Ethylbenzene

Endpoint summary

Additional information

Two studies reported genotoxic effects in ethylbenzene exposed workers. Beskid et al. (2006) found specific chromosomal aberrations in peripheral blood lymphocytes and Holz et al. (1995) reported an increase in kinetochore positive micronuclei, but not of DNA adducts, DNA single strand breaks, sister chromatid exchanges, or total micronuclei. Because of coexposure to other chemicals, especially benzene, and missing information on former long-term exposure these studies do not allow an appropriate evaluation of effects due to ethylbenzene.

 

Ethylbenzene produced consistently negative results in bacterial gene mutation tests and in the yeast assay on mitotic recombination (Stanford Research Institute, 1976; Shell Oil Co., 1981a; Zeiger et al. 1992).

 

In mouse lymphoma mammalian mutation assays a weak positive response was reported but only at doses with strong cytotoxicity (McGregor et al., 1988). Examination of the thymidine kinase locus in L5178Y mouse lymphoma cells showed weak increases in mutant frequency only at the highest doses tested with S-9 mix which were also strongly cytotoxic. Without S-9 mix ethylbenzene was not mutagenic at doses without strong cytotoxicity. The negative results at non-toxic concentrations were confirmed in further experiments with and without S-9 mix (RCC, 2000). Clearly negative results were obtained in a recent study in the same test system following the current test guidelines (The Dow Chemical Company, 2006).

 

No clear conclusion can be drawn regarding in vitro chromosomal aberration. Without S-9 mix there were equivocal increases in chromosomal aberration frequencies and micronuclei in CHO (NTP, 1992) and SHE cells (Gibson et al., 1997), respectively, or a negative result in a rat liver cell line (Shell Oil Co., 1981). With S-9 mix ethylbenzene did not cause chromosomal aberrations in CHO cells (NTP, 1992). An in vitro SCE test was clearly negative with and without S-9 mix (NTP, 1992).

 

In vivo, ethylbenzene was clearly negative in two micronucleus assays, one in bone marrow after intraperitoneal application (Mohtashamipur et al., 1985) and one in peripheral blood after inhalation (NTP, 1992).A mouse liver UDS assay after inhalation exposure was negative (Central Toxicology Laboratory, 2001).

After oral application 1-phenylethanol, the major metabolite of ethylbenzene, did not lead to an increase of micronuclei in the bone marrow of mice (Engelhardt, 2006).

 

The total data base was recently reviewed by Henderson et al. (2007). The authors concluded that the composite set of results from both in vitro and in vivo tests have been predominantly negative in the absence of excessive toxicity.

In a recent publication not reviewed by Henderson et al. (2007),Chen et al. (2008) investigated the genotoxic potential of methyl-tert-butyl ether (MBTE), benzene, toluene, ethylbenzene, and the different isomers of xylene in human lymphocytes. All chemicals gave positive results in the alkaline comet assay indicative of single-strand breaks at concentrations of 100 and 200 µM (MBTE, benzene, toluene, and p-xylene already at 50 µM). In the neutral comet assay indicative of double-strand breaks (a more relevant genotoxic endpoint) all chemicals were positive, apart from ethylbenzene and toluene up to the high concentration of 200 µM. This supports the contention that ethylbenzene is devoid of a biologically relevant genotoxic potential even at high concentrations in vitro.

The following information is taken into account for any hazard/risk assessment:

Ethylbenzene has been extensively tested for toxicity to genetic material using nearly every available type of genetic toxicity test. On the basis of various mutagenicity tests in vitro and in vivo, there is currently no relevant indication that ethylbenzene is a germ cell mutagen. Based on the available data ethylbenzene should not be classified as a germ cell mutagen.

Short description of key information:
Ethylbenzene has been extensively tested for toxicity to genetic material using nearly every available type of genetic toxicity test. On the basis of various mutagenicity tests in vitro and in vivo, there is currently no relevant indication that ethylbenzene is genotoxic to somatic or germ cells.

Endpoint Conclusion: No adverse effect observed (negative)

Justification for classification or non-classification

Ethylbenzene produced consistently negative results in bacterial gene mutation tests and in the yeast assay on mitotic recombination. Former mouse lymphoma mutation assays (thymidine kinase locus in L5178 cells) gave inconsistent results (weakly positive / negative). Clearly negative results were obtained in a recent study in the same test system following current test guidelines. No clear conclusion can be drawn regarding in vitro chromosomal aberration with either equivocal or negative results. Recently positive results in the alkaline comet assay in human lymphocytes in vitro indicative of single-strand breaks were reported, while in the neutral comet assay indicative of double-strand breaks (a more relevant genotoxic endpoint) ethylbenzene was negative. This supports the contention that ethylbenzene is devoid of a biologically relevant genotoxic potential in vitro.

 

In vivo, ethylbenzene was clearly negative in two micronucleus assays, one in bone marrow after intraperitoneal application and one in peripheral blood after inhalation. A mouse liver UDS assay after inhalation exposure was negative.

 

The total data base (apart from the comet assay) was recently reviewed. The authors concluded that the composite set of results from both in vitro and in vivo tests have been predominantly negative for genotoxicity in the absence of excessive toxicity.

 

In conclusion, on the basis of various mutagenicity tests in vitro and in vivo, there is currently no relevant indication that ethylbenzene reacts with the DNA.

 

Based on the available data ethylbenzene should not be classified as a germ cell mutagen and classification for genotoxicity is not warranted in accordance with Directive 67/548/EEC,EU CLP (Regulation (EC) No. 1272/2008) and UN GHS.