This smelly substance, a gas with the distinctive odor of rotten eggs, is produced as a byproduct of burning sulfur-containing fossil fuels such as coal and natural gas. This substance changes into highly corrosive sulfuric acid when exposed to water vapor suspended in the air, and when it falls back to earth as acid rain, it can cause significant damage to virtually everything in the environment. Most airborne sulfur dioxide is produced by coal-fired generating plants