An antiknock agent is a gasoline additive used to reduce engine knocking and increase the fuel's octane rating by raising the temperature and pressure at which auto-ignition occurs. The mixture known as gasoline or petrol, when used in high compression internal combustion engines, has a tendency to knock (also called "pinging" or "pinking") and/or to ignite early before the correctly timed spark occurs (pre-ignition, refer to engine knocking). Notable early antiknock agents, especially Tetraethyllead, added to gasoline included large amounts of toxic lead. The chemical was responsible for global negative impacts on health, and the phase out of leaded gasoline from the 1970s onward was reported by the UN to be responsible for $2.4 trillion in annual benefits, 1.2 million fewer premature deaths, higher overall intelligence and 58 million fewer crimes," the United Nations Environmental Programme said. Some other chemicals used as gasoline additives are thought to be less toxic. Research Early research was led by A.H. Gibson and Harry Ricardo in England and Thomas Midgley, Jr. and Thomas Boyd in the United States. The discovery that lead additives modified this behavior led to the widespread adoption of the practice in the 1920s and therefore more powerful higher compression engines. The most popular additive was tetraethyllead. However, with the discovery of the environmental and health damage caused by the lead, attributed to Derek Bryce-Smith and Clair Cameron Patterson, and the incompatibility of lead with catalytic converters found on virtually all US automobiles since 1975, this practice began to wane in the 1980s. Most countries are phasing out leaded fuel although different additives still contain lead compounds. Other additives include aromatic hydrocarbons, ethers and alcohol (usually ethanol or methanol). Typical agents Typical agents that have been used for their antiknock properties are: Tetraethyllead (still in use as a high octane additive) MTBE Ethanol Methylcyclopentadienyl manganese tricarbonyl (MMT) Ferrocene Iron pentacarbonyl Toluene Isooctane BTEX - a hydrocarbon mixture of benzene, toluene, xylene and ethyl-benzene, also called gasoline aromatics Xylidine - any of a number of isomeric amines of xylene. Tetraethyllead In the U.S., where tetraethyllead had been blended with gasoline (primarily to boost octane levels) since the early 1920s, standards to phase out leaded gasoline were first implemented in 1973. In 1995, leaded fuel accounted for only 0.6% of total gasoline sales and less than 2,000 tons of lead per year. From January 1, 1996, the Clean Air Act banned the sale