
Erythrocytes ↓ expansion of human erythroid progenitor cells in vitro Ji et al. 2013
MSCs ↓ viability of bone marrow stromal cells She et al. 2013

aAll significant effects reported with p <0.05.
Source: Committee generated.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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Given the variability of blood measures in any person over time and the heterogeneity among people in a population, it is difficult to find statistically significant changes in blood measures in human studies. Thus, it is notable that despite the inherent limitations of studying hematologic measures, over 14 recently published studies reported statistically significant hematologic effects on multiple hematopoietic-cell types. Although there are valid concerns about some results in individual studies (for example, the authors of one study used the consequences of the thalassemia trait for mean corpuscular volume to explain the findings), it is unlikely that most of these studies have been confounded by such issues. In light of the numerous studies that have reported significant differences in multiple measures, there is a strong association between inhaled formaldehyde exposure in humans and hematologic effects.

Although confounding exposures may complicate the interpretation of some studies, most of the studies documented efforts to identify possible confounding factors. Several studies were conducted in occupations in which formaldehyde was probably the predominant exposure during the period of study. One study showed that hematologic changes occurred in individual subjects over a limited period of exposure (Ying et al. 1999). Thus, the hematologic effects observed in those studies establish a specific association with inhaled formaldehyde in humans. Establishing the temporal relationship of exposure and effect is difficult in most human-exposure studies. Several studies report an association between duration of employment and exposure to formaldehyde, and an 8-week anatomy-laboratory exposure study (Ying et al. 1999) supports a temporal relationship. There is evidence from one human study that supports a biologic gradient of formaldehyde exposure and hematologic effects. In this study, increases in T cells and decreases in NK cells were proportional to formaldehyde exposure level (Costa et al. 2013). Those findings are supported by findings in animal-exposure studies (see below).

Hematologic Effects in Animals Exposed to Formaldehyde

Experimental-animal studies are informative with regard to the specificity, temporal relationship, and exposure–response relationship between formaldehyde and hematologic effects. It can be argued that rodents and humans differ in the mechanics of inhalation, the physiology of hematopoietic-cell turnover, and DNA-repair mechanisms. Therefore, results of animal studies were evaluated as supporting data, whereas the human data presented above are considered the primary source of evidence of potential associations of formaldehyde exposure and hematologic effects.

Six studies addressed the hematologic effects of exposure to formaldehyde in animals in vivo, of which four were published after the publication of the 12th RoC (Tables 3-11 and 3-12). There is poor agreement between individual studies as to the direction of hematologic effects induced by inhaled formaldehyde in animals. In particular, increased or decreased effects on total white-cell counts,

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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total lymphocyte counts, and percentage limit the ability to interpret the results. In addition, other hematologic effects have been reported in only one study, so the consistency of the findings cannot be assessed. The committee finds limited evidence of consistent hematologic effects in the few available studies of formaldehyde-exposed animal models alone.

In the experimental-animal studies, the associations that were observed were often strong in magnitude or level of statistical significance, although the clinical and biologic significance is unknown (Katsnelson et al. 2013; Kim et al. 2013). Thus, the strength of those specific associations is quite high, even if the consistency of the findings is limited. As is expected in experimental-animal studies, the observed multiple hematologic effects can be closely linked to the tested agent, and this establishes a specific association with formaldehyde. By their nature, the animal-exposure studies establish the temporal relationship between inhaled formaldehyde exposure and multiple hematologic effects. In particular, specific hematologic effects were shown to depend on the duration of exposure (Kim et al. 2013). Two animal studies reported multiple hematologic measures, and effects on them were proportional to formaldehyde concentrations (Kim et al. 2013; Zhang et al. 2013). The results suggest an exposure–response relationship between formaldehyde exposure and hematologic effects.

Hematologic Effects on Isolated Animal or Human Cells

In vitro studies of hematologic effects are of limited utility because they evaluate a nonintact hematopoietic system, which ignores the complex interplay between various cell types and the vascular and lymphohematopoietic organs. Such studies do not account for the complex dynamics between the portal of entry and the systemic distribution of formaldehyde.

