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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 5 9 1,2 - Dichloroethane Raghupathy Ramanathan, Ph.D. Toxicology Group Habitability and Environmental Factors Division Johnson Space Center National Aeronautics and Space Administration Houston, Texas BACKGROUND 1,2-dichloroethane (EDC; ethylene dichloride) is a colorless liquid with an odor characteristic of a chlorinated hydrocarbon. It has molecular structure CH2Cl-CH2Cl and CAS number 107-06-2. It has a vapor pressure of 87 mmHg at 25°C. The conversion factors are 1 part per million (ppm) = 4.05 milligrams per cubic meter (mg/m3) and 1 mg/m3 = 0.25 ppm. NASA reviewed the toxicologic properties of this compound with respect to various exposure durations at different atmospheric concentrations. Spacecraft maximum allowable concentrations (SMACs) were derived by NASA, reviewed by the National Research Council (NRC) Committee on Toxicology, and published by the National Academy Press in 1996 in Volume 3, Appendix B6, of Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants (Wong 1996). That document listed SMACs for 1 h, 24 h, 7 d, 30 d, and 180 d. With NASA now focusing on exploration missions going beyond low Earth orbit and targeted at the Moon and Mars, there is a need to reevaluate existing SMACs to derive acceptable concentrations (ACs) for long-duration missions, such as 1,000 d. The effort presented in this chapter consisted of identifying new (since 1996) toxicology literature on EDC and evaluating data most appropriate for deriving a 1,000-d SMAC as well as identifying a need to update previously derived SMACs or approaches taken. OCCURRENCE AND USE EDC is used on Earth to manufacture vinyl chloride and as a solvent, degreaser, and fumigant. This compound is present in the air in the International Space Station and the Space Shuttle as a result of outgassing from experimental and system hardware. Because the materials designed for exploration missions
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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 5 remain under development, future sources of such solvents in the spacecraft cannot be determined. SUMMARY OF ORIGINAL APPROACH The literature indicated that EDC can be absorbed from various exposure routes such as inhalation, oral, and dermal (IARC 1999). Exposure can lead to adverse effects, such as central nervous system (CNS) depression, corneal opacity, gastrointestinal (GI) irritation, hepatic necrosis, renal tubular necrosis, and neurotoxicity, and can result in death. The severity and incidence of these effects depend on exposure route, concentration, and duration. Acute inhalation exposure studies showed that EDC, in addition to causing neurotoxic, nephrotoxic, and hepatotoxic effects, also caused respiratory distress, congestion of the lungs, pulmonary edema, cardiac arrhythmia, nausea, and vomiting. In rodents, EDC impaired immune defense mechanisms and produced carcinogenic effects, especially by the oral route. No adequate data were available to evaluate the carcinogenicity of EDC in humans. Coexposure to other solvents may have confounded such evaluations (see NTP 2005). EDC has been reported to be a weak mutagen in the standard bacterial mutation assay (McCann et al. 1975), but it was mutagenic when bacteria were incubated with the liver S-9 fraction and glutathione and it was found to bind to DNA and proteins (Rannug et al. 1978, Banerjee et al. 1980, Guengerich et al. 1980). EDC has also been shown to be genotoxic in vivo (Reitz et al. 1982, Storer et al. 1984). Notably, although DNA lesions were observed in mice 4 h after exposure to EDC by intraperitoneal injection or given as gavage (Storer et al. 1984), no evidence of DNA damage was seen after a 4-h inhalation exposure of mice to up to 500 ppm. EDC is metabolized by a cytochrome P-450-dependent microsomal oxidation system as well as by the cytosolic enzyme glutathione S-transferase, which conjugates EDC with glutathione, leading to the formation of S-(2-chloroethyl)glutathione (Reitz et al. 1982). These conjugates are excreted in the urine. D’Souza et al. (1988) and Reitz et al. (1982) proposed that conjugation with glutathione produced an alkylating agent—an episulfonium ion that is primarily responsible for EDC genotoxicity—but no later papers were found to confirm the supposition. Nevertheless, glutathione conjugation has been shown to result in genotoxic metabolites for other chlorinated alkanes. In the 1996 SMAC derivations, Wong (1996) derived ACs for critical effects including CNS effects, GI symptoms, liver toxicity, and impaired immune defenses. He reviewed the data from a Russian paper (Kozik 1957) describing the effects of EDC exposure in an occupational setting in the Russian aviation industry. The original article was in Russian and, according to a translation of the essential parts of the paper, the study provided data collected from workers in the aviation industry during 1951 to 1955. The author reported that the num-
