Ethylene, a naturally occurring substance found in coal gas, is a colorless gas with an odor similar to that of ether. Often synthesized from petroleum hydrocarbons, ethylene has derivatives that are used to produce various rubbers, plastics, detergents, and antifreeze. Though utilized widely as an anesthetic before the discovery of halothane, the high reactivity between ethylene and oxygen has caused deaths due to the resulting explosive mixtures. Ethylene has rarely been used as an anesthetic since the 1970s.