The committee examined six studies that reported cytotoxic effects on or functional consequences for hematopoietic cells or bone marrow stromal cells, of which five were published after publication of the 12th RoC (Table 3-10 and 3-11). All six studies reported deleterious effects of formaldehyde exposure on T cells, B cells, NK cells, or bone marrow stromal cells; this suggests that formaldehyde may have hematologic effects if it comes into direct contact with these cell types. However, given the unclear relevance of direct exposure in in vitro studies, particularly exposure to formaldehyde, the committee concludes that although the available literature demonstrates a deleterious effect of formaldehyde exposure on hematologic cells in vitro, it is difficult to draw firm conclusions regarding the hematologic effects of formaldehyde on isolated animal and human cells. The direct effects reported on several hematopoietic cell types raise important questions, but additional studies are needed that account for the physiologic exposure of hematopoietic cells to formaldehyde and its metabolites and for poorly understood systemic consequences.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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Conclusions and Considerations for Hematologic Effects

The committee concludes that the association of inhalation formaldehyde exposure and diverse hematologic effects is supported by evidence from human studies. Studies in experimental animals provide some additional support. The consistency of individual hematologic effects varied among multiple human and animal studies, and many reported decreases in hematologic measures. The strength of the association in multiple reports of hematologic effects in multiple populations is convincing. The specificity of findings in exposed humans is challenging, but select human studies and experimental-animal studies support the specificity of the association. The temporal relationship is adequately addressed in most studies, and the biologic gradient is addressed in some studies, particularly in animal studies. Taken as a whole, the body of evidence from studies of exposed humans and animals indicates broad and strong associations between exposure to inhaled formaldehyde and hematologic effects.

Toxicogenomics

Toxicogenomics is the study of gene-expression changes elicited by a toxicant. The committee reviewed recent toxicogenomic publications to gain a better understanding of changes in gene expression after formaldehyde exposure. The committee looked specifically at toxicogenomic studies and identified eight publications that had microarray data. Those publications provided information on the genomewide expression of mRNA transcripts in humans, experimental animals, or cultured cells after exposure to formaldehyde. Five of the publications were identified through the committee’s independent literature search for genotoxicity and mutagenicity studies (Andersen et al. 2010; Zeller et al. 2011a; Cheah et al. 2013; Neuss et al. 2010b; Kuehner et al. 2013) (see Figure D-4), and two additional publications were identified from the reference lists of those relevant publications (Hester et al. 2003; Andersen et al. 2008). One publication was identified during the committee’s secondary ad hoc effort to identify relevant literature (Rager et al. 2013). Five of the eight publications described exposures in humans or experimental animals (Hester et al. 2003; Andersen et al. 2008, 2010; Zeller et al. 2011a; Rager et al. 2013), and the remaining three used cell culture (Hester et al. 2003; Neuss et al. 2010b; Cheah et al. 2013). The eight studies are described in more detail in this section and in Table 3-13.

Zeller et al. (2011a) used volunteer human subjects to examine transcriptomal changes in nasal inferior turbinate biopsies and peripheral blood samples after inhalation of formaldehyde vapor at up to 0.8 ppm 4 hours/day for 5 days. This is the only study that the committee identified that attempted to examine both portal-of-entry and systemic transcriptomal effects of formaldehyde. The authors reported that 27 mRNA transcripts were differentially expressed between exposed and nonexposed conditions in the nasal specimens. In

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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TABLE 3-13 Transcriptomal Profiling Studies