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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 5 ber of workers who complained of GI disorders and in whom CNS effects were measured (increased errors in hand-eye coordination tests) and who reported they were sick depended on the number of years they worked in the industry. The time-weighted average (TWA) workplace concentration associated with such effects was found to be 15 ppm (NIOSH 1976). Using 15 ppm as the lowest-observed-adverse-effect level (LOAEL) for CNS effects, Wong (1996) calculated the 1-d AC using CNS effects as the adverse end point. Even though this study is not an acute exposure study, because exposure continued over several months and perhaps years, Wong (1996) concluded that this level of EDC can be used to evaluate CNS effects for all exposure durations. The rationale was based on the pharmacokinetic behavior of inhaled EDC—specifically, the finding that blood concentrations reach steady-state within 2 h of exposure at or above 150 ppm (Reitz et al. 1982), continuous exposures are not cumulative in blood, and the fact that EDC in blood affects the CNS. This also implies that CNS effects did not need to have a time adjustment factor for continuous exposures. After a factor of 10 was applied for extrapolating from the LOAEL to no-observed-adverse-effect level (NOAEL), the 1-d AC was derived as 1.5 ppm. Wong (1996) also used the GI effects data from the Russian occupational exposure study to derive a 180-d AC, which he adopted for 1 h, 24 h, 7 d, and 30 d for GI symptoms. Wong used the NOAEL adjusted concentrations for continuous exposure, taking into account 40 h/wk as the number of work hours. The calculation is as follows: The GI effects are supported by Byers’ report (1943) that U.S. workers exposed to about 100 ppm of EDC for 7.5 h/d developed nausea, vomiting, and abdominal pain within a few hours after they left work each day. The concentration of 15 ppm from the Kozik study is much lower. As the Kozik data involved only one LOAEL (TWA of 15 ppm) concentration and the reports of illnesses could not be quantitative, the data were not amenable to benchmark dose (BMD) modeling. Because the publication is quite old and the documentation and references are in a foreign language, NASA did not analyze these data further. Several animal studies evaluated for acute toxicity effects were considered unsuitable because they used single high doses and had very high mortalities (Heppel et al. 1946; Spencer et al. 1951). NASA reviewed the literature for toxicology data available since 1995 that can be used to derive 7-, 30-, and 180-d ACs, and no data were found. In addition to CNS and GI effects, another toxicologic end point that Wong (1996) used for deriving 1-h, 24-h, 7-d, 30-d, and 180-d ACs was hepatotoxicity reported in studies by Spencer et al. (1951) and Heppel et al. (1946) on monkeys, rats, and guinea pigs. Exposing monkeys to 400 or 200 ppm of EDC leads to fatty liver. One of these studies also noted degeneration of renal tubules. Spencer et al.
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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 5 (1951) reported that monkeys exposed to 100 ppm of EDC for 7 h/d, 5 d/wk, for 29 wk showed no adverse effects on behavior, gross and microscopic tissue morphology, and hematologic parameters. They also studied the effects of a series of concentrations of EDC on rats and guinea pigs. Wong (1996) determined ACs for various durations using the NOAEL dose of 100 ppm for hepatotoxicity for rats and guinea pigs reported by Spencer et al. (1951). As NOAELs were found to be 100 ppm for 15 and 30 wk EDC exposures, Wong (1996) calculated the 24 h and 7-d ACs conservatively without adjusting for discontinuous-to-continuous exposure. Thus, the 24-h and 7-d ACs for liver toxicity were derived as follows: Using the 15 wk (105 d) NOAEL of 100 ppm Wong (1996) calculated the 30-d AC for liver toxicity as follows after applying a factor for discontinuous-continuous exposure. Using a NOAEL of 100 ppm based on exposure of rats and guinea pigs for 7h/d, 5 d/wk, for 30 wk (210 d) a 180-d AC for hepatotoxicity was calculated: NASA evaluated the hepatotoxicity data used for these durations to determine whether BMD modeling could be used. Although Spencer et al. (1951) used various concentrations of EDC (100, 200, and 400 ppm for repetitive exposures) in guinea pigs and rats and stated that they had measured several parameters to assess toxicity, the only quantitative data they presented were changes in body weight and tissue weight. In general, the NRC SMAC and spacecraft water exposure guideline committees do not consider body weight changes or organ weight changes to be robust variables for calculating ACs (NRC 1992, 2000). Thus, because of the absence of quantitative dose-response data, BMD modeling could not be carried out on the results from this study. Wong (1996) also used impaired immune defense as an adverse end point and derived 1-h, 24-h, 7-d, 30-d, and 180-d ACs from the results of Sherwood et al. (1987), who reported that a single 3-h exposure of young mice to 5-11 ppm of EDC by inhalation increased mortality after the mice were infected (challenge) with Streptococcus zooepidemicus. However, at 2.5 ppm this effect was not seen (NOAEL is 2.5 ppm for 3 h). Sherwood et al. also pointed out that the immunotoxic response differed among species; young male rats exposed to 200
