Genetic toxicology of trichloroethylene (TCE)

Nephrotoxicity and Renal Pathophysiology: A Contemporary Perspective

The kidney consists of numerous cell types organized into the nephron, which is the basic functional unit of the kidney. Any stimuli that induce loss of these cells can induce kidney damage and renal failure. The cause of renal failure can be intrinsic or extrinsic. Extrinsic causes include cardiovascular disease, obesity, diabetes, sepsis, and lung and liver failure. Intrinsic causes include glomerular nephritis, polycystic kidney disease, renal fibrosis, tubular cell death, and stones. The kidney plays a prominent role in mediating the toxicity of numerous drugs, environmental pollutants and natural substances. Drugs known to be nephrotoxic include several cancer therapeutics, drugs of abuse, antibiotics, and radiocontrast agents. Environmental pollutants known to target the kidney include cadmium, mercury, arsenic, lead, trichloroethylene, bromate, brominated-flame retardants, diglycolic acid, and ethylene glycol. Natural nephrotoxicants include aristolochic acids and mycotoxins such as ochratoxin, fumonisin B1, and citrinin. There are several common characteristics between mechanisms of renal failure induced by nephrotoxicants and extrinsic causes. This common ground exists primarily due to similarities in the molecular mechanisms mediating renal cell death. This review summarizes the current state of the field of nephrotoxicity. It emphasizes integrating our understanding of nephrotoxicity with pathological-induced renal failure. Such approaches are needed to address major questions in the field, which include the diagnosis, prognosis and treatment of both acute and chronic renal failure, and the progression of acute kidney injury to chronic kidney disease.

Micronucleus induction by oxidative metabolites of trichloroethylene in cultured human peripheral blood lymphocytes: a comparative genotoxicity study

The genotoxic effects of oxidative metabolites of trichloroethylene (TCE), namely chloral hydrate, trichloroacetic acid (TCA), dichloroacetic acid (DCA), and trichloroethanol (TCEOH) were examined in human peripheral blood lymphocytes. In this context, lymphocytes were exposed in vitro to 25, 50, and 100 μg/ml concentrations of these metabolites separately for a period of 48 h and examined for micronucleus (MN) induction through flow cytometer. At 50 μg/ml TCE metabolites, TCA (6.33 ± 0.56 %), DCA (5.06 ± 0.55), and TCEOH (4.70 ± 1.73) induced highly significant (p<0.001) frequency of MN in comparison to control (1.03 ± 0.40) suggestive of their genotoxic potential. However, exposure of 100 μg/ml of all the metabolites consistently declined the frequencies of MN which in some cases was equable to that of observed at 25 μg/ml. Further, cytotoxicity and cell cycle disturbances were also measured to find out the association of these endpoints with the MN induction. DNA content analysis revealed 3-4-fold elevation of S-phase at all the concentrations tested. Particularly, at 100 μg/ml, treatment elevation of S-phase was significantly (p<0.0001) higher as compared to the control. Present findings together with earlier reports indicate that TCE induces genotoxicity through its metabolites. Interaction of these metabolites with DNA, as evident by elevated S-phase, seems to be the major cause of MN induction. However, involvement of spindle disruption cannot be ruled out. This comparative study also suggests that after TCE exposure, the metabolic efficiency of human to generate oxidative metabolites determines the extent of genotoxicity.

Genotoxicity of trichloroethylene in the natural milieu

Trichloroethylene (TCE) is a suspected genotoxic and carcinogenic compound which is usually present in the air, soil and water as pollutant. To estimate the genotoxic potential of TCE in a pure chemical form as well as an ingredient of the complex sample, Ames fluctuation test using TA98 and TA100 strains and Allium cepa genotoxicity assay were performed. For the genotoxicity analysis of TCE in natural milieu, the above mentioned tests were performed on the waste waters collected from two different stations of northern India namely Saharanpur and Aligarh, U.P., and these waste waters were supplemented with 50 and 100 mg/l of trichloroethylene. TCE alone was found to be non-genotoxic by both the testing system up to the range of 1000 mg/l concentration (data not shown). However, the test water samples supplemented with 100 mg/l of TCE, exhibited a significant increase in the genotoxicity compared with control by both the testing systems. In Ames fluctuation test, Mi(f) value was found to be increased by 41% and 53% with 100 mg/l of TCE supplemented Saharanpur and Aligarh waste water samples respectively, in the presence of S9 fraction compared with their respective controls. Allium cepa genotoxicity test also showed around 25% increase in total chromosomal aberration frequency following 100 mg/l TCE supplementation. However, supplementation of 50 mg/l TCE to the test water samples could not enhance the genotoxicity to a significant extent. From these results, we can conclude that TCE itself was non-genotoxic but it may promote mutation and/or DNA damage at a concentration of 100 mg/l under certain environmental conditions. We suggest that some chemicals in the test water samples might be interacting with TCE and/or metabolite(s) to cause the enhancement in genotoxicity. The mechanism of these synergistic effects should be explored further.

