Its major use is a captive intermediate in the production of sulfuric acid. In the pulp and paper industry, sulfur dioxide is used to produce other chemicals such as chlorine dioxide and sodium hyposulfite and is also used in the bleaching of pulp. In food processing, sulfur dioxide is used for fumigating, preserving, bleaching and steeping. It is also used to reduce residual chlorine in potable water, treated sewage and industrial effluent, as an oxygen scavenger, a selective extraction solvent and as a catalyst in chemical processes. In the presence of a catalyst (e.g. nitrogen compounds), sulfur dioxide can be oxidized to sulfuric acid. Occurs as a by-product during the burning (combustion) of sulfur containing organic compounds (e.g. coal). A common component of air pollution, it is a major contributor to acid rain.