What Is Benzene?

Benzene is a colorless, flammable liquid with a sweet odor. It is a volatile chemical that evaporates quickly. Benzene is formed from natural processes, such as volcanoes and forest fires, as well as from human activities. Benzene is even a component of cigarette smoke. Benzene is widely used in the United States and ranks among the top 20 chemicals produced. Benzene is primarily used as a solvent, as a starting material for the synthesis of other chemicals and as a gasoline additive. It is found in a number of hazardous waste sites in the United States identified by the Environmental Protection Agency (EPA).

How Are People Exposed to Benzene?

People are exposed to benzene in 3 ways: at work, in the general environment, and through the use of some consumer products. The greatest risk for exposure to high doses of benzene occurs from workplace exposures, but the most common exposure to lower doses of benzene occurs in the general environment. The 2 routes of exposure to benzene are inhalation and skin absorption. However since liquid benzene evaporates quickly, skin absorption, which requires contact with a source such as gasoline, is less common. Therefore, inhalation of contaminated air is the primary route of exposure.

Occupational exposures. Workers in industries that make or use benzene may be exposed to high levels of this chemical. These industries include the rubber industry, oil refineries, chemical plants, shoe manufacturers, and gasoline related industries. In 1987, OSHA estimated that about 237,000 workers in the United States were potentially exposed to benzene. It is not known if this number has changed since that time.

Environmental exposures. Sources of benzene in the environment include gasoline, automobile exhaust fumes, cigarette smoke, emissions from coke ovens and other industrial processes, and waste water from certain industries. While benzene is commonly found in air in both urban and rural areas, the levels are usually very low. Areas of heavy vehicular traffic, gasoline stations, and areas near industrial sources may have higher air levels. Cigarettes have been found to release between 50 and 150 micrograms of benzene per cigarette, so smoking and second-hand smoke are important sources of exposure to benzene. Cigarette smoke accounts for about half of the US national exposure to benzene and for about 89% of benzene exposure among smokers. Secondhand smoke accounts for 10% of benzene exposure among nonsmokers. Benzene has also been identified in contaminated water and food.

Consumer products. Some consumer household products, such as glues, cleaning products, detergents, art supplies, and paint strippers, contain benzene.

Does Benzene Cause Cancer?

The evidence linking benzene and cancer predominantly comes from studies of workers, and relates to leukemia, particularly with 2 types called acute myeloid leukemia (AML) and to a lesser degree, chronic lymphocytic leukemia (CLL). Leukemia is a cancer of blood-forming cells in the bone marrow.

What Do the Epidemiologic Studies Say?

A considerable number of human studies provide evidence linking benzene and cancer. Initially, increased risks of leukemia, chiefly AML, were reported among workers with high levels of benzene exposure in the chemical, shoemaking, and oil refining industries. More recently, studies have focused on workers with relatively lower exposure. Two examples are highlighted here.

One is a study of approximately 750 Pliofilm rubber workers at three facilities in Ohio, whose causes of death were followed over three decades. This study had several strengths: relatively little exposure to dangerous chemicals other than benzene that might act as confounders (and create a false impression about benzene), a wide range of benzene exposures, and very complete follow-up. It showed an increase in leukemia and multiple myeloma (a cancer of immune system cells in the bone marrow) among exposed workers.

Another important study was conducted by the National Cancer Institute (NCI) in collaboration with the Chinese Academy of Preventive Medicine. Investigators followed almost 30,000 workers exposed to benzene in 233 factories in China, and a similar group of unexposed control workers. The large sample size permitted elevated risks to be detected at low levels of exposure. This study suggested an increased risk of leukemia in workers exposed to less than 10 ppm (parts per million).

These and other epidemiologic studies of benzene show a fairly consistent excess risk to exposed workers of leukemia, and suggestions of excesses in other blood and bone marrow cancers as well.

What Does the Animal and Laboratory Evidence Say?

The human data are supported by animal studies. There is sufficient evidence for the carcinogenicity of benzene in experimental animals. Key animal studies support the finding of an excess risk of leukemia in humans from exposure to benzene by inhalation and ingestion. The details of these studies have been reviewed and found to support the association between benzene and cancer.

Benzene has been shown to induce chromosomal aberrations, or changes, in mammalian cells in vitro (outside a living organism). In vivo (within a living organism) studies have shown that benzene exposure leads to chromosome changes in bone-marrow cells. Such changes are commonly found in human leukemia cells.

What Do the Expert Agencies Say?

Based on animal and human evidence like the examples above, expert agencies have evaluated the carcinogenicity of benzene.

The National Toxicology Program (NTP) evaluates exposures that may be carcinogenic. Exposures that are thought to be carcinogenic are included in the Reports on Carcinogens, published every 2 years. Each exposure is assigned to one of two categories: "known to be human carcinogens," and "reasonably anticipated to be human carcinogens." The first category includes substances for which human studies (epidemiology studies and/or experimental studies) provide "sufficient evidence" of carcinogenicity in humans. The second category includes substances for which there is limited evidence of carcinogenicity in humans and/or sufficient evidence of carcinogenicity in experimental animals. Using this scheme, the National Toxicology Program classifies benzene a "known carcinogen.”

