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--> SPACECRAFT MAXIMUM ALLOWABLE CONCENTRATIONS FOR SELECTED AIRBORNE CONTAMINANTS Introduction The space station—a multinational effort—is expected to be launched in 1997 and, in its present configuration, is expected to carry a crew of four to eight astronauts for up to 180 days. Because the space station will be a closed and complex environment, some contamination of its internal atmosphere is unavoidable. Several hundred chemical contaminants are likely to be found in the closed-loop atmosphere of the space station, most at very low concentrations. Important sources of atmospheric contaminants include off-gassing of cabin materials, operation of equipment, and metabolic waste products of crew members. Other potential sources of contamination are releases of toxic chemicals from experiments and manufacturing activities performed on board the space station and accidental spills and fires. The water recycling system has also been shown to produce chemical contaminants that can enter the cabin air. Therefore, the astronauts potentially can be chronically exposed to low levels of airborne contaminants and occasionally to high levels of contaminants in the event of a leak, spill, or fire. The National Aeronautics and Space Administration (NASA) seeks to ensure the health, safety, and functional abilities of astronauts and seeks to prevent the exposure of astronauts to toxic levels of spacecraft contaminants. Consequently, exposure limits need to be established for continuous exposure of astronauts to spacecraft contaminants for up to 180 days (for normal space-station operations) and for short-term (1-24 hr) emergency exposures to high levels of chemical contaminants. Federal regulatory agencies, such as the U.S. Occupational Safety
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--> and Health Administration (OSHA) and the U.S. Environmental Protection Agency (EPA), have not promulgated exposure limits for the unique environment of spacecraft, nor are their existing standards appropriate for this environment. In 1972, the National Research Council's Committee on Toxicology (COT) first recommended maximum levels for continuous and emergency exposures to spacecraft contaminants (NRC, 1972). However, that early report did not provide documentation of toxicity data or the rationale for the recommended exposure levels. Toxicity data for most of the compounds were not well developed at that time, and the risk-assessment methods were rudimentary. Over the past several years, COT has recommended emergency exposure guidance levels (EEGLs) and continuous exposure guidance levels (CEGLs) for several hundred chemical substances for the U.S. Department of Defense (NRC, 1984a,b,c,d; 1985a,b; 1986; 1987; 1988). However, EEGLs and CEGLs are not available for most spacecraft contaminants. Because of the experience of COT in recommending EEGLs and CEGLs, NASA requested that COT establish guidelines for developing spacecraft maximum allowable concentrations (SMACs) that could be used uniformly by scientists involved in preparing SMACs for airborne contaminants and review the SMACs for individual contaminants to ascertain whether they are consistent with the guidelines. SMACs are intended to provide guidance on chemical exposures during normal operations of spacecraft as well as emergency situations. Short-term SMACs refer to concentrations of airborne substances (such as a gas, vapor, or aerosol) that will not compromise the performance of specific tasks by astronauts during emergency conditions or cause serious or permanent toxic effects. Such exposures might cause reversible effects, such as mild skin or eye irritation, but they are not expected to impair judgment or interfere with proper responses to emergencies. Long-term SMACs are intended to avoid adverse health effects (either immediate or delayed) and to prevent decremental change in crew performance under continuous exposure to chemicals in the closed environment of the space station for as long as 180 days. In response to NASA's request to establish guidelines for developing SMACs and to review SMAC documents for selected spacecraft contaminants, COT organized the Subcommittee on Guidelines for Developing Spacecraft Maximum Allowable Concentrations for Space Station Contaminants. The subcommittee comprised experts in toxicology, epi-
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--> demiology, medicine, physiology, biochemistry, pathology, pharmacology, neurotoxicology, industrial hygiene, statistics, and risk assessment. In the first phase of the study, the subcommittee prepared Guidelines for Developing Spacecraft Maximum Allowable Concentrations for Space Station Contaminants (NRC, 1992). That report provided guidance for deriving SMACs from available toxicological and epidemiological data. It also provided guidance on what data to use, how to evaluate the data for appropriateness, how to perform risk assessment for carcinogenic and noncarcinogenic effects, and how to consider the effects of physiological changes induced by microgravity that might enhance the susceptibility of astronauts to certain spacecraft contaminants. The executive summary of that report is contained in Appendix A of this volume. Summary of Report on Guidelines for Developing SMACs As described in Appendix A, the first step in establishing SMACs for a chemical is to collect and review all relevant information available on a compound. Various types of evidence are assessed in establishing SMAC values for a chemical contaminant. These include information from (1) chemical-physical characterizations, (2) structure-activity relationships, (3) in vitro toxicity studies, (4) animal toxicity studies, (5) human clinical studies, and (6) epidemiological studies. For chemical contaminants, dose-response data from human exposure are most applicable and are used when available in preference to data from animal studies and in vitro studies. Toxicity data from inhalation exposures are most useful because inhalation is the most likely route of exposure. For most chemicals, actual human toxicity data are not available. Therefore, toxicity data from studies conducted in animals are extrapolated to estimate the potential toxicity in humans. Extrapolation requires experienced scientific judgment. The toxicity data from animal species most representative of humans in terms of pharmacodynamic and pharmacokinetic properties are used for determining SMACs. If data are not available on which species best represents humans, the data from the most sensitive animal species are used to set SMACs. Safety or uncertainty factors are commonly used when animal data are extrap-