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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 5 ppm of EDC for 5 h or to 100 ppm for 5 h/d for 12 days were unaffected when challenged via inhalation with Klebsiella pneumoniae, whereas young mice showed infection in response to this organism even with just 11 ppm of EDC for 3 h. The most sensitive response was used to derive the AC very conservatively for humans. For all durations up to 180 d (1 h, 24 h, 7 d, 30 d, and 180 d ), Wong (1996) used the impaired host defense end point described above. He also used a space flight factor of 3 to protect against microgravity-induced impairment of the cell-mediated immune response (Taylor 1993). The AC for the immunologic end point for all durations was calculated as follows: Thus, an AC of 0.8 ppm for impaired host defense was derived for all durations up to 180 d. Wong (1996) also derived a 180-d AC for carcinogenic effects. He used the report of Ward (1980), who presented a National Cancer Institute (NCI) study (NCI 1978) showing that EDC produced tumors in multiple organs of Osborne-Mendel rats (50/dose/sex) and in B6C3F1 mice (50/dose/sex) that were administered EDC as a daily gavage in corn oil, 5 d/wk, for 78 wk. The two estimated TWA doses were 47 and 97 mg per kg of body weight for male and female rats, 97 and 195 mg/kg for male mice, and 149 and 299 mg/kg for female mice. Observed tumors included squamous cell carcinomas of the forestomach, hemangiosarcomas of the circulatory system, and fibrosarcomas of the subcutaneous tissue. Tumors were also found at other organ sites. Hepatocellular carcinomas and alveolar/bronchiolar adenomas were seen in male and female mice. In female rats and mice, mammary carcinomas were also noted. A summary of these results is presented in Table 9-1. TABLE 9-1 Tumors Found in NCI Bioassay of EDCa Species/Sex Adverse Effect Site Rat/male Squamous cell carcinoma Forestomach Hemangiosarcoma Circulatory system Fibroma Subcutaneous tissue Rat/female Adenocarcinoma Mammary gland Mouse/female Adenocarcinoma Mammary gland Mouse/female Stromal polyp Endometrium Stromal sarcoma Endometrium Mouse/male & female Adenoma Alveoli and bronchioli aThe observed incidence rate of tumors in exposed animals is statistically different from that of controls. Source: NCI 1978.
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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 5 Although these serious tumorigenic responses were found in the gavage study, Maltoni et al. (1980) found no evidence of carcinogenicity in a lifetime EDC inhalation exposure study in which Sprague-Dawley rats and Swiss mice were exposed to EDC at 5 to 150 ppm (20 to 600 mg/m3). Another study by Cheever et al. (1990), in which groups of 50 male and 50 female Sprague-Dawley rats were exposed by inhalation to 50 ppm EDC for 7 h/d, 5 d/wk, for 2 y, showed no significant increase in incidence of any tumor type in male and female rats. However, the study included only one dose and one species. Reitz et al. (1982) compared the pharmacokinetics of EDC in rats after oral and inhalation routes of administration. The oral dose selected (150 mg/kg in corn oil) was the same as that of the National Toxicology Program’s (NTP 1991) EDC gavage study. The concentration and duration selected for inhalation exposure (150 ppm for 6 h) was the same as that of Maltoni et al.’s EDC inhalation study (1980), except Maltoni et al. exposed rats for 7 h/d. The peak blood concentrations were about 5-fold higher in the EDC-gavaged animals than in rats exposed by inhalation. The amounts of EDC metabolites binding to liver, spleen, kidney, and stomach DNA were 2 to 5 times greater in the gavaged animals than in those exposed by inhalation; however, the overall extent of DNA alkylation was low. In spite of this notable difference in cancer incidence between the gavage studies and inhalation exposure studies, and although no adequate data were available to evaluate the carcinogenicity of EDC in humans, both the NCI and the International Agency for Research on Cancer (IARC) (NCI 1978, IARC 1999) declared that EDC is reasonably anticipated to be a human carcinogen based on sufficient evidence of carcinogenicity in experimental animals. IARC (1999) classified EDC as Group 2B (the agent [mixture] is possibly carcinogenic to humans). The U.S. Environmental Protection Agency (EPA) classified EDC as B2, a probable human carcinogen. They extrapolated the gavage data to an inhalation exposure scenario and calculated an inhalation unit risk (unit risk = upper-bound excess lifetime cancer risk estimated to result from continuous exposure to an agent at a concentration of 1 microgram [μg]/m3 in air) using the linearized multistage procedure extrapolation method. One hundred percent absorption from inhalation and metabolism at low dose were assumed when oral data were used to calculate inhalation unit risk. Wong (1996) used the tumor data from the NCI gavage study to derive an AC for inhalation exposure, from which U.S. EPA estimated that 4 μg/m3 would yield an excess tumor risk of less than 1 in 10,000 for continuous lifetime exposure of humans. Following the approach recommended by the NRC (1992), which assumes the earliest age of exposure is 30 y and the average life span of an astronaut is 70 y, an adjustment factor of 146.7 (see NRC 1992, pp. 88-89) was calculated and used to compress the EPA estimated value of 4 mg/m3 into a much shorter continuous exposure of 180 d that would yield the same tumor risk. With these adjustment factors, the concentration of EDC for a risk of 1 × 10−4 was calculated to be 0.2 ppm (0.8 mg/m3).
