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Agency for Toxic Substances and Disease Registry
Case Studies in Environmental Medicine (CSEM)
Benzene Toxicity
Exposure Pathways
Benzene (C6H6) is the first member of a series of aromatic hydrocarbons recovered from refinery streams during catalytic reformation and other petroleum processes. It is a clear, colorless, highly flammable liquid at room temperature. Its vapor is heavier than air and can travel to a source of ignition and flash back. It has a pleasant, aromatic odor detectable at concentrations of 1.5 to 4.7 parts per million (ppm). (The workplace permissible exposure level [PEL] is 1 ppm). Common synonyms for benzene include benzol, cyclohexatriene, phenyl hydride, and coal tar naphtha.
Benzene is one of the world's major commodity chemicals. Its primary use (85% of production) is as an intermediate in the production of other chemicals, predominantly styrene (for styrofoam and other plastics), cumene (for various resins), and cyclohexane (for nylon and other synthetic fibers). Benzene is an important raw material for the manufacture of synthetic rubbers, gums, lubricants, dyes, pharmaceuticals, and agricultural chemicals. Benzene is a natural component of crude and refined petroleum. The mandatory decrease of
lead alkyls in gasoline has led to an increase in the aromatic hydrocarbon content of gasoline to maintain high octane levels and antiknock properties. In the United States, gasoline typically contains less than 2% benzene by volume, but in other countries the benzene concentration may be as high as 5%.
Because of its lipophilic nature, benzene is an excellent solvent. Its use in paints, thinners, inks, adhesives, and rubbers, however, is decreasing and now accounts for less than 2% of current benzene production. Benzene was also an important component of many industrial cleaning and degreasing formulations, but now has been replaced mostly by
toluene, chlorinated solvents, or mineral spirits. Although benzene is no longer added in significant quantities to most commercial products, traces of it may still be present as a contaminant.
Benzene is widespread in the environment. Airborne benzene is usually produced by processes associated with chemical manufacturing or the gasoline industry, including gasoline bulk-loading and discharging facilities and combustion engines (e.g., automobiles, lawn mowers, and snow blowers). Benzene is a component of both indoor and outdoor air pollution. Benzene levels measured in ambient outdoor air have a global average of 6 micrograms per cubic meter (µg/m3) (range 2-9 µg/m3). In almost all cases, benzene levels inside residences or offices are higher than levels outside and still higher in homes with attached garages and those occupied by smokers. Seasonal variations also affect benzene levels, with higher levels found in the fall and winter when buildings are less well ventilated. People living around hazardous waste sites, petroleum-refining operations, petrochemical manufacturing sites, or gas stations may be exposed to higher levels of benzene in air. In addition to being inhaled, airborne benzene is absorbed across intact skin in experimental animals. For most people, the level of exposure to benzene through food, beverages, or drinking water is not as high as their exposure through air.
Leakage from underground storage tanks and seepage from landfills or improper disposal of hazardous wastes has resulted in benzene contamination of groundwater used for drinking. Effluent from industries is also a source of groundwater contamination. In addition to being ingested, benzene in water can also be absorbed through wet skin and inhaled as it volatilizes during showering, laundering, or cooking. Typical drinking water contains less than 0.1 parts per billion (ppb) benzene. Benzene has been detected in bottled water, liquor, and food.
Cigarette smoke is another common source of personal and environmental benzene exposure, representing about half of the benzene to which the general population is exposed. Persons who smoke one pack of cigarettes a day inhale a daily dose of approximately 1 milligram (mg) of benzene, about 3 to 4% of the amount inhaled daily by a worker exposed at the current occupational PEL. Nonsmokers who live with smokers and who are passively exposed to environmental tobacco smoke typically experience 50% greater exposure to benzene than do nonsmokers who live in a smoke-free environment.
- Later, the patient in the case study tells you that his well water has always tasted "funny" and smells like "solvent." You learn that a chemical plant was near his property until 9 years ago, when the company moved. You are concerned about your patient's description of his drinking water, and you request that the state health department investigate the problem. The investigator contacts the chemical company that owns the abandoned site and learns that benzene is stored at the site in tanks that are above and below ground. Laboratory analyses of the patient's well water reveal an average concentration of 20 ppm benzene and traces of 1,1,1-trichloroethane and toluene.
What questions will you ask to gauge the extent of the patient's exposure to benzene?
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