Sulfur dioxide is formed when materials containing sulfur are burned. It is a primary air pollutant emitted by smelters and electrical power plants that burn coal or oil. Sulfur dioxide is found at concentrations of 1–10 parts per billion (ppb) in clean ambient air, and at 20–200 ppb in polluted air (Seinfeld 1986). Sulfur dioxide also is used in treating wood pulp for paper manufacturing; in ore and metal refining; in extraction of lubricating oils; as a bleaching, disinfecting, and fumigating agent; as a food additive and preservative; and as a reducing agent. Sulfur dioxide is a precursor to acid sulfates, which generally are more toxic; therefore, recent research has focused on those compounds (Costa and Amdur 1996).

Sulfur dioxide is primarily an upper airway and eye irritant. In the airways, it produces bronchoconstriction and mucous secretion. Because of its high water