Sulfur in crude oil is an impurity. It can give off the smell of rotten eggs when found as hydrogen sulfide. It can also be found as elemental sulfur. Sulfur can go on to become a secondary air pollutant when the sulfur combines with water in the atmosphere producing sulfuric acid. This is a compound that is found in acid rain and is very corrosive. In order to process crude oil into gasoline, sulfur is removed first. Sulfur in fuels can lead to respiratory problems, asthma, chronic fatigue syndrome and can affect the sense of smell. As pollution laws have become stricter in the United States, the gasoline products have become much cleaner.