Sulfur oxides—notably sulfur dioxide—are among the most dangerous and irritating of all air pollutants. Factories and electric power plants that use sulfur-containing coal or oil as fuel are major sources of sulfur oxides. In the air, some sulfur dioxide is converted to sulfuric acid, which is then deposited on the earth's surface, usually by rain. Such acid rain has caused serious damage to the environment in many parts of the world.