Sulfur dioxide, or SO₂, belongs to the family of sulfur oxide gases (SOx).  These gases dissolve easily in water.  Sulfur is prevalent in all raw materials, including crude oil, coal, and ore that contains common metals like aluminum, copper, zinc, lead, and iron. SOx gases are formed when fuel containing sulfur, such as coal and oil, is burned, or when gasoline is extracted from oil refinement, or when metals are extracted or isolated from ore. SO₂ dissolves in water vapor to form Sulfuric acid, which can interact with other gases and particles in the air to form sulfates and other products, that can be harmful to people and their environment.

Over 65% of SO₂ released to the air, or more than 13 million tons per year, comes from electric utilities, especially those that burn coal.  Other sources of SO₂ are industrial facilities that derive their products from raw materials like metallic ore, coal, and crude oil, or that burn coal or oil to produce process heat, as in Power generation. Examples are: petroleum refineries, cement manufacturing, and metal processing facilities. Also, locomotives, large ships, and some nonroad diesel equipment currently burn fuel, containing moderate to high amounts of sulfur and emit SO₂ into the air (atmosphere) in large quantities.