Model Subjects Exposure Sample Criteriaa Main conclusions Reference
Animals or humans Human volunteers: male nonsmokers or ex-smokers Formaldehyde vapor Before and after exposure (paired) nasal biopsy (inferior turbinate); venous whole blood 2-fold or 1.5-fold; p < 0.05 (paired t); no FDR correction Formaldehyde exposure affected mRNA expression in nasal biopsy or blood samples only marginally. There were 2–17 and 25–67 differentially expressed genes identified in biopsies with 2.0- and 1.5-fold difference criteria, respectively. Results identified 0–9 and 6–39 differentially expressed genes in the blood with 2.0- and 1.5-fold difference criteria, respectively. Differentially expressed genes identified in the three exposure groups showed little overlap. No significant specific pathways involving differentially expressed genes were apparent. When FDR cutoff (less than 10%) was applied in addition to 1.5-fold change cutoff, no differentially expressed genes were detected. Zeller et al. 2011a
Up to 0.8 ppm 4 hours/day for 5 days
3 groups (5–8/group)
Nonhuman primates: male Cynomolgus macaques Formaldehyde vapor 0 (n = 2), 2 (n = 3), and 6 ppm (n = 3) 6 hours/day for 2 days Nasal epithelial tissue from maxilloturbinate region collected by necropsy 1.5-fold; p < 0.05 (ANOVA); FDR corrected q < 0.1 Low (2 ppm) and high (6 ppm) doses of formaldehyde changed 3 and 13 micro-RNA expressions, respectively. Suppression of transcriptional targets of most significantly increased miRNA (miR-125b) was confirmed by real-time PCR. Induction of transcriptional targets of most robustly decreased miRNA (miR-142-3p) was also confirmed by real-time PCR. Four miR-125b targets encoding proapoptotic regulators BAK1, CASP2, MAP2K7, and MCL1 b were downregulated. Thus, formaldehyde Rager et al. 2013
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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exposure disrupts miRNA expression in nasal epithelium and probably affects apoptosis.
Rats: male F344/CrlBR Formaldehyde vapor Nasal surface epithelial cells (lateral meatus and nasoturbinate encompassing area between levels II and III) selectively isolated by incubating necropsy tissues in protease mixture 2-fold; BenjaminiHochberg; FDR < 0.05 Exposure to formaldehyde at 2 ppm caused induction of genes involved in cellular stress responses—thiol transport/reduction, inflammation, and cell proliferation—at all exposure durations. Exposure to formaldehyde at 6 ppm or greater resulted in changes in expression of genes involved in cell-cycle regulation, DNA repair, and apoptosis. Andersen et al. 2010
0, 0.7, 2, 6, 10, and 15 ppm 6 hours/day for 1, 4, 13 weeks
(15 per dose per time)
Rats: male F344/CrlBR Formaldehyde vapor or instillation Nasal surface epithelial cells (lateral meatus and nasoturbinate encompassing area between levels II and III) selectively isolated by incubating necropsy tissues in protease mixture 1.5-fold; BenjaminiHochberg; FDR < 0.05 No differentially expressed genes were detected after exposure to formaldehyde vapor at 0.7 ppm. Exposure at 2 and 6 ppm resulted in up to 15 and 54 differentially expressed genes, respectively, at different timings over the course of the 3-week exposure. Exposure at 15 ppm caused 745 differentially expressed genes within 24-hour period, and exposure by instillation (400 mM x 40 µL per nostril) caused 2,553 differentially expressed genes within 24-hour period. About 75% of differentially expressed genes caused by exposure at 15 ppm were also affected by exposure via instillation, and these genes were enriched in gene ontology categories of wound response, apoptotic regulation, inflammation, and receptor tyrosine kinase signaling. Andersen et al. 2008
Vapor: 0, 0.7, 2, and 6 ppm 6 hours/day for 5 days/week for up to 3 weeks (5 per dose per time)
Vapor: 15 ppm for 6 hours (10 exposed, 5 controls)
Instillation: 400 mM x 40 μL per nostril, 6 hours (10 exposed, 5 controls)
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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Model Subjects Exposure Sample Criteriaa Main conclusions Reference
Rats: male F344 Formaldehyde instillation Nasal epithelial cell lysis by direct instillation of Trizol reagent BenjaminiHochberg; FDR < 0.05 or 0.1 Exposure to formaldehyde caused differential gene expression. These genes were enriched in pathways relevant to xenobiotic metabolism, cell cycle, apoptosis, and DNA repair. Hester et al. 2003
400 mM formaldehyde (n = 3) or water (n = 4) x 40 μL per nostril, 24 hours
Cell culture Primary culture human nasal epithelial cells (commercial product, derived from three Caucasian women) 20 or 100 μM for 2 hours; 50, 100, 200 μM for 4 hours; 100 or 200 μM for 24 hours; 20 or 50 μM for 24 hours with 4 consecutive repeats; no exposure control Total cell lysate 2-fold; p < 0.05 (t test); no FDR correction Exposure to 100 and 200 μM formaldehyde for 4 hours changed expression of 153 and 887 genes, respectively. Exposure to 50 μM formaldehyde for 24 hours with 4 repeats changed expression of 143 genes. Less than 10 differentially expressed genes were observed with all other conditions. Genes upregulated by exposure to 200 μM formaldehyde for 4 hours were enriched for apoptosis regulation and stress response. Neuss et al. 2010b
Human A549 lung-cancer cell line (adenocarcinoma, alveolar basal epithelial) 0 or 83.2 μM for 2 hours Total cell lysate 1.5-fold; BenjaminiHochberg;FDR < 0.05 Exposure to 83.2 μM formaldehyde for 2 hours caused 66 differential gene expressions, which were enriched for apoptosis regulation, transcription, and DNA damage (upregulated genes) or transcription (downregulated genes). Cheah et al. 2013
Human TK6 B lymphoblastoid cells 0, 50, 100, or 200 uM for 4 or 24 hours Total cell lysate 1.5-fold and 2-fold; p < 0.05 (t test); FDR < 0.1 (multi-variable permutation test) Exposure to 50 μM formaldehyde did not cause significant transcriptomal changes. Exposure to 200 μM formaldehyde caused 2,147 and 2,502 differentially expressed genes after 4 or 24 hours of exposure, respectively. Exposure to 100 μM formaldehyde for 4 hours caused 1,367 differentially expressed genes, whereas Kuehner et al. 2013
Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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exposure to the same concentration of formaldehyde for 24 hours caused only 2 differentially expressed genes. Genes upregulated after exposure to 200 μM formaldehyde for 24 hours were enriched for transcription, transport, protein phosphorylation, signal transduction, and apoptosis.