Toxicity, biomarkers, genotoxicity, and carcinogenicity of trichloroethylene and its metabolites: a review

Trichloroethylene (TCE) is a prevalent occupational and environmental contaminant that has been reported to cause a variety of toxic effects. This article reviews toxicity, mutagenicity, and carcinogenicity caused by the exposure of TCE and its metabolites in the living system as well as on their (TCE and its metabolites) toxicity biomarkers.

Detailed review of transgenic rodent mutation assays

Induced chromosomal and gene mutations play a role in carcinogenesis and may be involved in the production of birth defects and other disease conditions. While it is widely accepted that in vivo mutation assays are more relevant to the human condition than are in vitro assays, our ability to evaluate mutagenesis in vivo in a broad range of tissues has historically been quite limited. The development of transgenic rodent (TGR) mutation models has given us the ability to detect, quantify, and sequence mutations in a range of somatic and germ cells. This document provides a comprehensive review of the TGR mutation assay literature and assesses the potential use of these assays in a regulatory context. The information is arranged as follows. (1) TGR mutagenicity models and their use for the analysis of gene and chromosomal mutation are fully described. (2) The principles underlying current OECD tests for the assessment of genotoxicity in vitro and in vivo, and also nontransgenic assays available for assessment of gene mutation, are described. (3) All available information pertaining to the conduct of TGR assays and important parameters of assay performance have been tabulated and analyzed. (4) The performance of TGR assays, both in isolation and as part of a battery of in vitro and in vivo short-term genotoxicity tests, in predicting carcinogenicity is described. (5) Recommendations are made regarding the experimental parameters for TGR assays, and the use of TGR assays in a regulatory context.

The in vivo comet assay: use and status in genotoxicity testing

The in vivo comet assay (single cell gel electrophoresis assay) in its alkaline version (pH >13) is being increasingly used in genotoxicity testing of substances such as industrial chemicals, biocides, agrochemicals, food additives and pharmaceuticals. Recommendations for an appropriate performance of the test using OECD guidelines for other in vivo genotoxicity tests have been published. In this review, we critically discuss the biological significance of comet assay effects in general and the status of the test in current strategies for genotoxicity testing. Examples for practical applications of the in vivo comet assay and potential consequences of positive and negative test results are given. The significance of comet assay results for hazard identification and risk assessment is discussed. In accordance with international guidelines for genotoxicity testing the in vivo comet assay is recommended for follow-up testing of positive in vitro findings. It is particularly useful as a tool for the evaluation of local genotoxicity, especially for organs/cell types which cannot easily be evaluated with other standard tests. A positive result in an appropriately performed in vivo comet assay indicates genotoxicity of the test compound in the tissue tested and gains particular significance when a mutagenic potential of the test compound has already been demonstrated in vitro. Such findings will have practical consequences in the risk assessment processes and further development of substances.

Lack of direct mitogenic activity of dichloroacetate and trichloroacetate in cultured rat hepatocytes

