The National Aeronautics and Space Administration (NASA) has measured numerous airborne contaminants in spacecraft during space missions because of the potential toxicological hazards to humans that might be associated with prolonged spacecraft missions. This volume reviews the spacecraft maximum allowable concentrations (SMACs) for various contaminants to determine whether NASA's recommended exposure limits are consistent with recommendations in the National Research Council's 1992 volume Guidelines for Developing Spacecraft Maximum Allowable Concentrations for Space Station Contaminants.