Abbreviation: FDR, false discovery rate.
aCriteria for defining differentially expressed genes.
bMCL1 isoform 1 is antiapoptotic, whereas isoform 2 is proapoptotic.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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the blood specimens, statistically significant differential expression of 11 mRNA transcripts was observed. However, the authors concluded that these were “minor” effects that reflected assay variability and that inhalation of formaldehyde did not cause alterations in the expression of genes in either the nasal or blood samples. In the absence of appropriate negative exposure control groups, appropriate positive controls, or detailed power-analysis discussion, the committee was unable to determine whether the results of this study supported the absence of transcriptomal effects after exposure to formaldehyde or whether the study design provided sufficient discovery power in light of the small number of study subjects (six to eight per group).

Rager et al. (2013) examined maxilloturbinate necropsy specimens of nasal epithelial tissues from macaques and observed significant changes in expression of micro-RNAs after exposure to formaldehyde at 6 ppm 6 hours/day for 2 days. Using real-time quantitative polymerase chain reaction methods, the authors confirmed significant induction of miR-125b expression and concomitant suppression of its target mRNA transcripts, including proapoptotic genes BAK1, CASP2, MAP2K7, and MCL1.

Two other studies examined transcriptomal effects in nasal epithelial cells of F344 rats that were exposed to formaldehyde via vapor or instillation into the nostrils (Hester et al. 2003; Andersen et al. 2010). These studies collectively demonstrated that exposure to formaldehyde, either by inhalation (2 ppm or higher for 6 hours or longer) or by intranasal instillation (40 μL of a 400 mM solution for 6 hours or longer), resulted in significant changes in expression of the mRNA transcripts that encode proteins involved in cell-cycle regulation, DNA repair, wound response, inflammation, and regulation of apoptosis. In comparison, data obtained after exposure to lower doses of formaldehyde were mostly insignificant.

Three cell-culture experiments—one that used primary cultures of human nasal epithelial cells (Neuss et al. 2010a), one that used human A549 lung alveolar basal epithelial cancer cells (Cheah et al. 2013), and one that used human TK6 lymphoblastoid cells (Kuehner et al. 2013)—demonstrated significant formaldehyde-related changes in expression of mRNA transcripts that encode proteins involved in apoptosis regulation, stress response, transcription, DNA damage, transport, and signal transduction. Relatively high concentrations of formaldehyde—greater than 83.2 μM for 2 hours (Cheah et al. 2013) or greater than 100 μM for 4 hours (Neuss et al. 2010a; Kuehner et al. 2013)—resulted in transcriptomal changes, whereas exposure to lower concentrations of formaldehyde did not have detectable effects even after prolonged exposure.

