INTRODUCTION The chemical form of@ sulfur oxide or oxides in the ambient air cab be associated by means of epidemiologic studies with morbidity and mortality has not yet been identified. Various sulfur containing products have been implicated. These have included sulfur dioxide@ sulfuric acid and a number of inorganic sulfates such as ammonium sulfate. The major source of the sulfur compounds has been the burning of fossil fuels containing sulfur. Other more localized sources have been smelters and pulp mills. The former produces sulfur compounds in the process of recovering the metal from the ores where they occur as sulfides. Pulp mills employing the Kraft process can emit considerable amounts of inorganic particulates usually as sodium sulfate. The sulfur dioxide that is emitted may remain in the ambient air for a few days. During this time it can be converted to sulfuric acid and sulfates by sunlight@ photochemical oxidants or by catalytic action of various particulates in the air. These are complex reactions that are not yet fully understood. Thus@ the sulfur dioxide that is emitted into the ambient air provides a reservoir from which more toxic sulfates and sulfuric acid can be generated. Throughout the United States the average ambient air concentrations of sulfur dioxide have been decreasing for the past few years as a result of using fossil fuels with a lower sulfur content. On the other hand the concentration of suspended sulfates in the ambient air has not fallen in a parallel fashion and the sulfate levels are highest over the northeast section of the United States.