Sulfur dioxide (SO 2 ) enters the atmosphere through natural and anthropogenic processes. Mitigation of SO 2 emissions from many industrial activities has produced by-product sulfur and by-product synthetic gypsum essentially mineralogically identical to the primary materials extracted using mines and wells. Regulation to reduce anthropogenic SO 2 emissions was one of the first environmental protection efforts in the United States, which later became mandated under the Clean Air Act Amendments of 1990. The availability of by-product sulfur has increased over the years, and following the closure of the last domestic sulfur mine in 2000, it became the only domestic source of elemental sulfur in the United States. The most widely adopted means of reducing SO 2 emissions from coal-burning facilities has been to install flue gas desulfurization (FGD) equipment, which produces synthetic FGD gypsum. The decrease in SO 2 emissions since 1980 has significantly improved air quality in parts of the United States. By-products from these activities have replaced the supply of products, such as elemental sulfur, sulfuric acid (H 2 SO 4 ), and gypsum, through substitution of by-product for primary mining of these mineral commodities. The cascading effect of efforts in the United States to mitigate SO 2 emissions from multiple sources through the enactment of the Clean Air Act, and its amendments, has resulted in more than improved air quality alone. It has also, through the increasing availability of environmental products of SO 2 mitigation, such as by-product H 2 SO 4 , elemental sulfur, and by-product synthetic gypsum, reduced the environmental impacts of mining these materials from mineral deposits.