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Methyl tertiary butyl ether (MTBE)
A substance that can be added to gasoline to increase its oxygen content and thereby make it burn more cleanly. MTBE has been used in some areas of the US since 1990, from which time the Clean Air Act required the use of gasoline with a 2% oxygen content in areas of high pollution. More recently, MTBE has been linked to water pollution incidents and is being phased out in favour of ethanol in many areas.