FIGURE 4.1 Structure of a) benzene, b) toluene, and c) xylenes.

Data from laboratory animals and humans show that benzene affects the bone marrow in a dose-dependent manner, causing anemia, leukopenia, and thrombocytopenia; continued exposure causes aplasia and pancytopenia (Bruckner and Warren, 2001). Benzene also has carcinogenic properties. In experimental animals, an increased incidence of malignant lymphomas and some solid tumors have been seen after exposure to high doses of benzene. As discussed in Chapter 6, benzene has also been associated with some types of leukemia in humans. Differences in cancer sites among species indicate that species differences in the carcinogenicity of benzene exist.

Because of the health concerns associated with benzene, it has been replaced in many uses with other solvents, especially toluene and xylenes (Bruckner and Warren, 2001). Toluene and xylenes are widely used in the production of other chemicals and, like benzene, are components of gasoline. Toluene is also present in paints, thinners, cleaning agents, and glue and is widely abused as an inhalant. Toluene and xylenes have the same general toxicity as many solvents, but animal data do not indicate that they are hematopoetic toxicants; the tumors associated with exposure to benzene do not appear to be associated with exposure to toluene and xylenes. Auditory toxicity has been demonstrated in animals following toluene exposure. In rats, intermediate exposures (1000 and 1200 ppm for 14 hours/day for 2 to 9 weeks) resulted in permanent loss of hearing in the high frequency range. A loss of hair cells has been seen following exposure to auditory-toxic concentrations of toluene and could be involved in the underlying mechanism (ATSDR, 2000). Toluene and xylenes are methylated benzenes and, unlike benzene, are metabolized at the methyl group(s) and then readily eliminated.

Halogenated hydrocarbons contain at least one halogen atom (such as chlorine, bromine or fluorine). Four halogenated hydrocarbons are discussed in this section: tetrachloroethylene, trichloroethylene, methylene chloride, and chloroform.