<unk>  (commonly styled<unk> , abbreviated<unk> , is an organolead compound with the formula Pb(C2H5)4. It is a fuel additive, first being mixed with gasoline beginning in the 1920s as a patented octane rating booster that allowed engine compression to be raised substantially. This in turn increased vehicle performance and fuel economy.<unk>  had been identified chemically in the mid-19th century, but General Motors had discovered its effectiveness as an antiknock agent in 1921, after spending several years attempting to find an additive that was both highly effective and inexpensive. Concerns were later raised over the toxic effects of lead, especially on children. Lead and lead oxides also poison catalytic converters and are a major cause of spark plug fouling. From the 1970s many countries began phasing out and eventually banning<unk>  in automotive fuel. In 2011, a study backed by the United Nations estimated that the removal of<unk>  had resulted in $2.4 trillion in annual benefits, and 1.2 million fewer premature deaths.<unk>  is still used as an additive in some grades of aviation gasoline. Innospec has claimed to be the last firm legally making<unk> , and ,<unk>  was being produced illegally by several companies in China. In July 2021, the sale of leaded gasoline for cars was completely phased out worldwide, prompting the United Nations Environment Programme (UNEP) to declare an "official end" of its use in cars on August 30 (although it is still available in aviation fuel for light aircraft).
Tetraethyllead
Tetrachlorodecaoxide
Hydrogen fluoride
Lewis lead
Nickel tetracarbonyl
Selenium disulfide
Dichloroacetic acid
Triphenylchloroethylene