The committee found multiple studies that reported transcriptional responses in nasal cavity epithelial cells from experimental animals exposed to formaldehyde vapor at doses of 2 ppm or greater. The transcriptomal responses were indicative of cell apoptosis, DNA damage, and proliferation, which are relevant to carcinogenesis. The committee notes that the doses are relevant to occupational human exposure to formaldehyde. The committee did not identify studies that considered the transcriptomal effects of chronic, low-dose exposure

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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to formaldehyde in the nasal epithelial cells, peripheral blood, or any other tissues of human or animal models.

SUMMARY OF EVIDENCE

The statement of task specifically asked the committee to “integrate the level-of-evidence conclusions, and considering all relevant information in accordance with the RoC listing criteria, make an independent listing recommendation for formaldehyde and provide scientific justification for its recommendation” (Appendix B). The committee notes that the term integrate does not have a standard definition in the context of hazard assessment. The committee understood the term in its conventional sense of bringing together parts into a whole. To be listed as “reasonably anticipated as a human carcinogen” or “known to be a human carcinogen”, the RoC listing criteria only requires information to be integrated across human studies or across animal studies, and supporting information can be derived from mechanistic studies. Mechanistic information “can be useful for evaluating whether a relevant cancer mechanism is operating in people” (NTP 2010, p. iv), but a known mechanism is not required for a substance to be listed in the RoC. In the subsections below, the committee summarizes human, experimental animal, and mechanistic information on nasopharyngeal and sinonasal cancer and myeloid leukemia. Summaries were not presented for other kinds of cancer because of a lack of strong evidence that formaldehyde exposure causes other kinds of cancer in humans.

Nasopharyngeal and Sinonasal Cancers

The committee found clear and convincing epidemiologic evidence of an association between formaldehyde exposure and nasopharyngeal cancer and sinonasal cancer in humans. On the basis of evidence of an association between nasopharyngeal cancer and exposure to formaldehyde in two strong studies—a large case–control study (Vaughan et al. 2000) and a large cohort study (Beane Freeman et al. 2013)—and other supporting studies that were judged to be moderately strong (Vaughan et al. 1986a,b; West et al. 1993; Hildesheim et al. 2001; Siew et al. 2012), the committee concludes that the relationship is causal and chance, bias, and confounding factors can be ruled out with reasonable confidence. For sinonasal cancer, there is evidence of an association based on a strong, well-conducted pooled case–control study (Luce et al. 2002) and other, corroborating studies that were judged to be moderately strong (Hayes et al. 1986; Olsen and Asnaes 1986; Vaughan et al. 1986a,b; Luce et al. 1993; Siew et al. 2012). The committee concludes that the relationship between formaldehyde and sinonasal cancer is causal and chance, bias, and confounding factors can be ruled out with reasonable confidence.

Several well-conducted studies in experimental animal models demonstrate an increase in nasal squamous-cell carcinoma after inhalation exposure to formaldehyde (Kerns et al.1983; Sellakumar et al. 1985; Monticello et al. 1996).

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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Two of the studies used F344 rats (Kerns et al. 1983; Monticello et al. 1996), and one used Sprague Dawley rats (Sellakumar et al. 1985). The evidence is corroborated by other rat studies (Feron et al. 1988; Soffritti et al. 1989; Woutersen et al. 1989; Kamata et al. 1997) and by a mouse study (Kerns et al. 1983). Although there are limitations in extrapolating findings on nasal tumors in rodents to nasopharyngeal and sinonasal cancer in humans, the experimental-animal evidence indicates that exposure to inhaled formaldehyde is associated with carcinogenic effects on tissues at the portal of entry.

