The purpose of this document is to review and evaluate available information on the biological effects of sulfur oxides and suspended particulate matter including suspended sulfates and sulfuric acid aerosols, and to provide a scientific basis for decisions aimed at the protection of human health from the adverse consequences of exposure to these substances in both occupational and general environments. Although there are various routes of exposure, such as inhalation, ingestion (World Health Organization, 1971, 1974) and contact with skin, attention in this report has been concentrated upon the effects of inhalation of these substances, since this is the most important route of exposure. The discussion has also been limited to sulfur dioxide, sulfur trioxide, sulfate ions, and particulate matter primarily resulting from the combustion of fossil fuels. The sulfate ion has been considered in the variety of forms in winch it occurs in the atmosphere, e.g., sulfuric acid and various sulfate salts.