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Benzene

BENZENE

Benzene is a ubiquitous component of the petrochemical era. Present in crude petroleum, benzene is produced from the combustion of fossil fuels. It has been known to cause toxicity to human bone marrow since the late nineteenth century, at high levels destroying the bone marrow machinery responsible for the production of mature red blood cells, white blood cells, and platelets. When severe, this is a frequently fatal condition known as aplastic anemia. Lesser levels of benzene exposure result in sufficient bone marrow destruction to cause partial decrements in the levels of circulating blood cells, a condition known as pancytopenia.

Benzene is also a known cause of acute myelogenous leukemia, the adult form of acute leukemia, and a more than probable cause of other forms of blood and bone marrow cancers, including non-Hodgkin's lymphoma, multiple myeloma, and acute lymphatic leukemia, the childhood form of acute leukemia. There are recent indications that subpopulations vary in their susceptibility to benzene toxicity based upon their metabolic capabilities.

In the latter half of the twentieth century there was a dramatic decline in the allowable levels of benzene at the workplace. To protect workers from this known human carcinogen, the United States permissible standard progressively decreased from 100 parts per million (ppm) to 1 ppm on an eight-hour time-weighted average. There was a corresponding fall in shorter term exposure limits, and an increase in requirements for industrial hygiene monitoring and in the use of respirators and other personal protective equipment. Even more stringent standards have been proposed.

At high concentrations, well above 100 ppm, benzene is also a central nervous system anesthetic-like agent. This effect is due to its solubility in lipids and its other physicochemical characteristics, and it is predictable based upon what is known about analogous compounds such as toluene and xylenes. In contrast, the bone marrow toxicity of benzene is a result of its metabolism and this toxicity does not occur with toluene, xylenes, and other related compounds that are metabolized differently. In fact, at high concentrations toluene is known to protect against the bone marrow toxicity of benzene because it occupies the metabolic machinery that otherwise would produce toxic benzene metabolites. However, concentrations in the general environment are too low to produce this result. For benzene, outdoor environmental exposure is usually in the 1 to 5 parts per billion (ppb) range in the United States. Benzene levels from natural sources are negligible in comparison.

For most nonsmoking individuals in the general population, it is indoor exposure that is the most dominant source of benzene, often reflecting the storage of gasoline or of benzene-containing consumer products within the home. Gasoline in the United States contains about 1 to 2 percent benzene, and higher levels are present elsewhere. Cigarette smokers inhale benzene directly in tobacco smoke, causing contamination of indoor air with benzene that is then inhaled by nonsmokers. Drinking water supplies are sometimes contaminated with benzene, most frequently from leaking underground petroleum storage tanks. This can also lead to inhalation of benzene through offgassing from contaminated water during cooking or showering. Skin absorption can occur in those working with products that contain benzene, as well as during the refueling of automobiles with gasoline.

As with other cancer-causing agents, it is unclear what level of exposure, if any, can be considered completely safe, or what level might be certain to cause cancer. As benzene is a component of gasoline, a useful solvent, and an organic building block in many chemical reactions, it cannot simply be banned. However, there have been many actions taken to decrease the extent to which the general population is exposed to benzene from gasoline and from industrial effluents.

Bernard D. Goldstein

(see also: Cancer; Carcinogen; Carcinogenesis; Environmental Tobacco Smoke; Fuel Additives; Groundwater Contamination; Hazardous Air Pollutants; One-Hit Model )

Bibliography

Goldstein, B. D., and Witz, G. (1999). "Benzene." In Environmental Toxicants: Human Exposures and Their Health Effects, 2nd edition, ed. M. Lippman. New York: John Wiley.

Krewski, D., and Snyder, R., eds. (2000). "Assessing the Health Risks of Benzene: A Report on the Benzene State-of-the-Science Workshop." Journal of Toxicology and Environmental Health 61:307338.

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benzene

benzene (bĕn´zēn, bĕnzēn´), colorless, flammable, toxic liquid with a pleasant aromatic odor. It boils at 80.1°C and solidifies at 5.5°C. Benzene is a hydrocarbon, with formula C6H6. The simplest picture of the benzene molecule, proposed by the German chemist Friedrich Kekulé (1865), is a hexagon of six carbon atoms joined by alternating single and double bonds and each bearing one hydrogen atom, symbolized by [symbol]. However, modern studies have shown that the six carbon-carbon bonds are all of equal strength and distance; thus the double-bond electrons do not belong to any particular bonds but rather are delocalized about the ring, with the result that the strength of each bond is between that of a single bond and that of a double bond (see chemical bond).

Benzene is the parent substance of the aromatic compounds, a large and important group of organic compounds. It is the first of a series of hydrocarbons known as the benzene series, formed by the substitution of methyl groups, CH3, for the hydrogen atoms of the benzene molecule. The second member of the series is toluene, C6H5CH3, from which trinitrotoluene is derived, and the third member is xylene, C6H4(CH3)2, a solvent. In xylene and other benzene derivatives in which two of the hydrogens have been replaced, there are three possible arrangements of the substitution groups; in the ortho (o) configuration the groups are on adjacent carbon atoms, in the meta (m) configuration the groups are separated by one carbon atom, and in the para (p) configuration the groups are on opposite sides of the ring. The three forms of xylene (dimethylbenzene) are shown below:

In addition to derivatives formed by the substitution of other groups for one or more of the hydrogen atoms of the benzene ring, two or more rings may be joined together, as in naphthalene, anthracene, and phenanthrene; or other atoms, such as nitrogen, may be substituted for carbon atoms in the ring, as in pyridine (C5H5N) and pyrimidine (C4H4N2). Among the important derivatives of benzene are phenol, aniline, and picric acid. Benzene and the other aromatic hydrocarbons are obtained for industrial purposes from the distillation of coal tar, a byproduct in the manufacture of coke, and from petroleum by special reforming methods. They are used in the manufacture of plastics, synthetic rubber, dyes, and drugs. Benzene is a known carcinogen.

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benzene

benzene Colourless, volatile, sweet-smelling, flammable liquid hydrocarbon (C6H6), a product of petroleum refining. A benzene molecule is a hexagonal ring of six unsaturated carbon atoms (benzene ring). It is a raw material for manufacturing many organic chemicals and plastics, drugs and dyes. Properties: r.d. 0.88; m.p. 5.5°C (41.9°F); b.p. 80.1°C (176.2°F). Benzene is carcinogenic and should be handled with caution.

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benzene

ben·zene / ˈbenˌzēn; benˈzēn/ • n. a colorless volatile liquid hydrocarbon, C6H6, present in coal tar and petroleum, used in chemical synthesis.

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benzene

benzene aromatic liquid hydrocarbon XIX. f. BENZOIC acid, whence it is derived; earlier benzine (now used for a mixture of petroleum hydrocarbons).
So benzol (esp. unrefined) benzene.

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benzene

benzene (ben-zeen) n. a toxic liquid hydrocarbon. Continued inhalation of benzene vapour may result in aplastic anaemia or a form of leukaemia. Formula: C6H6.

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benzene

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