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A Review of the NIOSH Roadmap for Research on Asbestos Fibers and Other Elongate Mineral Particles 1 Introduction Although asbestos is no longer mined in the United States, prior and ongoing exposures to asbestos continue to contribute to respiratory diseases, including mesothelioma, lung cancer, and asbestosis. Asbestos exposures are estimated to have contributed to 18,068 deaths from mesothelioma in the United States from 1999–2005; asbestos-related diseases continue to be diagnosed due to the long latency period for their manifestation (MMWR, 2009). U.S. workers and residents, for example, may continue to undergo hazardous exposures due to unremediated asbestoscontaining materials, imported asbestos-containing products, and natural environmental occurrences. Internationally, asbestos continues to be mined and used in manufacturing in a number of countries because of its desirable commercial properties such as strength and heat resistance. Ongoing issues include potential health effects in workplaces and in situ environmental settings as well as exposures to mineralogical mixtures that may contain asbestos and exposures to nonasbestiform elongate mineral particles of similar size and shape to asbestos particles. To examine ongoing issues and concerns in this field, the National Institute for Occupational Safety and Health (NIOSH) drafted a research roadmap that provides an overview of the state of the science and a plan for future research in areas including toxicology, mineralogy, epidemiology, and exposure assessment. In 2008, NIOSH asked the Institute of Medicine (IOM) and the National Research Council (NRC) to form a committee to review the Roadmap and provide recommendations on necessary changes to the document to improve its clarity, comprehensiveness, and accuracy. This report is the result of an 11-month study conducted by an ad hoc IOM-NRC committee composed of experts in the fields of toxicology, miner-
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A Review of the NIOSH Roadmap for Research on Asbestos Fibers and Other Elongate Mineral Particles alogy, exposure assessment, public health, occupational safety and health, clinical medicine, industrial hygiene, biostatistics, and pulmonary medicine. The committee’s task was to provide a review of the scientific and technical quality of the January 2009 NIOSH revised draft document Asbestos Fibers and Other Elongated Mineral Particles: State of the Science and Roadmap for Research, with a focus on proposed research intended to clarify the relationship between health effects and the physical and chemical characteristics (e.g., mineralogy, morphology, dimensions, surface properties) of a wide range of elongate mineral particles.1 In particular, the committee was asked to address the following questions: Is the document consistent with the state of scientific understanding of the toxicity, occupational exposures, epidemiology, and sampling or analytical methods? Should any of the content of this section be modified, based on the state of scientific understanding of these issues? Are there any significant studies that have been overlooked? Does the document clearly and adequately explain the scientific rationale for research on the mineralogy, morphology, dimensions, and surface characteristics of elongate mineral particles, and is its treatment of this issue consistent with the state of scientific understanding of the toxicity, occupational exposures, and epidemiology of elongate mineral particles? Does the document discuss the most significant issues regarding mineralogy, morphology, dimensions, and surface characteristics of elongate mineral particles? Should any of the discussed issues be omitted or revised, based on the state of scientific understanding of these issues? Are there any significant issues that should be added? Is the research proposed likely to effectively address the most significant issues regarding mineralogy, morphology, dimensions, and surface characteristics of elongate mineral particles? Should any of the discussed research be omitted or revised, based on the state of scientific understanding of these issues? Is there any significant research that should be added? 1 The committee urges the use of the adjective elongate rather than elongated, so as to describe the physical appearance of the particles as opposed to implying that they have been actively lengthened by some means. The committee uses the term elongate hereafter in this report.
