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 convened 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. 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. This report, Volume 4, covers 15 compounds: acetone, C3 to C8 aliphatic saturated aldehydes, hydrogen chloride, isoprene, methylhyrazine, perfluoropropane and other aliphatic perfluoroalkanes, polydimethylcyclosiloxanes, dichlorofluoromethane (Freon 21), chlorodifluoromethane (Freon 22), trichlorofluoromethane (Freon 11), dichlorodifluoromethane (Freon 12), 4-methyl-2-pentanone, chloroform, furan, and hydrogen cyanide.
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 concludes that the SMAC reports on 15 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.
