NASA has unique requirements for the development and application of air quality standards for human space flight. Such standards must take into account the continuous nature of exposures, the possibility of increased susceptibility of crewmembers to the adverse effects of air pollutants because of the stresses of space flight, and the recognition that rescue options may be severely limited in remote habitats. NASA has worked with the National Research Council Committee on Toxicology (NRCCOT) since the early 1990s to set and document appropriate standards. The process has evolved through 2 rounds. The first was to set standards for the space station era, and the second was to set standards for longer stays in space and update the original space station standards. The update was to be driven by new toxicological data and by new methods of risk assessment for predicting safe levels from available data. The last phase of this effort has been completed. The final version of air quality standards for exposure durations from 1 hour to 1000 days is presented for selected interesting compounds. My goal is to convey to the community of vehicle engineers, medical specialists, payload providers, and safety experts how the air quality values are designed to be used. These values are called spacecraft maximum allowable concentrations (SMACs).