Although outdoor air pollution has been recognized as a threat to cultural property for many years, damage to works of art from indoor air pollutants is now receiving attention. In this study, samples of air from inside and immediately outside nine southern California museums was analyzed for a number of pollutants. A database consisting of some 4300 analyses of peroxyacetyl nitrate (PAN) and chlorinated hydrocarbons, 1100 analyses of nitrogen dioxide, and 180 analyses of nitric acid. Ozone, formaldehyde, and particulate matter had been measured in previous Getty Conservation Institute-sponsored research. The measured pollutants could be detected indoors at all locations. At all but one location, concentrations of nitrogen dioxide and PAN were comparable to outdoor levels. At six of nine locations, indoor chlorinated hydrocarbons exceeded outdoor levels, suggesting existence of indoor sources. Indoor levels of most pollutants followed outdoor levels, indicating rapid air exchange. Air conditioning, which removes ozone effectively, has little effect on pollutants measured in this study. Of three locations equiped with chemical filtration, only one yielded the expected low indoor pollutant levels. Ozone damages museum collections, as does nitrogen dioxide to a limited extent. The effect of nitric acid is currently being studied by GCI. The effects of PAN, chorinated hydrocarbons, and other pollutants remain to be investigated. -- CAL