<P> Antiknock agents are classed as high - percentage additives, such as alcohol, and low - percentage additives based on heavy elements . Since the main problem with TEL is its lead content, many alternative additives that contain less poisonous metals have been examined . A manganese - carrying additive, methylcyclopentadienyl manganese tricarbonyl (MMT or methylcymantrene), was used for a time as an antiknock agent, though its safety is controversial and it has been the subject of bans and lawsuits . Ferrocene, an organometallic compound of iron, is also used as an antiknock agent although with some significant drawbacks . </P> <P> High - percentage additives are organic compounds that do not contain metals, but require much higher blending ratios, such as 20--30% for benzene and ethanol . It had been established by 1921 that ethanol was an effective antiknock agent, but TEL was introduced instead mainly for commercial reasons . Oxygenates such as TAME derived from natural gas, MTBE made from methanol, and ethanol - derived ETBE, have largely supplanted TEL . MTBE has environmental risks of its own and there are also bans on its use . </P> <P> Improvements to gasoline itself decrease the need for antiknock additives . Synthetic iso - octane and alkylate are examples of such blending stocks . Benzene and other high - octane aromatics can be also blended to raise the octane number, but they are disfavored today because of toxicity and carcinogenicity . </P> <P> TEL was supplied for mixing with raw gasoline in the form of ethyl fluid, which was TEL blended with the lead scavengers 1, 2 - dibromoethane and 1, 2 - dichloroethane . Ethyl fluid also contained a reddish dye to distinguish treated from untreated gasoline and discourage the use of leaded gasoline for other purposes such as cleaning . </P>

When did the us stop selling leaded gas