Inhalation of formaldehyde at sufficient concentrations substantially increases formaldehyde to above the total endogenous concentration in tissues at the portal of entry in both animal and human studies. There is experimental evidence that, due to its chemical reactivity, formaldehyde exerts genotoxic and mutagenic effects and cytotoxicity followed by compensatory cell proliferation at the portal of entry3 in animals and humans exposed to formaldehyde; this provides biologic plausibility of a relationship between formaldehyde exposure and cancer. The evidence on formaldehyde-associated DNA adducts, DNA–protein cross-links, DNA strand breaks, mutations, micronuclei, and chromosomal aberrations is consistent, strong, and specific. In addition, both temporal and exposure–response relationships have been established, most strongly in studies of rodents and nonhuman primates.

Myeloid Leukemia

The committee found clear and convincing epidemiologic evidence of an association between formaldehyde exposure and myeloid leukemia. There may also be an increase of other lymphohematopoietic cancers, although the evidence is less robust. On the basis of three strong studies with widely different coexposures (the NCI formaldehyde-industry cohort [Beane Freeman et al. 2009], the NIOSH garment-worker cohort [Meyers et al. 2013], and the NCI funeral-industry cohort [Hauptmann et al. 2009]) and several moderately strong studies (Walrath and Fraumeni 1983, 1984; Stroup et al. 1986; Coggon et al. 2014), the committee concludes that there is a causal association between formaldehyde exposure and myeloid leukemia. Chance, bias, and confounding factors can be ruled out with reasonable confidence given the consistent pattern of association in the larger studies that had good exposure assessment.

Although multiple lines of reasoning and experimental evidence indicate that it is unlikely that inhalation exposure to formaldehyde will increase formaldehyde to substantially above endogenous concentrations in tissues distant from the site of entry, there is a robust database of experimental studies of systemic4

__________________

3Defined as effects that arise from direct interaction of inhaled or ingested formaldehyde with cells or tissues.

4Defined as effects that occur beyond cells or tissues that have direct interaction with inhaled or ingested formaldehyde.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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mechanistic events that have been observed after exposure to formaldehyde. The committee notes that it is plausible that some of the systemic effects, notably findings of genotoxicity and transcriptional changes in circulating blood cells, may have resulted from the exposure of the cells at the portal of entry (for example, lymphoid tissue in the nasal mucosa). The mechanistic events that were considered by the committee as relevant to the plausibility of formaldehyde-associated tumors beyond the portal of entry included genotoxicity and mutagenicity, hematologic effects, and effects on gene expression. Overall, in mechanistic studies of experimental animals and exposed humans, the evidence is largely consistent and strong. As shown in Table 3-10, a majority of the mammalian in vivo studies resulted in positive findings compared to negative findings (60 and 38 studies, respectively), particularly in humans (49 and 19 studies, respectively). Both temporal and exposure–response relationships have been demonstrated in studies of humans and animals exposed to formaldehyde. The committee concludes that these findings provide plausible mechanistic pathways supporting a relationship between formaldehyde exposure and cancer, even though the potential mechanisms of how formaldehyde may cause such systemic effects are not fully understood. It would be desirable to have a more complete understanding about how formaldehyde exposure may cause systemic effects, but the lack of known mechanisms should not detract from the findings of an association between formaldehyde exposure and myeloid leukemia in epidemiology studies.

The animal cancer bioassay literature provided some information relevant to myeloid leukemia. One drinking water study (Soffritti et al. 2002) reported a significant increase in lymphohematopoietic cancers following long-term exposure to formaldehyde in drinking water, but there is uncertainty regarding the finding. Of the three inhalation studies that included histopathologic examinations of non–respiratory tract tissues, two did not report leukemia (Sellakumar et al. 1985; Kamata et al. 1997). The full laboratory report (Battelle 1981) of a third study (Kerns et al. 1983) discussed findings of leukemia and lymphoma that were not found to be compound related. However, diffuse multifocal bone marrow hyperplasia in rats exposed to 15 ppm of formaldehyde for 18 months was increased in both treated males (p = 0.0001) and females (p = 0.0001). Although the Battelle finding was not a finding of malignancy, it does indicate that long-term inhaled formaldehyde may cause effects in bone marrow.

CONCLUSIONS AND LISTING RECOMMENDATION

The committee identified and evaluated relevant, publicly available, peer-reviewed literature on formaldehyde, including attention to literature published between June 10, 2011 (the release date of the substance profile for formaldehyde in the 12th RoC), and November 8, 2013. The committee applied NTP’s established RoC listing criteria to the scientific evidence on formaldehyde from

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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studies of humans, studies of experimental animals, and other studies relevant to mechanisms of carcinogenesis.

