Tetraethyllead

Tetraethyllead (commonly styled tetraethyl lead), abbreviated TEL, is an organolead compound with the formula Pb(C₂H₅)₄. It was widely used as a fuel additive for much of the 20th century, first being mixed with gasoline beginning in the 1920s. This "leaded gasoline" had an increased octane rating that allowed engine compression to be raised substantially and in turn increased vehicle performance and fuel economy. TEL was first synthesized by German chemist Carl Jacob Löwig in 1853. American chemical engineer Thomas Midgley Jr., who was working for the U.S. corporation General Motors, was the first to discover its effectiveness as an knock inhibitor on December 9th, 1921, after spending six years attempting to find an additive that was both highly effective and inexpensive.

Of the some 33,000 substances in total screened, lead was found to be the most effective antiknock agent, in that it necessitated the smallest concentrations necessary; a treatment of 1 part TEL to 1300 parts gasoline by weight is sufficient to suppress detonation. The four ethyl groups in the compound served to dissolve the active lead atom within the fuel. When injected into the combustion chamber, tetraethyllead decomposed upon heating into ethyl radicals, lead, and lead oxide. The lead oxide scavenges radicals and therefore inhibits a flame from developing until full compression has been achieved, allowing the optimal timing of ignition, as well as the lowering of fuel consumption. Throughout the sixty year period from 1926 to 1985, an estimated 20 trillion liters of leaded gasoline at an average lead concentration of 0.4 g/L were produced and sold in the United States alone, or an equivalent of 8 million tons of inorganic lead, three quarters of which would have been emitted in the form of lead chloride and lead bromide. Estimating a similar amount of lead to have come from other countries' emissions, a total of more than 15 million tonnes of lead may have been released into the atmosphere.

In the mid-20th century, scientists discovered that TEL caused lead poisoning and was highly neurotoxic to the human brain, especially in children. Approximately 90% of the total lead in a human is present in the bones, deposited in the form of insoluble Lead(II) phosphate salt, with a half-life longer than twenty years. The United States and many other countries began phasing out the use of TEL in automotive fuel in the 1970s. With EPA guidance and oversight, the US achieved the total elimination of sales of leaded gasoline for on-road vehicles on January 1st, 1996. By the early 2000s, most countries had banned the use of TEL in gasoline. In July 2021, the sale of leaded gasoline for cars was completely phased out worldwide following the termination of production by Algeria, prompting the United Nations Environment Program (UNEP) to declare an "official end" of its use in cars on August 30, 2021.

In 2011, researchers retroactively estimated the annual impact of tetraethyl lead worldwide to be the following: 1.1 million excess deaths, 322 million lost IQ points, 60+ million crimes, and 4% of worldwide GDP (around 2.4 trillion United States dollars per year).