From Ohio History Central
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Unfortunately, most Ohio coal has about 3.5 percent sulfur, a compound that forms sulfur dioxide when the coal is burned. Most swamps in which Ohio coal formed were near the sea. Periodically, sea water flooded the swamps resulting in the formation of pyrite (iron sulfide) and other sulfur compounds. The Clean Air Act of 1970, and later amendments, caused a rapid decline in production of Ohio coal from the all-time annual high of 55 million tons in 1970. Coal-cleaning techniques to remove sulfur compounds and techniques to remove sulfur dioxide from power-plant emissions have helped to keep Ohio coal a marketable commodity. About 25,000 tons are produced annually.
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