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Introduction The National Aeronautics and Space Administration (NASA) is preparing to launch a manned space station by the year 1996. Because of concerns about the health, safety, and functioning abilities of the crews, NASA has requested that the National Research Council (NRC) through the Board on Environmental Studies and Toxicology (BEST) provide advice on toxicological matters for the space-station program. The Subcommittee on Guidelines for Developing Spacecraft Maximum Allowable Concentrations for Space Station Contaminants was es- tablished by the Committee on Toxicology (COT) to address NASA's concerns. Spacecraft maximum allowable concentrations (SMACs) are defined as the maximum concentrations of airborne substances (such as gas, vapor, or aerosol) that will not cause adverse health effects, significant discomfort, or degradation in crew performance. SMACs are further classified into 1- or 24-hr emergency SMACs and 7-, 30-, or 180-day continuous SMACs. The 1- or 24-hr SMACs are to be used in emer- gency situations such as accidental spills or fire. The 7-, 30-, and 180-day SMACs are guidance levels intended to avoid adverse health effects, either immediate or delayed, and to avoid degradation in per- formance of crew after continuous exposure for as long as 180 days in the space station; these values will be used for normal operations of the space station. NASA expects that each crew for the space station will stay aloft for up to 180 days. Limits for continuous exposure to toxic substances for up to 1,000 days have been recommended for a few chemicals by NRC (1968,1972). However, the rationale for these limits was not discussed in these reports. More recently, COT has recommended exposure guidance levels for about 70 chemicals for up to 90 days for crews of submarines (NRC, 1984a,b,c, 1985a,b, 1986a, 1987a, 1988a) with extensive documentation and rationale. In setting 7-, 30-, and 180-day SMACs for the space station, the subcommittee will have to consider not only a longer exposure period but also the
10 GUIDELINES FOR DEVELOPING SMACS physiological changes induced by the spaceflight and the susceptibility of the crew to the effects of these changes. The subcommittee will also consider the question of exposure to mixtures of chemicals (approximately 300 substances have been identified that are expected to be found in the space station). The current space-station program began with President Reagan's 1984 State of the Union address in which he directed NASA to de- velop a permanently manned space station within a decade. The space station that NASA is now planning in response to this directive will be 353 feet long. It will be resuppliable and capable of staying in orbit for about 30 years. Many of its systems will be highly automated, leaving the crew more time for the tasks that can be performed only by humans. The space station will be a multipurpose national research facility with the following functions: ⢠A laboratory in which long-term research and technology devel- opment studies can be conducted in the almost perfect vacuum and microgravity of space. ⢠A manufacturing facility to make pure Pharmaceuticals, perfect crystals, and exotic metal alloys. ⢠A laboratory for life sciences research conducted in microgravity. ⢠A permanent observatory for earth or stellar viewing in low earth orbit. The space station will be continuously habitable; it will depend on the space shuttle for launch and assembly as well as for crew rotation. As currently planned, it will take nearly 20 space-shuttle flights to assemble and outfit the space station in orbit. Initial plans call for a crew of four persons in the permanently manned phase of the opera- tional space station, supported by resupply and crew-rotation flights of the space shuttle every 3 months. Wash water and condensate will be purified and reused. Present planning calls for food and nitrogen to be periodically resupplied from earth. Other materials to be brought to the space station include oxygen to replace gradual loss through leakage, raw materials for materials processing, and propel- lants for engines needed to correct orbital decay; miscellaneous sup- plies also will be brought up every few months. The crew of the space station will live and operate primarily in four (or more) pressurized modules. The U.S. laboratory and habitation modules will be approximately 27 feet long and 15 feet in diameter. The modules will be interconnected by resource nodes and various
INTRODUCTION 11 support structures. At PMC (permanently manned capability) the pressure level inside will be the same as on earth (14.7 psi). The space station will operate at an altitude between I5O and 270 nautical miles. The space-station program involves international participation. NASA has signed memoranda of understandings with Canada, the European Space Agency, and Japan. These countries and space or- ganizations will work with NASA to design and build some com- ponents of the space station. Personnel from these countries also will be included in the space-station crews. The Space Station Freedom design currently comprises one module for habitation and three mod- ules for laboratories. All four of the modules will be interconnected. The space station will be an essentially closed and very complex environment; therefore, some contamination of its atmosphere is inev- itable. Several hundred chemicals have been found in samples of the atmosphere from space-shuttle missions. Even more chemicals are likely to be found in the space station. The contaminants come from humans, from materials making up the spacecraft, from operation of equipment, from the atmospheric gas supply, from food, from sup- plies, from experiments or manufacturing activities on the space sta- tion, and from reactions and interactions that occur between ordinarily nontoxic constituents within a closed space. As manned space mis- sions increase in duration, so will the severity and complexity of the contamination problem and the necessity for control and removal of these contaminants. Before an appropriate trace contaminant control system can be designed, the generation of atmospheric contaminants must be understood and their allowable exposure levels must be prop- erly defined. In response to NASA's request, this report was prepared to set forth guidelines for establishing SMACs for up to 180 days for potential atmospheric contaminants in the closed environments of the space station. Where appropriate knowledge exists, these guidelines take into account the pronounced physiological changes and stresses that occur in microgravity environments during spacef light and the behav- ior of the contaminants under these conditions. Some of the issues that should be addressed in developing these guidelines include the following: ⢠Converting animal or human toxicity data derived from in- halation, oral, or parenteral exposures to human 1-hr, 24-hr, 7-day, 30-day, and 180-day SMACs.
12 GUIDELINES FOR DEVELOPING SMACS ⢠Examining the issue of exposure to mixtures of chemicals (simul- taneous exposure to approximately 300 chemicals) and proposing max- imum exposure levels to these mixtures for the crew. ⢠Modifying the current methods for developing appropriate ex- posure criteria by taking into account the pronounced physiological changes and stresses that occur in the microgravity of space.
