Gasoline is one of the most widely used and highest volume chemical mixture to which the public is exposed, which justifies intense public health scrutiny of its composition. The widespread and inherently dispersive use of fuel additives has resulted in significant global contamination, as well as occupational and environmental exposures. Fuel additives are used to improve gasoline performance in vehicles and to reduce specific emissions. They include octane enhancers, antiknock compounds and oxygenates, as well as corrosion inhibitors, detergents, and dyes. Notable in terms of public health are the antiknock additives tetra-ethyl lead (TEL) and the oxygenate MTBE—both of which have been added to gasoline and later banned or restricted because of their adverse health effects— and MMT, which has been the subject of controversy, especially in Canada.
Tetra-ethyl lead, which prevents precombustion in higher compression-ratio engines, was introduced in the United States in the 1920s and adopted worldwide by the 1950s. Although the toxic effects of lead exposure were well known by the 1920s, the potential for exposure and toxicity from this new use was not generally understood, although public health experts, including Alice Hamilton and Yandell Henderson, argued against the initial approval of TEL. By the early 1970s, when the U.S. Clean Air Act was passed, lead additives were increasingly recognized as a problem for two reasons. First, the catalytic converters adopted in the United States to reduce automotive emissions of CO, NOx, and VOCs were damaged by lead. Second, blood lead levels were found to be associated with lead in air, and a growing biomedical literature demonstrated that even low levels of lead adversely affected growth as well as neurobehavioral and intelligence development in children and increased risks of hypertension in adults. As a consequence, unleaded gasoline was introduced in the United States and the amount of lead in leaded gasoline was reduced in a series of steps during the 1970s and 1980s. Lead was finally eliminated from motor gasoline in the United States in 1995.
This phaseout revealed the true impact of lead in gasoline on public health. In the United States, population blood lead levels fell from about 15 micrograms per deciliter (mg/dL) in 1976 to about 9 mg/dL in 1980, and to about 3 mg/dL in 1990. Much of this decrease has been attributed to the elimination of lead from gasoline. By 1991, scientific consensus established that blood lead levels as low as 5 to 10 mg/dL have a range of health impacts, including detrimental effects on fetal and child development.
Lead additives were replaced by new gasoline mixtures, some with a higher content of the carcinogen benzene and by the additive MTBE (methyl tertiary butyl ether). In addition to its antiknock properties, MTBE is also an oxygenate—an oxygen-containing compound intended to reduce emissions of carbon monoxide and ozone-forming compounds. MTBE use in the United States increased greatly after 1990 to meet mandates to reduce carbon monoxide and ozone air pollution. However, data soon demonstrated that oxygenates are not very effective in reducing ozone. Moreover, MTBE, which is highly soluble in water, has been found to have contaminated drinking water supplies. In animals, MTBE causes cancer, and even at very low concentrations MTBE affects the taste of water. As of 2001, a number of states were acting to ban MTBE and the U.S. Environmental Protection Agency (EPA) had urged a substantial reduction in its use.
MMT (methyl cyclopentadienyl manganese tricarbonyl), another organometallic antiknock additive, has been widely used in Canada since 1978. Levels of manganese in urban air in Canada have increased as a consequence. Similar to lead, manganese is highly neurotoxic at high levels of exposure, but the effects of the long-term low-level exposures that would result from widespread use in gasoline are incompletely understood. In 1996, Canada banned MMT, but lifted its ban in 1998 after a challenge to its risk assessment. The EPA effectively prevented the introduction of MMT in the United States from the late 1970s through the mid-1990s, but it was overruled by the U.S. courts in 1995. As of 2000, MMT is used in less than 1 percent of U.S. gasoline; the automobile industry opposes its use because it could damage vehicle engines and emissions control systems.
The history of fuel additive use in the twentieth century reflects the interplay between engine and fuel technologies and public health concerns related to air pollution. Often, decisions to utilize
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