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A Review of the NIOSH Roadmap for Research on Asbestos Fibers and Other Elongate Mineral Particles Was the process that was used to develop and revise the document and that is described in the Foreword, including the mechanisms for input from the scientific and stakeholder communities, appropriate from a scientific perspective? The task stipulated that, in addressing these questions, the committee was not to undertake its own assessment of the potential occupational health risks from exposure to asbestos and other elongate mineral particles or to conduct a formal literature review on these topics. To accomplish its charge, the committee held three meetings and gathered information through a scientific workshop that included a public comment session (Appendix A) and through discussions with individuals in relevant fields. The committee provides its assessment of the January 2009 draft NIOSH Roadmap document in this report. The remainder of this chapter introduces some of the complexities involved in discussions about research in this field. Chapter 2 sets the context for this Roadmap within efforts to develop and assess roadmaps for other areas of research. In Chapter 3, the committee reviews the major scientific issues and research directions that shape the Roadmap. The report concludes with Chapter 4, which provides the committee’s recommendations for strengthening the Roadmap and increasing its utility for work by NIOSH, other federal agencies, the private sector, and other stakeholders. COMPLEXITIES OF THE ISSUES The following brief overview identifies several issues that highlight the challenges faced in conducting research in this field and provides some introductory material for readers unfamiliar with these topics. The committee provides comments on the scope of the Roadmap and the terminology in Chapter 3. Background to the Use of Terminology in the Roadmap Because the term asbestos does not denote a single mineral but rather is used to encompass a set of minerals with specific industrial characteristics and commercial value, there have been challenges and controversies in determining both what specific set of minerals and what set of
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A Review of the NIOSH Roadmap for Research on Asbestos Fibers and Other Elongate Mineral Particles characteristics should be included in a definition of asbestos. The current regulatory definitions used by the Occupational Safety and Health Administration (OSHA) and the Mine Safety and Health Administration (MSHA) define six recognized minerals as varieties of asbestos:2 chrysotile (a member of the serpentine group of sheet silicates) and five fibrous forms of the amphibole group of double-chain silicates: riebeckite asbestos (also termed crocidolite), cummingtonite-grunerite asbestos (also commercially termed amosite), anthophyllite asbestos, tremolite asbestos, and actinolite asbestos (29 CFR 1910.1001(b); 29 CFR 1926.1101(b); 30 CFR 56.5001(b)(1); 30 CFR 57.5001(b)(1); 30 CFR 71.702(a)). The 1990 NIOSH Recommended Exposure Limit (REL) outlines limits for airborne asbestos fibers, a term that encompasses the six minerals defined as asbestos by OSHA and MSHA and particles greater than 5 μm in length and having (1) an aspect ratio of 3:1 or greater and (2) the mineralogical characteristics (i.e., crystal structure, elemental composition) of their nonasbestiform analogs (NIOSH, 1990a,b). Table 1-1 replicates and compares the terminology used by the regulatory agencies, OSHA and MSHA, for asbestos and that suggested by NIOSH in its 1990 recommended exposure limit for airborne asbestos fibers. The central focus of the Roadmap is on establishing research to assess the potential for asbestos, asbestos analogs, or other mineral particles with specific physical or chemical characteristics to impact human health. The generic term elongated mineral particles is used by NIOSH in the Roadmap to encompass a broad spectrum of mineral particles of a specific size and aspect ratio but the term has no current regulatory or mineralogical connotation. As noted above, the committee prefers the term elongate mineral particles (see Chapter 3 for discussion). Increasing the specificity of the terminology to the extent possible will aid in clarifying the central message of the Roadmap. Within the range of vari- 2 29 CFR 1910.1001(b) for General Industry: “Asbestos includes chrysotile, amosite, crocidolite, tremolite asbestos, anthophyllite asbestos, actinolite asbestos, and any of these minerals that have been chemically treated and/or altered.” 29 CFR 1926.1101(b) for Construction Industry: “Asbestos includes chrysotile, amosite, crocidolite, tremolite asbestos, anthophyllite asbestos, actinolite asbestos, and any of these minerals that has been chemically treated and/or altered. For purposes of this standard, ‘asbestos’ includes PACM, as defined below.” Presumed Asbestos Containing Material (PACM) is defined as “thermal system insulation and surfacing material found in buildings constructed no later than 1980.” 30 CFR 56.5001(b)(1); 30 CFR 57.5001(b)(1); 30 CFR 71.702(a): “Asbestos means chrysotile, cummingtonite-grunerite asbestos (amosite), crocidolite, anthophylite asbestos, tremolite asbestos, and actinolite asbestos.”
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A Review of the NIOSH Roadmap for Research on Asbestos Fibers and Other Elongate Mineral Particles ous types of elongate mineral particles, the potential health impact of exposures may vary widely, although this is yet to be fully explored. TABLE 1-1 Minerals Addressed by OSHA and MSHA Regulations and the NIOSH Recommended Exposure Limit OSHA and MSHA Permissible Exposure Limit for Asbestos NIOSH 1990 Recommended Exposure Limit for Airborne Asbestos Fibersa Serpentine Minerals Serpentine Minerals Chrysotile Chrysotile Amphibole minerals Amphibole minerals Crocidolite Amositeb Anthophyllite asbestos Tremolite asbestos Actinolite asbestos Crocidolite Amosite (cummingtonite-grunerite) Anthophyllite Tremolite Actinolite And any of these minerals that have been chemically treated and/or altered.c In addition, airborne cleavage fragments from the nonasbestiform habits of the serpentine minerals antigorite and lizardite, and the amphibole minerals contained in the series cummingtonite-grunerite, tremolite-ferroactinolite, and glaucophane-riebeckite are counted as fibers provided they meet the criteria for a fiber when viewed microscopically (NIOSH, 1990b). aNIOSH defines airborne asbestos fibers as those particles greater than 5 μm in length and having an aspect ratio of 3:1 or greater and the mineralogical characteristics (i.e., crystal structure, elemental composition) of the asbestos minerals and their nonasbestiform analogs. bMSHA regulations state “cummingtonite-grunerite asbestos (amosite).” cSpecific to the OSHA regulations (29 CFR 1910.1001(b); 29 CFR 1926.1101(b)). SOURCE: 29 CFR 1910.1001(b); 29 CFR 1926.1101(b); 30 CFR 56.5001(b)(1); 30 CFR 57.5001(b)(1); 30 CFR 71.702(a); NIOSH, 1990a,b.
