Sulfur dioxide (SO2) is a water soluble gas commonly emitted into ambient air by coal fired power plants, refineries, smelters, paper and pulp mills, and food processing plants. As you recall from Chapter 4, SO2is part of sulfurous air pollution that has plagued human populations for centuries. SO2was one of the primary pollutants elevated during the 1952 killer fog in London (discussed in detail in Ch 11). And SO2is a precursor for sulfuric acid, an air pollutant that plays a major role in the adverse respiratory effects of air pollution. This chapter summarizes what is known about the health effects of both SO2and sulfuric acid through a review of controlled and epidemiologic studies.