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 presently planned 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) is seeking 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 of astronauts to high levels of chemical contaminants.



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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants 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 presently planned 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) is seeking 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 of astronauts to high levels of chemical contaminants.

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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants 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 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 Con-