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 periods are developed, and a rationale is provided for each recommendation. One- or 24-hr emergency SMACs are usually derived from acute exposure toxicity studies. 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 (C × T = k) when applicable. Detoxification or recovery and data available on 24-hr exposures are taken into account in modifying Haber's law.
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 assessment is done, and the SMAC is set so that the estimated 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.
In the second phase of the study, the Subcommittee on Spacecraft Maximum Allowable Concentrations reviewed reports for 35 spacecraft contaminants to determine whether the recommended exposure limits were consistent with the 1992 guidelines (see Appendix A). One of the SMAC reports was prepared with the collaboration of members of this subcommittee to serve as an example for NASA scientists in preparing their reports on other substances. The remaining SMAC reports were prepared solely by NASA scientists or contractors.
These SMAC reports are intended for use by engineers in develop-