Dichloroacetate (DCA) and trichloroacetate (TCA) are hepatocarcinogenic metabolites of the common groundwater contaminant, 1,1,2-trichloroethylene. DCA and TCA have been shown to induce hepatocyte proliferation in vivo, but it is not known if this response is the result of direct mitogenic activity or whether cell replication occurs indirectly in response to tissue injury or inflammation. In this study we used primary cultures of rat hepatocytes, a species susceptible to DCA- but not TCA-induced hepatocarcinogenesis, to determine whether DCA and TCA are direct hepatocyte mitogens. Rat hepatocytes, cultured in growth factor-free medium, were treated with 0.01-1.0 mM DCA or TCA for 10-40 h; cell replication was then assessed by measuring incorporation of 3H-thymidine into DNA and by cell counts. DCA or TCA treatment did not alter 3H-thymidine incorporation in the cultured hepatocytes. Although an increase in cell number was not observed, DCA treatment significantly abrogated the normal background cell loss, suggesting an ability to inhibit apoptotic cell death in primary hepatocyte cultures. Furthermore, treatment with DCA synergistically enhanced the mitogenic response to epidermal growth factor. The data indicate that DCA and TCA are not direct mitogens in hepatocyte cultures, which is of interest in view of their ability to stimulate hepatocyte replication in vivo. Nevertheless, the synergistic enhancement of epidermal growth factor-induced hepatocyte replication by DCA is of particular interest and warrants further study.

The assessment of genotoxic effects in lymphocyte cultures of infants treated with chloral hydrate

Chloral hydrate is a sedative commonly used in pediatric medicine. It was evaluated for genotoxicity in cultured peripheral blood lymphocytes of infants who were given chloral hydrate for sedation. Sister chromatid exchange and micronucleus frequencies were determined before and after chloral hydrate administration. After treatment, the frequencies of sister chromatid exchange and micronuclei were significantly increased, suggesting that chloral hydrate has moderate genotoxic potential in infants.

Induction of peroxisome proliferation in cultured hepatocytes by a series of halogenated acetates

Trichloroacetate (TCA) and dichloroacetate (DCA) are hepatocarcinogenic metabolites of the environmental pollutant trichloroethylene (TCE) and are common water contaminants. Induction of peroxisome proliferation via activation of the peroxisome proliferator-activated receptor alpha (PPARalpha) has been proposed as a mechanism for their hepatocarcinogenic action. However, it is unclear whether these compounds are direct ligands of PPARalpha or whether activation occurs by a ligand-independent process. The present studies were undertaken to determine whether a primary rat hepatocyte model system could be used to examine structure-activity relationships of haloacetates for the induction of peroxisomal palmitoyl-CoA oxidation. The haloacetates tested differed in both type (iodo, bromo, chloro and fluoro) and extent (mono, di and tri) substitution. Significant differences were observed in both potency and efficacy. Potency varied over about two orders of magnitude, in the order of mono > di = tri. Within the monohalo-substituted series, the order of potency was iodo > bromo > chloro, with the fluoro analog being essentially inactive. The monoiodo- and monobromo-derivatives showed significant induction at 50 and 100 microM, respectively, but cytotoxicity precluded obtaining full concentration-response curves. The dihalo- and trihalo-acetates had generally similar potency, and, with the exception of the diflouro- and dibromoacetates, showed a maximal induction of two- to three-fold. Difluoroacetate and dibromoacetate induced palmitoyl-CoA oxidation by nine- and six-fold, respectively, approaching the effectiveness of Wy-14,643 (50 microM) in this system. Of interest, the slopes of the concentration-dependence lines of the difluoro- and dibromo-acetates were markedly dissimilar from the other di- and tri-haloacetates, suggesting either a marked difference in the way they activate the PPARalpha receptor or a substantial difference in the way they are metabolized or transported by the hepatocytes.

Science and transscience in carcinogen risk assessment--the European Union regulatory process for trichloroethylene

This is a study of carcinogen risk assessment of the chlorinated solvent trichloroethylene within the European Union existing substances program and the classification and labeling process. The focus is on the most active and influential participants of this process, namely, those from the United Kingdom, Germany, and Sweden, and from industry. The member state and other experts have different opinions regarding the appropriate classification of trichloroethylene for mutagenicity (no classification or category 3) and carcinogenicity (category 3, 2, or 1). In this article these differences are described, as well as how the primary carcinogenicity and mutagenicity data have been interpreted and evaluated by these participants. It is concluded that underlying the different assessments are disagreements about issues that to some degree lie outside the scope of purely scientific considerations.

