Sulfur dioxide (SO₂) enters the atmosphere through natural and anthropogenic processes. Mitigation of SO₂ 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 SO₂ 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...