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A Review of the NIOSH Roadmap for Research on Asbestos Fibers and Other Elongate Mineral Particles Mineral Variability Approximately 3,000 minerals exist in nature. Mineral identification is based on mineral crystal structure and crystal chemistry. Mineral crystal growth habits3 and the chemical substitutions in the crystal structure, influenced by the variable conditions of growth of a mineral, are additional characteristics that can be used to describe a mineral and distinguish it from another. The mineral growth environment and chemical substitutions have the potential to cause variations in a mineral’s optical properties, surface chemistry, and crystal structure, as well its tendency to break down or decompose. Within a particular mineral deposit or rock, the constituent minerals can be heterogeneous, even at the micrometer scale, varying in composition, crystal structure, and/or habit, for example. The potential diversity of minerals occurring in mineral deposits and rocks creates challenges when identifying those minerals associated with health impacts and when characterizing human exposures, whether at the mine site or during or following processing into manufactured products. Nature of the Exposures The nature and extent of human exposures to asbestos in the United States have changed over the past 50 years. In the 1970s, most asbestos mining ceased in the United States, with the final mine closing in 2002 (NTP, 2005; Virta, 2006). This ended more than a hundred years of production (1890 to 2003) of an estimated 3.29 million metric tons of asbestos. Additionally during that time, 29.6 million metric tons of asbestos were imported into the United States for industrial uses, including roofing materials, flooring, friction materials in brakes and clutches, and asbestos-cement pipes (Virta, 2006). Health consequences of exposure to asbestos, particularly its association with the development of asbestosis, lung cancer, and mesothelioma after a long latency period, became known in the 1950s and 1960s and led to regulations in the United States in 1971. Use of asbestos in the United States peaked in 1973 at 803,000 metric tons per year, and by 2003, U.S. consumption was at 4,650 metric tons (Virta, 2006). The European Union banned new uses of asbestos in 2005, and asbestos is fully or partially banned in many other countries. 3 The habit is the general shape of the crystals, e.g., acicular, prismatic, fibrous. For a given type of crystal, the habit may vary from locality to locality depending on the environment of growth (Neuendorf et al., 2005).
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A Review of the NIOSH Roadmap for Research on Asbestos Fibers and Other Elongate Mineral Particles Changes in occupational exposures have generally followed the trajectory of the use and regulation of asbestos. Mining, milling, and manufacturing exposures in the United States have declined, leading to reductions in long-term occupational exposures, although some exposures continue with imported asbestos-containing products. Limited information is available on the number of workers still exposed to asbestos. OSHA (2008) estimates that 1.3 million U.S. workers in construction and general industry face asbestos exposure on the job. However, exposures generally occur during remediation and abatement efforts, through demolition of buildings with asbestos-containing materials, and during excavation in areas where asbestos occurs naturally; thus, exposures tend to be shorter term and more intermittent. The Roadmap acknowledges the limitations in current understanding of exposures and recommends research on the nature and levels of occupational exposures to asbestos and other elongate mineral particles and on the number of workers exposed. Further, in the United States, although generally below the levels of occupational exposures in mines, nonoccupational exposures can occur in homes, schools, public buildings, and other locations with unabated asbestos insulation or from other exposures. Environmental exposures are possible for residents or workers near asbestos-containing waste sites and active or inactive mines; the most recent well known issues occurred in Libby, Montana, where mined vermiculite was contaminated with asbestos. Internationally, asbestos-containing products (particularly asbestos-cement products) continue to be produced and used in some countries; in 2003, the leading consumers of asbestos were Brazil, China, India, Iran, Kazakhstan, Russia, Thailand, and Ukraine (Virta, 2006). Because the latency period between asbestos exposure and onset of disease may be several decades, the public health impact of the disease lags behind exposure reductions. For example, the number of asbestosis deaths in the United States reached a plateau in the late 1990s, in part because of the long latency period and the fact that individuals survive many years after disease onset (Moolgavkar et al., 2009). The annual number of asbestosis deaths is anticipated to decrease substantially due to exposure reductions (NIOSH, 2009).
