<P> TEL remains an ingredient of 100 octane avgas for piston - engine aircraft . The current formulation of 100LL (low lead, blue) aviation gasoline contains 2.12 grams of TEL per gallon, half the amount of the previous 100 / 130 (green) octane avgas (at 4.24 grams per gallon), but only slightly less than the 2.2 grams per gallon historically permitted in automotive leaded gasoline and substantially greater than the allowed 0.001 grams per gallon in automotive unleaded gasoline sold in the United States today . The United States Environmental Protection Agency, FAA, and others are working on economically feasible replacements for leaded avgas, which still releases 100 tons of lead every year . </P> <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>

When was lead removed from petrol in australia