Sulphur dioxide is produced by the combustion of fossil fuels, such as coal and oil, which contain sulphur impurities. When these fuels are burned, the sulphur in the fuel reacts with oxygen in the air to form sulphur dioxide gas. Sulphur dioxide is also a product of the smelting of certain metals, such as copper, nickel, and zinc. In addition, it is released naturally by volcanoes and during forest fires.