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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 5 RATIONALE The original SMACs were set in 1996, before the current NRC approaches to data analysis such as BMD modeling came to be used by many regulatory organizations in risk assessment. Thus, the 1996 calculations of ACs and SMACs were based on the conventional NOAEL-LOAEL approach. In derivation of the 1,000-d AC, the dose-response data were evaluated for applicability to dose-response modeling. Exposure limits and recommended amounts set by other organizations are shown in Table 9-2. The EPA did not derive an oral reference dose or an inhalation reference concentration for non-caricogenic effects, but did calculate an oral slope factor for the carcinogenic potency of EDC based on the oral carcinogenicity data from the NCI (1978) study in rats described elsewhere in this document. The EPA (1991) arrived at an oral slope factor (an upper-bound estimate of the human cancer risk per mg of agent/kg body weight/d) of 0.091 (mg/kg/d)−1 by using a linearized multistage procedure. This corresponds to a drinking water unit risk of 2.6 × 10−6 μg/liter. An inhalation unit risk of 2.6 × 10−5 μg/m3 was derived from the oral data assuming 100% absorption from inhalation. This equals a risk of 4 μg/m3 (EPA 1991) using a nominal adult body weight of 70 kg and a daily respiratory volume of 20 m3. Inhalation unit risk represents the potential excess cancer risk for a person exposed for a lifetime to EDC at 1 μg/m3 and is at most 22 in 1,000,000. The EPA’s estimated inhalation carcinogenic risks and associated EDC air concentrations, summarized in IRIS (1991), are shown in Table 9-3. TABLE 9-2 A Summary of Exposure Standards or Recommended Levels by Other Organizations for EDC Vapors Organization, Standard Exposure Limit (ppm) References ACGIH ACGIH 1996 TLV-TWA 10 OSHA NIOSH 2005 PEL TWA 50 STEL ceiling 100 NIOSH REL NIOSH 2005 TWA 1 ceiling 2 NIOSH IDLH NIOSH 1996 Originala 1,000 Revisedb 50 NIOSH STEL 2 NIOSH 2005 Conversion: 1 ppm = 4.05 mg/m3. aOriginal IDLH was based on rat data of Spencer et al. (1951).
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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 5 bBasis for revised IDLH: acute inhalation toxicity data from Polish agricultural workers (Brzozowski et al. 1954). Abbreviations: ACGIH, American Conference of Governmental Industrial Hygienists; IDLH, immediately dangerous to life and health; NIOSH, National Institute for Occupational Safety and Health; OSHA, Occupational Safety and Health Administration; PEL, permissible exposure limit; ; REL, recommended exposure limit; STEL, short-term exposure limit; TLV, threshold limit value; TWA, time-weighted average. TABLE 9-3 Air Concentration and Specified Carcinogenic Risk Levels Carcinogenic Risk EDC Concentration (μg/m3) 1 in 10,000 4 1 in 100,000 0.4 1 in 1,000,000 0.04 Source: EPA 1991. Minimal Risk Levels from Agency for Toxic Substances and Disease Registry An acute-duration inhalation minimal risk level (MRL) for EDC has not been derived by the Agency for Toxic Substances and Disease Registry (ATSDR). An intermediate-duration MRL (15-364 days) adopted the chronic-duration MRL (ATSDR 2001). An MRL of 0.6 ppm (2.4 mg/m3) was derived for chronic-duration inhalation exposure to EDC, which will be protective for intermediate-duration inhalation exposure to EDC. This was derived by using the NOAEL for liver histopathology data from the study of Cheever et al. (1990), which exposed rats to EDC at 50 ppm for 7 h/d, 5 d/wk, for 2 y. The details of the Cheever et al. (1990) study are described elsewhere in this document. ATSDR used an uncertainty factor of 90 (3 for species extrapolation, 10 for human variability, and 3 as a modifying factor for database deficiency including lack of dose-response data). Using liver as the target organ for EDC, toxicity was justified because several studies have shown hepatotoxicity after exposure to EDC. In deriving this MRL, ATSDR did not use a conversion factor for adjusting the intermittent exposure to a continuous exposure and did not derive a carcinogen potency factor even though the literature on cancer from exposures to EDC was discussed in the ATSDR toxicology profile for EDC (ATSDR 2001). Changes to Previously Established SMACs for EDC As mentioned previously, none of the AC values calculated in the previous EDC SMAC document could be recalculated by using newer methods such as
