Exposure Limits for Airborne Contaminants in Spacecraft Atmospheres

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Abstract
Toxicologists at Johnson Space Center, in cooperation with the National Research Council Committee on Toxicology, have been setting limits for airborne toxicants for exposure periods from 1 hour to 180 days to protect astronaut health and performance during spaceflights. Limits are used to provide a database for designing spacecraft air revitalization systems, assessing health risks based on analytical measurements obtained during spaceflights, and acceptance testing hardware that out-gasses into the spacecraft atmosphere. Chemical contaminants can accumulate in the air over long periods, or they can be released suddenly from a variety of sources. The International Space Station will be the first spacecraft equipped for onboard analysis of a broad range of airborne chemical contaminants; hence, exposure standards must be available to interpret the measurements and, if necessary, to change operational procedures. Unique limits are required because astronauts are exposed continuously to airborne chemicals, and they may be more susceptible to certain toxicants because of spaceflight-induced physiological effects. Further, the adverse effects acceptable to NASA differ from those of other agencies. Exposure limits are set on the basis of well-documented human or animal studies involving cumulative exposures of appropriate lengths. Short-term limits are set to prevent effects that could degrade performance; however, mild, transient effects such as headache or irritation are considered acceptable risks. Long-term limits are set sufficiently low that noncancer adverse effects do not pose appreciable risks, and the risk of acquiring cancer as a result of chemical exposure is limited to <0.01 percent per mission. The spaceflight-induced effects that have caused limits to be set lower than for Earth-based populations include loss of red cell mass, cardiac rhythm disturbances, immune function changes, and radiation exposure. At the writing of this review article, a set of limits has been established for approximately 40 compounds, and documentation of 23 of these limits can be obtained from the National Research Council.