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A Review of the NIOSH Roadmap for Research on Asbestos Fibers and Other Elongate Mineral Particles Exposure Assessment Further complicating the issues regarding asbestos are the challenges in exposure assessment that make it difficult to quantify current exposures and to understand how the levels of exposure were determined in prior studies. Some worker populations that have been studied in epidemiological investigations may have been exposed to complex mixtures of mineral particles, where the level of exposure may be difficult to ascertain in retrospective studies, data on cigarette smoking may not be available, and exposure assessments may have not used direct measurements of exposure but rather surrogates such as job description. Measures of the extent to which workers or residents are exposed to hazardous airborne particles are conducted primarily by collecting air samples and then counting those that occur as single crystals or in bundles in a representative section of the air sample filter. Cleavage fragments, when identifiable, may or may not be counted depending on the analytic methodology. Detailed methodologies for identifying and counting asbestos fibers have been developed by NIOSH, ASTM International, the International Organization for Standardization, and other organizations using phase contrast microscopy, polarized light microscopy, scanning electron microscopy, or transmission electron microscopy. Electron microscopy methods allow higher spatial resolution and can provide further definition to the nature of the crystalline habit, previously characterized through polarized light microscopy and/or the mineral’s chemical composition. Challenges in assessing exposure include variations in the particle distribution on sample filters, ensuring consistency in inter- and intralaboratory counting techniques, varying methods for preparation of samples, the assumption that small samples represent actual exposures, and subjectivity in assessing what particles are countable in a given analytical method (see Chapter 3). Broader Context of Research on Exposure to Airborne Particulates In addition to the research addressed in the NIOSH Roadmap, it is important to recognize the wide breadth of ongoing research on exposures to and health effects from airborne particulates. For example, occupational inhalation exposures to crystalline silica dust are well documented to contribute to silicosis, and coal dust exposure of miners is
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A Review of the NIOSH Roadmap for Research on Asbestos Fibers and Other Elongate Mineral Particles closely associated with pneumoconiosis. Research on synthetic vitreous fibers is also relevant. Because of the relevance of related research on airborne particulates, it is important for the Roadmap to draw from these other experiences and to coordinate research using an interdisciplinary approach. Chapter 2 examines recent efforts to develop research roadmaps and discusses the key elements and larger context for framing a broader research strategy. REFERENCES MMWR (Morbidity and Mortality Weekly Report). 2009. Malignant mesothelioma mortality—United States, 1999-2005. MMWR 58(15): 393–396. Moolgavkar, S. H., R. H Meza, and J. Turim. 2009. Pleural and peritoneal mesotheliomas in SEER: Age effects and temporal trends, 1973-2005. Cancer Causes and Control 20(6):935–944. Neuendorf, K. K. E., J. P. Mehl, Jr., and J. A. Jackson, eds. 2005. Glossary of Geology, 5th edition. Alexandria, VA: American Geological Institute. NIOSH (National Institute for Occupational Safety and Health). 1990a. Comments of the National Institute for Occupational Safety and Health on the Occupational Safety and Health Administration’s notice of proposed rulemaking on occupational exposure to asbestos, tremolite, anthophyllite, and actinolite. Docket No. H-033d, April 9, 1990. http://www.cdc.gov/niosh/review/public/099/pdfs/asbestostestimony_April%209_1990.pdf (accessed June 29, 2009). NIOSH. 1990b. Testimony of the National Institute for Occupational Safety and Health on the Occupational Safety and Health Administration’s notice of proposed rulemaking on occupational exposure to asbestos, tremolite, anthophyllite, and actinolite. Docket No. H-033d, May 9, 1990. http://www.cdc.gov/niosh/review/public/099/pdfs/asbestos_testimony_May9.pdf (accessed June 29, 2009). NIOSH. 2009. Revised draft. NIOSH current intelligence bulletin. Asbestos fibers and other elongated mineral particles: State of the science and roadmap for research. January 2009. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health. http://www.cdc.gov/niosh/docket/pdfs/NIOSH-099b/099B-040109AsbestosNAreviewDoc.pdf
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A Review of the NIOSH Roadmap for Research on Asbestos Fibers and Other Elongate Mineral Particles (accessed September 18, 2009). NTP (National Toxicology Program). 2005. Asbestos. In NTP report on carcinogens, 11th edition. http://ntp.niehs.ni.gov/ntp/roc/eleventh/profiles/s016asbe.pdf (accessed September 17, 2009). OSHA (Occupational Safety and Health Administration). 2008. Safety and health topics: Asbestos. http://www.osha.gov/SLTC/asbestos/index.html (accessed May 7, 2009). Virta, R. L. 2006. Worldwide asbestos supply and consumption trends from 1900 through 2003. U.S. Geological Survey Circular 1298. http://pubs.usgs.gov/circ/2006/1298/c1298.pdf (accessed June 29, 2009).