When sulfur and oxygen are combined, they form a variety of compounds, including sulfur dioxide (SO2) and sulfur trioxide (SO3). Sulfur dioxide is a colorless, irritating gas with a pungent odor. It is produced by the combustion of fossil fuels and is a major contributor to air pollution. Sulfur trioxide is a colorless, fuming liquid that is highly corrosive. It is used in the production of sulfuric acid, which is one of the most important industrial chemicals.