MTBE (Methyl Tertiary-Butyl Ether) is a fuel additive widely used in the US, and has been widely used since its introduction in 1979, when it was added to petrol in an attempt to make it burn cleaner and in doing so reduce the lead content to cut air pollution. Petrol with MTBE added is sold under the name of RFG (reformulated gas) and is the only type of fuel allowed in 17 states

MTBE works by increasing the oxygen content of the petrol and therefore increasing the octane number, the industry measure of how likely a particular "blend" resist detonation, a form of abnormal combustion. Detonation occurs when the air-fuel mixture reaches a temperature and/or pressure at which it can no longer keep from self igniting

The main problems with MTBE is the fact that it is a known animal carcinogen, and like all ethers, it is a hydrophilic chemical. This makes it 30 times more soluble in water than the other byproducts of a petrol burning engine, and via various means it has managed to enter the groundwater in many areas. Once in groundwater it gives it a turpentine like odour, and is very hard to filter out.