Benzene is a cyclic hydrocarbon with the molecular formula C6H6, meaning that a benzene molecule consists of six carbon atoms arranged in a ring, with one hydrogen atom attached to each carbon atom. Benzene derivatives are chemical compounds made by replacing one or more hydrogen atoms in the benzene ring.
Benzene is a clear, colorless, highly flammable liquid with a sweet, petroleum-like odor. It has a boiling point of 176.2°F (80.1°C), a melting point of 42°F (5.5°C), and a density of 0.8765 g/mL. Benzene occurs naturally in crude oil, from which it is separated by fractional distillation. It usually ranks among the top 20 chemicals in the United States in terms of production. It is used primarily as a chemical intermediary in the production of other chemicals used to make a variety of products, such as plastics, synthetic elastomers (rubber), lubricants, dyes, drugs, synthetic detergents, insecticides, fumigants, solvents, paint removers, and gasoline.
Benzene is the parent of a group of compounds known as aromatic hydrocarbons. Aromatic hydrocarbons all contain some variation of the basic six-carbonring of benzene. Replacement of one hydrogen in the ring with a chlorine atom, for example, results in the formation of chlorobenzene, used primarily in the manufacture of dyestuffs and pesticides. The combination of two benzene rings with each other results in the formation of naphthalene, once the primary ingredient in mothballs, whereas the combination of three benzene rings results in the formation of anthracene, also used in the production of dyestuffs and pesticides. Thousands of other benzene derivatives are known.
The chronic effects of benzene exposure depend on the concentration of the compound to which one is exposed, the length of exposure, and individual factors, such as age and general health. The most serious effects are those involving the circulatory system. Benzene is known to interfere with the normal production of blood cells in bone marrow, producing a variety of effects. These effects may result in a decrease of any one of the three major blood cells, erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets), or any combination of the three. This process is reversible if exposure to benzene is discontinued but may result in permanent deficiency of blood cell production if exposure continues over an extended period of time. In such a case, the loss of normal blood cell production is manifested in a number of blood conditions, such as aplastic anemia and acute myeloid leukemia (AML, also known as acute non-lymphocytic leukemia).
Aplastic anemia results from a reduced production of all types of blood cells and is characterized by feelings of fatigue, an increased risk for infections, and uncontrolled bleeding. Acute myeloid leukemia (AML) may occur independently from, or after the earlier development of, aplastic anemia. It occurs when the process of cell production in bone marrow is disrupted, producing a flood of white blood cells that prevent the production and development of other blood cells. Undiagnosed or untreated AML is fatal within a matter of weeks or months. Benzene is implicated in the development of at least two other forms of leukemia also, acute lymphoblastic leukemia (ALL) and chronic myeloid leukemia (CML), presumably as the result of similar defective blood cell production in bone marrow.
Benzene's effect on blood cell production is thought to have deleterious effects also on the immune system. Studies on both humans and experimental animals have found that exposure to benzene results in a reduction of the number of leukocytes and antibodies used by the immune system to fight off disease. Benzene and some benzene derivatives are also well-known carcinogens. Research data on this point are somewhat uncertain because of confounding factors in most studies. But the data on benzene exposure appear to be consistent and convincing, and many health authorities list benzene and at least some of its chemical cousins as carcinogens. Benzene is also strongly suspected to be genotoxic, justifying its classification as a teratogen. This effect, as well as its carcinogenic effects, is thought to be a consequence of benzene's tendency to damage or destroy DNA molecules responsible for normal protein production in affected cells. The same mechanism may also account for benzene's tendency to produce changes in a woman's menstrual cycle, as well as to cause shrinking of the ovaries.
Individuals whose work causes increased exposure to benzene may also experience a variety of other health and medical conditions, including the following:
Limited evidence suggests that benzene may also have effects on the nervous system. The data suggest the possibility of damage by benzene inhalation or ingestion to the central nervous system that may result in neuropathy (nerve pain), mental confusion, interrupted sleep patterns, and memory loss.