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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 5 BMD modeling. There has been no study since 1996 that is suitable to rederive the ACs for these durations. Therefore, it was decided to leave the values as they are. Initially, Wong (1996) used CNS depression as an end point to derive 1- and 24-h ACs. This approach resulted in a value of 1.5 ppm for both 1- and 24-h durations. However, Wong (1996) also considered GI effects to derive the 1-and 24-h ACs, because GI effects had been the critical end point in deriving the 7-, 30-, and 180-d ACs. This approach resulted in all ACs (1 h to 180 d) having the same value of 0.4 ppm. The 180-d AC was lowered to 0.2 ppm when carcinogenesis was used as the critical end point for long-term derivation. Considerable concern has been raised about the validity and strength of the rationale behind using the oral data to compute the inhalation risk factor for cancer while experiments using inhalation exposure for a sufficiently long time did not provide convincing evidence that EDC could be carcinogenic to humans via inhalation. NASA decided to reassess this approach, as shown below under derivation of the 1,000-d AC. As a result, NASA decided to withdraw the 180-d AC value of 0.2 ppm derived for a carcinogenic end point, which was the driver for the 180-d SMAC. Relevant Data Since 1996 NASA reviewed the literature to find a long-term EDC inhalation exposure study to use for deriving a 1,000-d AC (and a SMAC) for both a noncarcinogenic toxicity end point and, if applicable, a carcinogenic risk factor for 1 in 10,000. NASA did not find a study that could be used since 1996, when the previous SMACs for durations were derived. Derivation of 1,000-Day ACs for EDC First, data from the human occupational exposure study published by a Russian investigator were considered for 1,000-d AC. Kozik (1957) studied workers in the aircraft industry in Russia who applied glue containing EDC as a solvent to large rubber sheets. On the basis of Kozik’s data, the National Institute for Occupational Safety and Health estimated that, in the first half of the shift, the TWA exposure concentrations of EDC were 28 ppm (113 mg/m3) during glue application and 16 ppm (65 mg/m3) during the time the glue dried (NIOSH 1976). In the second half of the shift, the TWA exposure concentration of EDC was 11 ppm (44.6 mg/m3). Therefore, the EDC TWA exposure concentration for the entire shift was 15 ppm (60.8 mg/m3) (NIOSH 1976). Effects reported by Kozik (1957) were likely caused by exposures to much higher (up to about 50 ppm) repeated short-term exposures for certain job categories rather than to the TWA exposure. Thus, the TWA of 15 ppm represents a very protective concentration to use as a LOAEL and may actually be a NOAEL for short-term exposures, although the data presented by this study are insufficient to determine that. Comparing the morbidity data of the gluers and the machinists,
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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 5 who were not exposed to EDC, Kozik (1957) reported that the EDC exposure increased both the number of cases of acute GI disorders per 100 workers and the number of workdays lost to acute GI disorders per 100 workers. Kozik also measured the hand-eye coordination speed of the gluers and machinists at the start and end of the workday for 14 d in 17 gluers and 10 machinists (as controls). The speed did not differ among the groups. However, the EDC-exposed gluers made more errors in the test than did the nonexposed machinists (error rates of 30% for the gluers and 10% for the machinists). A number of factors were considered in the study even though the concentration versus effect was not clearly discernable because of the uncertainty in the exposure concentration. NASA and the NRC Committee on Spacecraft Exposure Guidelines (SEGs) reevaluated the Kozik (1957) study for results on neurotoxicity and GI disturbances and concurred that the occupational exposure data from the Kozik (1957) study can be used for the 1,000-d AC. Kozik (1957) also included a laboratory study of changes in conditioned reflex activity in rats exposed to very low concentrations of EDC (2.5 ppm). However, because of a considerable lack of detail about the experimental design (e.g., sex, strain, age of rats, number of rats per group, presence of a control group, and details of EDC analysis and exposure data) and results (e.g., no data tables), it was concluded that the rat study should not be considered for AC derivation for any duration. The 1,000-d AC for GI disturbances and neurotoxicity can be calculated based on a LOAEL of 15 ppm as follows. AC is derived for a prolonged continuous duration of 1,000 d, and for GI effects and neurological effects the concentration was adjusted for discontinuous-to-continuous exposures. The 7-y exposure data are used for the shorter duration of only 1,000 d. Because of this margin of safety, 0.357 was rounded to 0.4 ppm. No time extrapolation is needed. Some rodent studies were also considered for 1,000-d AC derivation. It was decided to use the absence of abnormal liver histopathology data reported in the Cheever study for noncarcinogenic effects at 1,000 d. In the Cheever et al. (1990) study, groups of male and female Sprague-Dawley rats (50/sex/dose) were exposed to EDC by inhalation of 50 ppm for 7 h/d, 5 d/wk, for 2 y. Body weights, survival rates, and absolute and relative liver weights of animals were not affected. Gross pathology and histopathologic lesions were evaluated for incidence of intrahepatic bile duct cholangiomas in liver, mammary, and testicular tissues; incidence of subcutaneous fibromas, neoplastic nodules, and interstitial cell tumors in the testes; and incidence of mammary adenocarcinomas. No significant increase in the number of any tumor type was observed in rats ex-
