Oxygenates increase the octane rating in gasoline. Oxygenates include methyl tertiary butyl ether (MTBE), ethyl tertiary butyl ether (ETBE), tertiary amyl methyl ether (TAME), and ethanol. Gasoline burns cleaner when oxygenates are added, resulting in lower tailpipe emissions. Up until the late 1970s and early 1980s, tetraethyl lead was primarily used to increase octane levels. However, lead is toxic and shortened the life of catalytic converters by destroying the catalyst inside. After lead was phased out MTBE was primarily used, but it has also been banned in many places. MTBE has been found to contaminate drinking water from leaking gasoline storage tanks. Now ethanol use has significantly increased to replace MTBE. Often up to 10% of gasoline is ethanol. Ethanol is a grain alcohol, commonly made from corn or other starch-rich grains. When gasoline and ethanol are mixed, it is called gasohol. Gasohol burns cleaner, emitting less air pollutants and greenhouse gases than 100% gasoline.