The health effects of a number of benzene derivatives have also been studied in considerable detail. The most common subjects of such research have been phenol (hydroxybenzene), toluene (methylbenzene), xylene (dimethylbenzene), ethylbenzene, napthalene, anthracene, and polycyclic aromatic hydrocarbons (PAH). (Three forms of xylene are possible, known as m-xylene, o-xylene, and p-xylene.) The health data for each of these compounds differ from substance to substance, although some commonalities exist.
Research suggests that the health effects of the xylenes and ethylbenzene are broadly similar to those of toluene. All appear to result in damage to the neurological, optical, auditory, integumentary (skin), and reproductive systems similar to those produced by toluene.
The polycyclic aromatic hydrocarbons are of special interest to health researchers because of their role in air pollution. The PAHs are a group of several hundred complex aromatic compounds consisting of two or more benzene rings joined to each other. An example of a PAH is chrysene, whose molecules consist of four benzene rings fused together. PAHs are produced during the combustion of fossil fuels and tend to accumulate in the atmosphere where internal combustion engines are in common used. PAHs apparently interfere with the structure of cell membranes and with the action of enzymes. Individually they have a wide variety of specific health effects, ranging from no effect at all to well established carcinogenic, mutagenic, and teratogenic effects.
The average person tends to receive very low exposure to benzene and its derivatives in the course of everyday life. The two most common sources of exposure are cigarette smoking and refueling a vehicle at a gasoline station. In the former case, benzene exposure comes from the fact that benzene and a number of its derivatives are in tobacco smoke and thus are inhaled when a person smokes a cigarette or cigar. Nonsmokers may also inhale benzene and its derivatives in secondhand smoke, although the concentration in this form is much lower than it is for smokers. For example, a heavy smoker (two packs a day) receives ten times as much benzene from this source as a nonsmoker. The amount of benzene received during a stop at a gasoline service station is also low, but high enough to rank as the second most important source for the average person. For individuals who typically receive benzene only from such everyday sources as these, neither acute nor chronic health effects are likely to be a problem, so treatment is not a concern.
Exposure to benzene and derivatives is of greater concern to individuals who work with these compounds on a regular basis, such as those who work at a service station. Such individuals inhale very low concentrations of benzene at any one time. But the length of exposure can be great. In such cases, individuals who experience health problems related to benzene exposure receive treatment for the specific condition they have developed. Treatment for aplastic anemia, for example, depends on the stage of the disease once diagnosed. The condition can be treated, but not cured, with blood transfusions, or certain drugs may be prescribed to stimulate blood cell production in bone marrow. Sargramostim (Leukine), filgrastim (Neupogen) and pegfilgrastim (Neulasta), and epoetin alfa (Epogen, Procrit) are all available for this purpose. In the most severe cases of aplastic anemia, a bone marrow transplant may be necessary.
Prognosis for conditions resulting from exposure to benzene and its derivatives depends on the specific medical condition being treated and the stage at which the condition is diagnosed.
For individuals at greatest risk for benzenerelated health conditions, the obvious form of prevention is avoiding the environmental conditions under which benzene is inhaled or ingested. Gas station attendants, for example, can leave their work, which may be a reasonable health option, but not necessarily a reasonable economic option. For some types of exposure to benzene derivatives in the environment, however, a number of public health policies and programs are possible. For example, governmental agencies have developed rules and regulations for the method by which fossil fuels can be burned in various settings (as fuels for vehicles or in industrial plants) that reduce the concentration of PAHs in emitted gases.
See also Air pollution ; Cancer ; Leukemia ; Smoking ; Teratogen .
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Agency for Toxic Substances and Disease Registry, 4770 Buford Hwy. NE, Atlanta, GA, 30341, (800) 232-4636, ATSDRIC@cdc.gov, http://www.atsdr.cdc.gov .
David E. Newton, EdD