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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 5 posed to EDC. There were also no histological lesions of the respiratory tract. The limitations of this study are that no clinical biochemical variables were measured and no dose-response data were collected. Only a NOAEL has been identified, without a LOAEL; thus, only limited evaluations of toxicity could be derived. There may be some uncertainty about the NOAEL. As large numbers of both sexes of animals were used, it may not be a serious issue. No new methodology could be used to derive a 1,000-d AC. Using 50 ppm as a NOAEL for the absence of abnormal liver or lung histopathology, a 1,000-d AC can be calculated after using an appropriate adjustment factor (Adj) for exposure duration. For calculation of the NOAEL (Adj), the NRC Committee on Spacecraft Exposure Guidelines recommends modifying the previous approach so that when data are extrapolated from a chronic-duration animal study (especially a 2-y study) to 1,000 d for human exposure an additional time factor of 728 d (2 y) to 1,000 d is not necessary, because 2 y is a greater fraction of a rat’s lifetime than 1,000 d is of a human’s lifetime. The 1,000-d AC for histopathology and general toxicity can be calculated as follows: Thus, the 1,000-d AC for hepatotoxicity is 1.00 ppm. Another chronic exposure inhalation study by Spreafico et al. (1980) that showed both liver and kidney toxicity were used to derive a 1,000-d AC for a noncarcinogenesis end point. In this study, rats (8-10/sex/dose) were exposed to EDC at 0, 5, 10, 50, and 150-250 ppm for 7 h/d, 5 d/wk, for up to 18 mo. The authors did not state how many animals were exposed to 150 ppm after some deaths at 250 ppm and also did not say clearly how long after the deaths occurred the dose was reduced to 150 ppm. Serum chemistries were measured at 3, 6, 12, and 18 mo. Animals were exposed starting at 3 mo of age. In this study, some older animals (14 mo old) were also exposed to EDC but only for 12 mo. Serum chemistries were unremarkable up to and including the 50-ppm group. However, rats exposed to higher amounts of EDC for 12 mo had increased serum alanine transaminase activity at the two highest exposure concentrations, indicative of chronic liver damage. Changes in lactate dehydrogenase and aspartate transaminase concentrations did not appear to be dose related. Increased blood urea nitrogen concentrations in the 150-ppm group and increased uric acid levels at the two highest exposure groups indicated renal toxicity. This study
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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 5 identified an 18-mo NOAEL of 50 ppm. As 18 mo is a significant fraction of a rat’s lifetime compared with 1,000 d in a human lifetime, an additional time extrapolation factor for 18 mo to 1,000 d was not used. A 1,000-d AC for hepatotoxicity and nephrotoxicity as a critical adverse end point can be calculated after adjusting the NOAEL for going from a discontinuous to a continuous exposure dose. Thus, a 1,000-d AC for hepatotoxicity and nephrotoxicity was calculated as 1.00 ppm. The subchronic study of Spencer et al. (1951) was also evaluated for the 1,000-d AC derivation. In this study, male and female rats, guinea pigs, rabbits, and monkeys were exposed to EDC by inhalation at various concentrations. Rats were exposed to 400 and 100 ppm for 7 h/d, 5 d/wk, for 6 mo, and some additional rats were exposed to 200 ppm for 30 wk. Male and female guinea pigs were exposed to 200 ppm for 36 wk. In all the animal species tested, no adverse effects were observed in groups exposed to 100 ppm or less. Although no adverse effects were found in rats exposed to 200 ppm for 30 wk, mild hepatotoxic effects were noted in the guinea pigs (such as parenchymatous degeneration with some vacuolization). Severe effects, including hepatotoxicity and death, were observed in rats and guinea pigs exposed at 400 ppm. As the data indicated a clear NOAEL of 100 ppm for hepatotoxicity, this concentration was chosen for deriving a 1,000-d AC. A 1,000-d AC for hepatotoxicity as the critical effect can be calculated with data from the study by Spencer et al. (1951), as shown below. Concentration adjustments for intermittent to continuous exposures and factors for time extrapolation from 210 d to 1,000 d are used. A summary of ACs derived for 1,000 d for various end points is shown in Table 9-4. The 1,000-d SMAC is 0.4 ppm (1.6 mg/m3), based on the lowest 1,000-d AC for all end points.
