A comparison of air quality specifications from the first and second editions of G. Thomson's The Museum Environment and from the Nnational Research Council's Preservation of Historical Records (1986) shows a high degree of consensus on standards but less agreement on how to achieve them. Temperature, relative humidity, sulphur dioxide, norogen oxides, ozones, and particulates are discussed. A trend towards best available technology technology for gaseous polllutant removal and more stringent particulate removal for hivas systems in major collections is clear. More problematic are standards for smaller institutions. Tables references.