Indoor pollutants can damage cultural property stored in museums, art galleries, libraries and archives. The paper reviews the atmospheric substances of concern and their sources. A relatively small number of measurements available for the interiors are restricted to just a few gases. The review draws attention to the lack of information about the key factors that control the composition of museum interiors, chemical transformations of pollutants in indoor air, the nature of emissions from materials used in creating displays and deposition velocities onto indoor surfaces.