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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 5 TABLE 9-4 Summary of 1,000-d ACs for Vapors to EDC by Inhalation 1,000-d AC as ppm 1,000-d AC as mg/m3 Critical Adverse Effect Principal Study 0.40 1.6 GI effects and neurological effects Kozik 1957 1.00 4.0 Tissue histopathology Cheever et al. 1990 1.00 4.0 Hepatotoxicity Spreafico et al. 1980 0.45 1.8 Hepatotoxicity Spencer et al. 1951 Evaluation of Studies for Deriving a 1,000-Day AC for Carcinogenic Risk Level This discussion also applies to the derivation of a 180-d AC by Wong (1996) based on the NTP 2-y EDC oral carcinogenicity bioassay. NASA has decided to withdraw use of the oral carcinogenesis bioassay data for calculating an inhalation cancer risk factor for EDC for 1,000 d. The strength of the NCI cancer bioassay, which showed tumors in multiple sites, is that the bioassay was conducted in both sexes and in two species and with at least two doses of EDC. Nevertheless, one needs to provide a strong justification to resort to a “route-to-route extrapolation” method for extrapolation of results from this ingestion study to inhalation exposures, especially when sufficient data are already available from some long-term EDC inhalation exposure studies in which no tumors were found. Adequate understanding of the explanations offered for the striking differences in the tumor-induction response between these two routes of exposures is limited. However, NASA took several factors into account in its decision to not use the NTP cancer bioassay data to derive an inhalation cancer risk factor. Differences in the internal dose and pharmacokinetics between these two routes of exposure, the confounding factor of pharmacokinetic changes due to the use of corn oil for the gavage, the metabolic saturation behavior and potential consequences, and the levels of DNA alkylations, DNA damage, and in vitro and in vivo genotoxicity are the factors NASA considered in its decision. Supporting evidence includes pharmacokinetic data and comparative toxicity studies of bolus dosing of solvents in corn oil versus administration in drinking water (more similar to dosing associated with inhalation). One can make certain inferences based on results from some existing EDC pharmacokinetic data from oral and inhalation studies (Reitz et al. 1982). When Reitz et al. (1982) administered EDC at dose rates approximately comparable to those used by NCI (1978) and Maltoni et al. (1980), the peak blood concentration of EDC from a one-time oral bolus was about 5 times higher than that from an inhalation exposure for 6 h, even though the EDC gavage dose of 150 mg/kg was only 1.3 times higher than the dose associated with the 150-ppm, 6-h EDC inhalation protocol (delivered dose estimated to be 113 mg/kg). When blood concentra-
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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 5 tions are high, saturation of the primary oxidation pathway via the microsomal mixed function oxidase system (followed by detoxification) will shunt the EDC to form reactive species via a direct glutathione binding pathway. In spite of a significant difference in EDC pharmacokinetics between ingestion and inhalation exposures, and even though the amounts of DNA alkylations measured in various target and nontarget tissues (as a surrogate parameter for genetic damage and potency for cancer induction) were 2- to 5-fold higher in rats dosed with EDC by gavage than in those exposed by inhalation, the levels of DNA alkylations in these tissues were found to be on the order of only 2 to 20 per 106 nucleotides (Reitz et al. 1982). According to Reitz et al. (1982), it has been argued that the differences lie mostly in the metabolic saturation kinetics and behavior of the two pathways that are known to be involved with the biotransformation of EDC in the body. They estimated that the metabolic pathway for EDC would be saturated after an animal received an EDC dose of 25 mg/kg by gavage or 150 ppm by inhalation. In short, Reitz et al. (1982) proposed that EDC exposure by inhalation did not result in high enough EDC concentrations in blood to saturate its oxidation and detoxification pathway. This limited the amount of EDC available for the formation of S-(2-chloroethyl)glutathione, a reactive intermediate formed by direct conjugation of EDC to glutathione, which can bind to cellular macromolecules including DNA (Reitz et al. 1982). This difference in the metabolic saturation can partly explain why no significant increase in tumors was found in the inhalation experiments. The data on metabolic saturation are consistent with the observation by Spreafico et al. (1980) that saturation occurs at inhalation concentrations around 150 to 250 ppm, based on the finding in rats that when the EDC concentration was increased from 50 to 250 ppm (5-fold) the blood EDC concentration increased 22-fold. Some studies reported in the literature on single versus continuous boluses indirectly support the conclusions from the Reitz et al. acute exposure pharmacokinetic study (see gavage versus drinking-water studies described below). The NCI cancer bioassay protocol involved gavage administration of EDC in corn oil. Whether the positive results for tumor incidence in the NTP oral ingestion study might be due to the vehicle used for administration has been questioned. NASA reviewed studies in which a few volatile organics were administered by gavage in corn oil that resulted in a large amount in the system in a short period of time, and in drinking water, which delivered the chemical in small doses over an extended time. A subchronic study of EDC compared the two different types of administration. For example, a 13-wk NTP drinking water and gavage toxicity study was conducted to investigate the potential differences in strain susceptibility to EDC toxicity in F344/N rats, Sprague-Dawley rats, and Osborne-Mendel rats. The study concluded that administration of EDC in drinking water resulted in less toxicity than administration of similar doses by gavage (in corn oil) (NTP 1991). Similar results were reported in studies using other haloalkanes. For example, Larson et al. (1994) showed that chloroform increased the incidence of liver tumors in B6C3F1 mice when administered in