Possible involvement of glutathione and p53 in trichloroethylene- and perchloroethylene-induced lipid peroxidation and apoptosis in human lung cancer cells

Trichloroethylene (TCE) and perchloroethylene (PERC) are volatile organic compounds (VOCs) that are primarily inhaled through the respiratory system. The aim of this study was to elucidate the role of glutathione (GSH) and p53 in TCE- and PERC-induced lung toxicity. Human lung adenocarcinoma cells NCI-H460 (p53-wild-type) have constitutively lower levels of GSH than NCI-H1299 (p53-null) cells. The results showed that exposure to vapor TCE and PERC produced a dose-dependent and more pronounced accumulation of H(2)O(2) in p53-WT H460 than p53-null H1299 cells. The accumulation of H(2)O(2) was accompanied by severe cellular damage, as indicated by the significant increase of lipid peroxidation and apoptosis in p53-WT H460 cells, but not p53-null H1299 cells. Cotreatment of p53-WT H460 cells with free radical scavengers, such as D-mannitol, uric acid, and sodium selenite, significantly attenuated the TCE- or PERC-induced lipid peroxidation. In contrast, depletion of GSH in p53-null H1299 cells enhanced TCE- or PERC-induced lipid peroxidation. The levels of p53 and Bax proteins were elevated, while Bcl-2 protein was downregulated in TCE- or PERC-treated p53-WT H460 cells. Activity of caspase 3, the apoptotic executioner, was also significantly enhanced in TCE- or PERC-treated cells. These data suggest that, in human lung cancer cells, GSH plays a vital role in the protection of TCE- and PERC-induced oxidative stress and apoptosis, which may be mediated through a p53-dependent pathway.

Requirement of DNA repair mechanisms for survival of Burkholderia cepacia G4 upon degradation of trichloroethylene

A Tn5-based mutagenesis strategy was used to generate a collection of trichloroethylene (TCE)-sensitive (TCS) mutants in order to identify repair systems or protective mechanisms that shield Burkholderia cepacia G4 from the toxic effects associated with TCE oxidation. Single Tn5 insertion sites were mapped within open reading frames putatively encoding enzymes involved in DNA repair (UvrB, RuvB, RecA, and RecG) in 7 of the 11 TCS strains obtained (4 of the TCS strains had a single Tn5 insertion within a uvrB homolog). The data revealed that the uvrB-disrupted strains were exceptionally susceptible to killing by TCE oxidation, followed by the recA strain, while the ruvB and recG strains were just slightly more sensitive to TCE than the wild type. The uvrB and recA strains were also extremely sensitive to UV light and, to a lesser extent, to exposure to mitomycin C and H(2)O(2). The data from this study establishes that there is a link between DNA repair and the ability of B. cepacia G4 cells to survive following TCE transformation. A possible role for nucleotide excision repair and recombination repair activities in TCE-damaged cells is discussed.

An in vitro model for evaluation of vaporous toxicity of trichloroethylene and tetrachloroethylene to CHO-K1 cells

Toxicokinetics of trichloroethylene (TCE) and tetrachloroethylene (PER) in culture medium and their toxicity to CHO-K1 cells were investigated by employing an in vitro vapor exposure system. Cells were cultured in a 60 mm petri dish with a 25 mm glass dish glued in the central area. TCE or PER was added to the central glass dish so that it would evaporate and dissolve in the surrounding medium in which cells were growing. The results showed that the concentration of TCE or PER in medium increased significantly within 20 min and then decreased very rapidly with time. After a 24 h incubation, the residual of TCE or PER in the medium was very low, but was displayed in a dose-dependent manner. Treatment of cells with either TCE or PER resulted in a dose- and time-dependent inhibition of cell growth. A significantly increase in the frequency of micronuclei (MN) was also observed with either TCE or PER treatment. Low doses of TCE (5-20 microl) or PER (1-5 microl) significantly enhanced the intracellular glutathione (GSH) level. However, the level of GSH rapidly decreased with higher doses of TCE (40-80 microl) or PER (10-20 microl). Depletion of cellular GSH showed no effect on the sensitivity of cells to TCE or PER treatment. GSH-conjugation has been proposed as an activation mechanism to account for the nephrotoxicity of TCE and PER, however the toxicity of TCE and PER to CHO-K1 cells is probably mediated through a distinct mechanism.

