Chart 6 - Ambient Sulfur Dioxide

Sulfur dioxide (SO2) is a colorless gas that forms from the burning of fuel containing sulfur, mainly coal and oil, as well as from industrial and manufacturing processes, particularly the generation of electrical power. Environmental factors such as temperature inversion, wind speed, and wind concentration also affect SO2 levels. Ambient levels of sulfur dioxide decreased 66 percent between 1976 and 1997, and the United States has met the EPA's designated "good" category since 1981.