In setting primary ambient air quality standards, EPA's responsibility under the law is to establish standards that protect public health, regardless of the costs of implementing a new standard. The Clean Air Act requires EPA, for each criteria pollutant, to set a standard that protects public health with "an adequate margin of safety." As interpreted by the Agency and the courts, the Act requires EPA to create standards based on health considerations only. This book provides illustrative estimates of the incremental costs and monetised human health benefits of attaining a revised short-term Sulphur Dioxide (SO2) National Ambient Air Quality Standard (NAAQS) within the current monitoring network.