The International Agency for Research on Cancer (IARC) also evaluates exposures that may be carcinogenic. IARC classifies exposures into 1 of 4 categories: Group 1 exposures are those "known to be carcinogenic to humans," usually based on "sufficient" human evidence, but sometimes based on "sufficient" evidence in experimental animals and "strong" human evidence. Group 2 exposures are divided into 2 categories. Group 2A ("probably carcinogenic to humans") has stronger evidence, and Group 2B ("possibly carcinogenic to humans") has weaker evidence. Group 3 exposures are not considered classifiable, because available evidence is limited or inadequate. Finally, Group 4 exposures are "probably not carcinogenic to humans," based on evidence suggesting lack of carcinogenicity in humans and in experimental animals. IARC rated benzene as "known to be carcinogenic to humans" (Group 1).

The Environmental Protection Agency (EPA), through its Integrated Risk Information System, uses a classification scheme very similar to that of IARC. It classifies exposures into 1 of 5 categories: (A) Known human carcinogen, (B) Probable human carcinogen, (C) Possible human carcinogen, (D) Not classifiable as to human carcinogenicity, and (E) Evidence of noncarcinogenicity for humans. EPA classifies benzene as "a known carcinogen" (Group A).

Does Benzene Cause Any Other Health Problems?

Short-term effects. Short-term exposure (less than one year) to high doses (700-3,000 ppm) of benzene may cause drowsiness, dizziness, headaches, tremors, confusion, and/or unconsciousness. Consuming foods or fluids contaminated with high levels of benzene can cause vomiting, irritation of the stomach, dizziness, sleepiness, convulsions, and rapid heart rate. In extreme cases, death may occur after oral ingestion or inhalation of very high concentrations (approximately 10,000-20,000 ppm) of benzene.

Long-term effects. Long-term and/or high level exposure (one year or longer) to benzene may interfere with normal blood cell production by hematopoietic (blood forming) cells in the b one marrow. This may result in anemia (decreased ability of blood to transport oxygen) and low white blood cell counts (decreased ability of blood to fight infections), and can even be life-threatening. There is some evidence that benzene may also be harmful to reproductive organs.

What Should I Do If I’ve Been Exposed to Benzene?

Benzene can be measured in the blood or breath. A metabolite of benzene, called phenol, can be measured in the urine and can serve as an indicator of the presence of benzene. These tests are only useful for recent exposures to benzene. Also, these tests cannot predict health effects; they can only detect levels of exposure to benzene.

If you are concerned about benzene exposure, there are several ways you can limit your exposure. First, if you are exposed on the job, try to prevent exposures through process changes (such as replacing the benzene with another solvent or enclosing the benzene source) or by using personal protective equipment. Second, if you are a smoker, stop smoking. Smoking greatly increases the blood level of benzene. Third, avoid gasoline fumes by pumping gasoline carefully and choosing gas stations with vapor recovery systems that capture the fumes. Finally, use common sense when around any chemicals that might contain benzene. For example, avoid unnecessary skin contact with gasoline.

Conclusion

Benzene is considered a human carcinogen based on experimental and epidemiological data. Steps have been taken to limit exposures to benzene both occupationally and environmentally. The EPA regulates limits concentrations of benzene in drinking water to 5 ppb (parts per billion) with an ultimate goal of 0 ppb. The National Institute of Occupational Safety and Health (NIOSH) and the Occupational Safety and Health Administration have limited occupational exposures to benzene to 1 ppm (part per million) during an average workday and also recommend personal protective equipment such as respirators.

Are There Other National Organizations Addressing Benzene?

Agency for Toxic Substances and Disease Registry (ATSDR). "ToxFaq's on Benzene". 2005.
Internet Address: www.atsdr.cdc.gov/tfacts3.html

American Cancer Society. Cancer Facts and Figures 2006. Atlanta: American Cancer Society, 2006.

Environmental Protection Agency (EPA). "Carcinogenic Effects of Benzene", 1998.
Internet Address: www.epa.gov/NCEA/pdfs/benzenef.pdf

International Agency for Research on Cancer.
Internet Address: Benzene Supplement 7: (1987). and Benzene VOL.: 29 (1982).

National Toxicology Program – Eleventh Report on Carcinogens, Benzene.

References

Agency for Toxic Substances and Disease Registry (ATSDR). Toxicological profile for benzene. U.S. Public Health Service, U.S. Department of Health and Human Services, Atlanta, GA. 1997.

Glass DC, Gray CN, Jolley DJ, et al. Leukemia risk associated with low-level benzene exposure. Epidemiology. 2003;14:569-77.

Environmental Protection Agency (EPA). Proposed Guidelines for carcinogen risk assessment. Federal Register. 1996; 61; 17960-18011..

International Agency for Research on Cancer (IARC). IARC monographs on the evaluation of the carcinogenic risk of chemicals to humans. Some industrial chemicals and dyestuffs. Vol. 29. Lyon, France: IARC, 1982.

National Center for Environmental Assessment (NCEA). Carcinogenic Effects of Benzene: An Update. Office of Research and Development, U.S. Environmental Protection Agency, Washington, D.C. April 1998.

National Toxicology Program (NTP). Eleventh report on carcinogens. Summary. U.S. Public Health Service, U.S. Department of Health and Human Services, Atlanta, GA. 2005.

Occupational Safety and Health Administration. Occupational exposure to benzene. Final Rule. Federal Register. 1987;52:34460-34578.

Rinsky RA, Smith AB, Hornung R, et al. Benzene and leukemia: an epidemiological risk assessment. New Engl J Med. 1987;316:1044-50.

Yin SN, Li GL, Tain FD, et al. Leukemia in benzene workers: a retrospective cohort study. Brit J Ind Med. 1987;44:124-28.

Yin SN, Li GL, Tain FD, et al. A retrospective cohort study of leukemia and other cancers in benzene workers. Environ Health Persp. 1989;82:207-13.

Revised: 02/02/2006