The acute toxic effect of benzene is depression of the CNS. Exposure to 7500 ppm for 30 minutes can be fatal. Exposure to concentrations greater than 3000 ppm may cause euphoria, nausea, locomotor problems, and coma. Vertigo, drowsiness, headache, and nausea may occur at concentrations ranging from 250 to 500 ppm. No specific treatment exists for the acute toxic effect of benzene.
Chronic exposure to benzene can result in very serious toxic effects, the most significant of which is bone marrow injury. Aplastic anemia, leukopenia, pancytopenia, and thrombocytopenia occur, as does leukemia. Chronic exposure to low levels of benzene has been associated with leukemia of several types as well as lymphomas, myeloma, and myelodysplastic syndrome. Recent studies have shown the occurrence of leukemia following exposures as low as 2 ppm-years. The pluripotent bone marrow stem cells appear to be targets of benzene or its metabolites and other stem cells may also be targets.
Benzene has long been known to be a potent clastogen, ie, a mutagen that acts by causing chromosomal breakage. Recent studies have suggested specific chromosome reorganization and genomic patterns that are associated with benzene-induced leukemia. Epidemiologic data confirm a causal association between benzene exposure and leukemia and other bone marrow cancers in workers. IARC classifies benzene as a class 1, known human carcinogen. Most national and international organizations classify benzene as a known human carcinogen.
Toluene (methylbenzene) does not possess the myelotoxic properties of benzene, nor has it been associated with leukemia. It is not carcinogenic and is listed as class 3 by IARC. It is, however, a CNS depressant and a skin and eye irritant. It is also fetotoxic. See Table 56–1 and OSHA Tables Z-1 and Z-2 (http:// www.osha.gov) for the PELs. Exposure to 800 ppm can lead to severe fatigue and ataxia; 10,000 ppm can produce rapid loss of consciousness. Chronic effects of long-term toluene exposure are unclear because human studies indicating behavioral effects usually concern exposures to several solvents. In limited occupational studies, however, metabolic interactions and modification of toluene’s effects have not been observed in workers also exposed to other solvents. Less refined grades of toluene contain benzene. If technical grade toluene is to be used where there is human contact or exposure, analysis of the material for benzene content is advisable.
Xylene (dimethylbenzene) has been substituted for benzene in many solvent degreasing operations. Like toluene, the three xylenes do not possess the myelotoxic properties of benzene, nor have they been associated with leukemia. Xylene is a CNS depressant and a skin irritant. Less refined grades of xylene contain benzene. Estimated TLV–time-weighted average and TLV–short-term exposure limit are 100 and 150 ppm, respectively. The current OSHA PELs may be found at http://www. osha.gov, Table Z-1.
TABLE 56–2 Organochlorine pesticides.