The type of information needed to meet the criteria for sufficient evidence in experimental animals is clear and transparent, as outlined in the section “Cancer Studies in Experimental Animals”. In contrast, the RoC listing criteria do not provide detailed guidance about how evidence should be assembled to meet the requirement of limited evidence or sufficient evidence of carcinogenicity from studies in humans, except to note that limited evidence cannot exclude alternative explanations, such as chance, bias, or confounding factors, and to note that conclusions should be based on “scientific judgment, with consideration given to all relevant information” (NTP 2010, p. iv). In the section “Cancer Studies in Humans”, the committee used scientific judgment to develop an approach to assessing the epidemiology evidence. The approach included careful review of individual studies, selection of studies that were most informative, and evaluation of informative studies on the basis of the strength, consistency, temporality, dose-response, and coherence of the evidence and on the considerations presented in Table 3-1.

The committee notes that evidence in experimental animals and a known mechanism of action is not required by the RoC listing criteria in making a listing recommendation that a substance is known to be a human carcinogen if the evidence from studies in humans is sufficient and indicates an association between exposure and human cancer. Also, and importantly, the RoC listing criteria require an association in only one type of cancer to make the determination. On the basis of the information summarized directly above for nasopharyngeal cancer, sinonasal cancer, and for myeloid leukemia, the committee makes its independent determinations as follows:

• There is sufficient evidence of carcinogenicity from studies of humans based on consistent epidemiologic findings on nasopharyngeal cancer, sinonasal cancer, and myeloid leukemia for which chance, bias, and confounding factors could be ruled out with reasonable confidence.

• There is sufficient evidence of carcinogenicity in animals based on malignant and benign tumors in multiple species, at multiple sites, by multiple routes of exposure, and to an unusual degree with regard to type of tumor.

• There is convincing relevant information that formaldehyde induces mechanistic events associated with the development of cancer in humans, specifically genotoxicity and mutagenicity, hematologic effects, and effects on gene expression.

Because there is sufficient evidence of carcinogenicity from studies in humans that indicates a causal relationship between exposure to formaldehyde and at least one type of human cancer, the committee concludes that formaldehyde should be listed in the RoC as “known to be a human carcinogen”.

Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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Suggested Citation:"3 Independent Assessment of Formaldehyde." National Research Council. 2014. Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens. Washington, DC: The National Academies Press. doi: 10.17226/18948.
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Next: Appendix A: Biographic Information on the Committee to Review the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens »
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Many people in the United States are exposed to formaldehyde. Exposure can occur from environmental sources (for example, combustion processes, building materials, and tobacco smoke) or in occupational settings (for example, the furniture, textile, and construction industries). Formaldehyde exposure also has endogenous sources--it is produced intracellularly as a component of the one carbon pool intermediary metabolism pathway. Scientists have studied formaldehyde for decades to determine whether exogenous formaldehyde exposure may be associated with cancer in humans. In 1981, The National Toxicology Program (NTP) first listed formaldehyde in the 2nd Report on Carcinogens as "reasonably anticipated to be a human carcinogen". In 2011, NTP upgraded the listing of formaldehyde to "known to be a human carcinogen". Following the new listing, Congress directed the Department of Health and Human Services to arrange for the National Academy of Sciences to independently review formaldehyde's substance profile and listing. This report presents the findings and conclusions of the committee formed in response to the congressional request.

Review of the Formaldehyde Assessment in the National Toxicology Program 12th Report on Carcinogens concurs with NTP that there is sufficient evidence in studies that had adequate characterization of relevant exposure metrics to enable a strong conclusion about the association between formaldehyde exposure and cancer in humans. Additionally, the authoring committee independently reviewed the scientific evidence from studies in humans, experimental animals, and other studies relevant to the mechanisms of carcinogenesis and made level-of-evidence conclusions. This report finds clear and convincing epidemiologic evidence of an association between formaldehyde exposure and nasopharyngeal and sinonasal cancers in humans.

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