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.

BENZENE

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