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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 5 corn oil by gavage but not when similar daily doses were given ad libitum in drinking water. Similarly, Geter et al. (2004) reported that two other haloalkanes, bromodichloromethane and tribromomethane, which produced neoplasia in the large intestine when given as a corn oil gavage, failed to elicit such effects in male rats when given via drinking water for 26 wk. EDC has been shown to cause mutagenesis in Salmonella typhimurium TA 1535 and TA 100 and in several short-term genotoxicity assays (see WHO 1996). Although EDC tested positive in the in vitro micronucleus test in isolated human lymphocytes, a clear dose-dependent mutagenic activity was not found (Tafazoli et al. 1998). More importantly, Storer et al. (1984) studied the in vivo genotoxicity of EDC in the livers of male mice (B6C3F1) after single oral, intraperitoneal, or inhalation exposures of 150 and 500 ppm for 4 h by measuring single-strand breaks in liver DNA (alkali-labile lesions). Although genotoxic effects were found after intraperitoneal and oral administration in corn oil (even at the lowest dose of 100 mg/kg), EDC failed to produce any evidence of a genotoxic effect (hepatic DNA damage) in mice exposed to it for 4 h at 500 ppm, a dose that all animals survived. In the gavage-dosed animals, the DNA damage persisted even at 24 h (Storer et al. 1984). When the genetic activity of EDC was tested in the germ cells and somatic tissue of Drosophila melanogaster after inhalation treatment with EDC, the induction of interchromosomal recombination was very low (Ballerring et al. 1993). The most important consideration is that the studies that primarily aimed to assess the carcinogenic potential of EDC by the inhalation route did not show carcinogenic activity even at maximal tolerated doses. One might argue that, in one study (Cheever et al. 1990), only one dose (50 ppm) and one species (rats, both sexes) were used. However, in the other study (Maltoni et al. 1980), doses close to the maximum tolerated doses, two species of animals (rats and mice), and both sexes were used, with 90 animals in each treatment group, and the animals were exposed for a fairly long time (78 wk), which is most of their life span. Thus, extrapolating oral ingestion study data to inhalation data to derive a carcinogen risk factor for inhalation exposure cannot be fully justified. NASA is interested in setting a SMAC for 1,000 d, which is only a fraction of a human lifetime. It is unlikely that a space crew would be chronically exposed to concentrations of EDC used in the inhalation studies. The absence of a tumorigenic response from the results of inhalation studies provides sufficient confidence to believe that there will not be a carcinogenic risk to the crew on a 1,000-d mission. If there is almost no probability of carcinogenic risk for 1,000 d, there is a larger margin of safety for a 180-d AC for this effect. Thus, the 180-d AC value for a carcinogenic end point will be withdrawn and will not be the basis for the 180-d SMAC. A comprehensive summary of SMACs for all durations from 1 h to 1,000 d is presented in Table 9-5.
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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 5 TABLE 9-5 A Summary of SMACs for EDC for Various Durations Duration Concentration (ppm) Concentration (mg/m3) Adverse Endpoint Principal Study 1 h 0.40 1.6 GI symptoms Kozik 1957 24 h 0.40 1.6 GI symptoms Kozik 1957 7 d 0.40 1.6 GI symptoms Kozik 1957 30 d 0.40 1.6 GI symptoms Kozik 1957 180 d 0.40 1.6 GI effects Kozik 1957 1,000 d 0.40 1.6 Hepatotoxicity and GI effects Spencer et al. 1951 and Kozik 1957 REFERENCES ACGIH (American Conference of Governmental Industrial Hygienists). 1996. Ethylene Dichloride. Documentation of the Threshold Limit Values and Biological Exposure Indices, 6th Ed. American Conference of Governmental Industrial Hygienists, Cincinnati, OH. ATSDR (Agency for Toxic Substances and Disease Registry). 2001. Toxicological Profile for 1,2-Dichloroethane (Update). U.S. Department of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry, Washington, DC [online]. Available: http://www.atsdr.cdc.gov/toxprofiles/tp38.html [accessed Oct. 3, 2007]. Ballering, L.A., M.J. Nivard, and E.W. Vogel. 1993. Characterization of the genotoxic action of three structurally related 1,2-dihaloalkanes in Drosophila melanogaster. Mutat. Res. 285(2):209-217. Banerjee, S., B.L. Van Duuren, and F.I. Oruambo. 1980. Microsome-mediated covalent binding of 1,2-dichloroethane to lung microsomal protein and salmon sperm DNA. Cancer Res. 40(7): 2170-2173. Brzozowski, J., J. Czajka, T. Dutkiewicz, I. Kesy, and J. Wojcik. 1954. Hygiene and the health condition of workers employed in eradication of potato beetle Leptinotarsa decemlineata with hexachlorocyclohexane and with dichloroethane [in Polish]. Med. Pr. 5(2): 89-98. Byers, D.H. 1943. Chlorinated solvents in common wartime use. Ind. Med. 12(7):440-443. Cheever, K.L., J.M. Cholakis, A.M. el-Hawari, R.M. Kovatch, and E.K. Weisburger. 1990. Ethylene dichloride: The influence of disulfiram or ethanol on oncogenicity, metabolism, and DNA covalent binding in rats. Fundam. Appl. Toxicol. 14(2):243-261. D’Souza, R.W., W.R. Francis, and M.E. Andersen. 1988. Physiological model for tissue glutathione depletion and increased resynthesis after ethylene dichloride exposure. J. Pharmacol. Exp. Ther. 245(2):563-568. EPA (U.S. Environmental Protection Agency). 1991. 1, 2-Dichloroethane (CASRN 107-06-2). Quantitative Estimate of Carcinogenic Risk from Inhalation Exposure. Integrated Risk Information System, U. S. Environmental Protection Agency [online]. Available: http://www.epa.gov/iris/subst/0149.htm [accessed Oct. 3, 2007]. Geter, D.R., M.H. George, T.M. Moore, S.R. Kilburn, G. Huggins-Clark, and A.B. DeAngelo. 2004. Vehicle and mode of administration effects on the induction of
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