SPACECRAFT MAXIMUM ALLOWABLE CONCENTRATIONS FOR SELECTED AIRBORNE CONTAMINANTS :
Introduction

Construction of the International Space Station (ISS)—a multinational effort—is expected to begin 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, 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 accidents, such as a leak, spill, or fire.

The National Aeronautics and Space Administration (NASA) seeks to ensure the health, safety, and functional abilities of astronauts and 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 contaminants.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 1
Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants: Volume 3 SPACECRAFT MAXIMUM ALLOWABLE CONCENTRATIONS FOR SELECTED AIRBORNE CONTAMINANTS :
Introduction Construction of the International Space Station (ISS)—a multinational effort—is expected to begin 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, 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 accidents, such as a leak, spill, or fire. The National Aeronautics and Space Administration (NASA) seeks to ensure the health, safety, and functional abilities of astronauts and 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 contaminants.

OCR for page 1
Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants: Volume 3 Federal regulatory agencies, such as the U.S. Occupational Safety and Health Administration (OSHA) and the U.S. Environmental Protection Agency (EPA), have not promulgated exposure limits for the durations of exposures encountered in the space station or for conditions of microgravity. 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 many 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 the NRC 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 (1 to 24 hr) 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 (up to 180 days) 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, NRC assigned the project to the COT, which convened the

OCR for page 1
Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants: Volume 3 Subcommittee on Guidelines for Developing Spacecraft Maximum Allowable Concentrations for Space Station Contaminants. The subcommittee included experts in toxicology, epidemiology, medicine, physiology, biochemistry, pathology, pharmacology, neurotoxicology, industrial hygiene, statistics, and risk assessment. The subcommittee prepared Guidelines for Developing Spacecraft Maximum Allowable Concentrations for Space Station Contaminants (NRC, 1992). That report provides guidance for deriving SMACs from available toxicological and epidemiological data. It also provides 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, toxicity data from human studies 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 for setting SMACs for airborne contaminants 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

OCR for page 1
Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants: Volume 3 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 used to estimate 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 exposure periods are developed, and a rationale is provided for each recommendation. One-or 24-hr emergency SMACs are derived from acute exposure toxicity studies whenever possible. 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 law (an effect level is directly proportional to exposure concentration multiplied by time, or C × T = k) when applicable. Detoxification or recovery and data available on 24-hr exposures are taken into account in modifying Haber's law. The subcommittee and NASA recognize the limitations associated with Haber's law and use it in accordance with the NRC (1992) guidelines for developing SMACs. 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 risks are estimated, and the SMAC is set so that the estimated increased 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 As NASA began developing chemical-specific SMACs, COT con-

OCR for page 1
Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants: Volume 3 vened the Subcommittee on Spacecraft Maximum Allowable Concentrations to review the NASA reports for consistency with the 1992 NRC guidelines (see Appendix A). The SMAC reports are prepared by NASA scientists or contractors. The first SMAC report, published in 1994, addresses 11 compounds: acetaldehyde, ammonia, carbon monoxide, formaldehyde, Freon 113, hydrogen, methane, methanol, octamethyltrisiloxane, trimethylsilanol, and vinyl chloride. Volume 2, published in 1996, covers an additional 12 compounds: acrolein, benzene, carbon dioxide, 2-ethoxyethanol, hydrazine, indole, mercury, methylene chloride, methyl ethyl ketone, nitromethane, 2-propanol, and toluene. This report, Volume 3, addresses another 12 compounds: bromotrifluoromethane (Halon 1301), 1-butanol, tert-butanol, diacetone alcohol, dichloroacetylene, 1, 2-dichloroethane, ethanol, ethylbenzene, ethylene glycol, glutaraldehyde, trichloroethylene, and xylene. The SMAC reports are intended for use by engineers in developing design criteria for the ISS. The SMAC reports will also be applicable to the space shuttle, because the recommended SMACs will cover exposure times that are of interest to the space-shuttle program—1-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 and NASA's contractors involved both oral and written presentations to the subcommittee by the authors of the reports. The subcommittee provided advice and recommendations for revisions to ensure consistency with the NRC (1992) SMAC guidelines. The subcommittee concludes that the SMAC reports on 12 spacecraft contaminants presented in Appendix B of this report are consistent with the 1992 NRC guidelines. The subcommittee recognizes that many factors, such as the changes 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 rationale for the proposed SMAC values.

OCR for page 1
Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants: Volume 3 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. 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 Al-

OCR for page 1
Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants: Volume 3 lowable Concentrations for Selected Airborne Contaminants, Vol. 1. Washington, D.C.: National Academy Press. NRC (National Research Council). 1996. Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Vol. 2. Washington, D.C.: National Academy Press.

OCR for page 1
Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants: Volume 3 This page in the original is blank.