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--> olated to a safe level for humans. The magnitude of uncertainty factors depends on the quality of the animal data used to determine the no-observed-adverse-effect level (NOAEL). Conversion from animals to humans is done on a body-weight or surface-area basis. When available, pharmacokinetic data on tissue doses are considered for use in species interconversion. Based on the review of the toxicity data and the use of appropriate safety factors, SMACs for different periods are developed, and a rationale is provided for each recommendation. One- or 24-hr emergency SMACs are usually derived from acute exposure toxicity studies. Development of 1- or 24-hr SMACs usually begins with providing a SMAC for the shortest exposure of 1 hr. Values for 24-hr SMACs might necessitate using Haber's rule (C x T = k) when applicable. Detoxification or recovery and data available on 24-hr exposures are taken into account in modifying Haber's rule. When data from chronic exposure studies are available, they are used to derive 7-, 30-, or 180-day SMACs, and safety factors are applied as needed. For substances that affect several organ systems or have multiple effects, all end points—including reproductive (in both sexes), developmental, carcinogenic, neurotoxic, respiratory, and other organ-related effects—are evaluated, the most important or most sensitive effects receiving the major attention. With carcinogenic chemicals, quantitative carcinogenic assessment is done, and the SMAC is set so that the estimated lifetime risk of a neoplasm is no more than 1 in 10,000 exposed persons. When a substance is known to cause an effect that will be aggravated by microgravity, additional safety factors are used. Review of SMAC Reports In the second phase of the study, the Subcommittee on Spacecraft Maximum Allowable Concentrations reviewed reports for approximately 35 spacecraft contaminants to determine whether the recommended exposure limits were consistent with the 1992 guidelines (see Appendix A). The SMAC reports were prepared solely by NASA scientists or contractors. These SMAC reports are intended for use by engineers in developing design criteria for the space station. The SMAC reports will also be applicable to the space shuttle, because the recommended SMACs will
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--> cover the exposure times that are of interest to the space-shuttle program—l-hr and 24-hr SMACs for emergencies and 7-day and 30-day SMACs for continuous exposures. The subcommittee's review of the SMAC reports prepared by NASA, NASA's contractors, and members of the subcommittee involved both oral and written presentations to the subcommittee by the authors of the reports. The subcommittee provided advice and recommendations for revisions. The authors of the SMAC reports presented their revised reports at subsequent meetings until the subcommittee agreed that the reports followed the guidelines developed in the first phase of the study (NRC, 1992). The subcommittee recognizes that many factors, such as the alterations in normal human physiological and biochemical processes associated with spaceflight, are not fully understood and could warrant revisions of proposed SMAC values as additional scientific data become available. Because of the enormous amount of data presented in the SMAC reports, the subcommittee could not verify all the data. The subcommittee relied on NASA scientists for the accuracy and completeness of the toxicity data cited in the SMAC reports. Although individual data points were not verified by the subcommittee, the subcommittee agrees with the proposed SMAC values. This report is the second volume in the series Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants. It contains SMAC reports on 12 spacecraft contaminants. The subcommittee concludes that these reports, presented in Appendix B, are consistent with the 1992 NRC guidelines. The first report in the series, Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 1, published in 1994, contains SMAC reports on 11 substances. SMAC reports for additional spacecraft contaminants will be presented in subsequent volumes. References NRC (National Research Council). 1968. Atmospheric Contaminants in Spacecraft. Washington, D.C.: National Academy of Sciences. NRC (National Research Council). 1972. Atmospheric Contaminants in Manned Spacecraft. Washington, D.C.: National Academy of Sciences.
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--> NRC (National Research Council). 1984a. Emergency and Continuous Exposure Limits for Selected Airborne Contaminants, Vol. 1. Washington, D.C.: National Academy Press. NRC (National Research Council). 1984b. Emergency and Continuous Exposure Limits for Selected Airborne Contaminants, Vol. 2. Washington, D.C.: National Academy Press. NRC (National Research Council). 1984c. Emergency and Continuous Exposure Limits for Selected Airborne Contaminants, Vol. 3. Washington, D.C.: National Academy Press. NRC (National Research Council). 1984d. Toxicity Testing: Strategies to Determine Needs and Priorities. Washington, D.C.: National Academy Press. NRC (National Research Council). 1985a. Emergency and Continuous Exposure Guidance Levels for Selected Airborne Contaminants, Vol. 4. Washington, D.C.: National Academy Press. NRC (National Research Council). 1985b. Emergency and Continuous Exposure Guidance Levels for Selected Airborne Contaminants, Vol. 5. Washington, D.C.: National Academy Press. NRC (National Research Council). 1986. Emergency and Continuous Exposure Guidance Levels for Selected Airborne Contaminants, Vol. 6. Washington, D.C.: National Academy Press. NRC (National Research Council). 1987. Emergency and Continuous Exposure Guidance Levels for Selected Airborne Contaminants, Vol. 7. Washington, D.C.: National Academy Press. NRC (National Research Council). 1988. Emergency and Continuous Exposure Guidance Levels for Selected Airborne Contaminants, Vol. 8. Washington, D.C.: National Academy Press. NRC (National Research Council). 1992. Guidelines for Developing Spacecraft Maximum Allowable Concentrations for Space Station Contaminants, Washington, D.C.: National Academy Press. NRC (National Research Council). 1994. Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 1, Washington, D.C.: National Academy Press.
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