Formation and detoxification of reactive intermediates in the metabolism of chlorinated ethenes

Short-chain halogenated aliphatics, such as chlorinated ethenes, constitute a large group of priority pollutants. This paper gives an overview on the chemical and physical properties of chlorinated aliphatics that are critical in determining their toxicological characteristics and recalcitrance to biodegradation. The toxic effects and principle metabolic pathways of halogenated ethenes in mammals are briefly discussed. Furthermore, the bacterial degradation of halogenated compounds is reviewed and it is described how product toxicity may explain why most chlorinated ethenes are only degraded cometabolically under aerobic conditions. The cometabolic degradation of chlorinated ethenes by oxygenase-producing microorganisms has been extensively studied. The physiology and bioremediation potential of methanotrophs has been well characterized and an overview of the available data on these organisms is presented. The sensitivity of methanotrophs to product toxicity is a major limitation for the transformation of chlorinated ethenes by these organisms. Most toxic effects arise from the inability to detoxify the reactive chlorinated epoxyethanes occurring as primary metabolites. Therefore, the last part of this review focuses on the metabolic reactions and enzymes that are involved in the detoxification of epoxides in mammals. A key role is played by glutathione S-transferases. Furthermore, an overview is presented on the current knowledge about bacterial enzymes involved in the metabolism of epoxides. Such enzymes might be useful for detoxifying chlorinated ethene epoxides and an example of a glutathione S-transferase with activity for dichloroepoxyethane is highlighted.

Dermal benzene and trichloroethylene induce aneuploidy in immature hematopoietic subpopulations in vivo

Accumulation of genetic damage in long-lived cell populations with proliferative capacity is implicated in tumorigenesis. Hematopoietic stem cells (hsc) maintain lifetime hematopoiesis, and recent studies demonstrate that hsc in leukemic patients are cytogenetically aberrant. We postulated that exposure to agents associated with increased leukemia risk would induce genomic changes in cells in the hsc compartment. Aneusomy involving chromosomes 2 and 11 in sorted hsc (Lin(-)c-kit(+)Sca-1(+)) and maturing lymphoid and myeloid cells from mice that received topical doses of benzene (bz) or trichloroethylene (TCE) was quantified using fluorescence in situ hybridization. Six days after bz or TCE exposure, aneuploid cells in the hsc compartment increase four- to eightfold in a dose- and schedule-independent manner. Aneuploid lymphoid and myeloid cells from bz- and TCE-treated mice approximate controls, except after repeated benzene exposures. Aneuploid cells are more frequent in the hsc compartment than in mature hematopoietic subpopulations. Hematotoxicity was also quantified in bz- and TCE-exposed hematopoietic subpopulations using two colony-forming assays: CFU-GM (colony-forming units/granulocyte-macrophage progenitors) and CAFC (cobblestone area-forming cells). Data indicate that bz is transiently cytotoxic (< or =1 week) to hsc subpopulations, and induces more persistent toxicity (>2 weeks) in maturing, committed progenitor subpopulations. TCE is not hematotoxic at the doses applied. In conclusion, we provide direct evidence for induction of aneuploidy in cells in the hsc compartment by topical exposure to bz and TCE. Disruption of genomic integrity and/or toxicity in hsc subpopulations may be one step in leukemic progression.

Effect of trichloroethylene on DNA methylation and expression of early-intermediate protooncogenes in the liver of B6C3F1 mice

Trichloroethylene (TCE) is a multimedia environmental pollution that is carcinogenic in mouse liver. The ability of TCE to modulate DNA methylation and the expression of immediate-early protooncogenes was evaluated. Female B6C3F1 mice were administered 1000 mg/kg TCE by gavage 5 days/week and killed after 5, 12, or 33 days of exposure. Methylation of DNA as 5-methylcytosine was decreased by 5 days of treatment with TCE and remained reduced for 33 days. TCE also decreased the methylation of the promoter regions for the protooncogenes, c-jun and c-myc. The expression of the mRNA for the two protooncogenes was increased between 60 and 120 minutes after administering the last dose of TCE and returned to control level by 24 hours. The expression of the mRNA for c-fos remained undetectable after administering TCE. Hence, TCE decreased the methylation both of total DNA and the promoters for the c-jun and c-myc genes and increased the expression of their mRNA. The decreased methylation and increased expression of the two immediate-early protooncogenes might be associated with TCE-induced increase in cell proliferation and promotion of tumors.

C-mitotic effects of trichloroethylene (TCE) on bone marrow cells of mice

The possible aneuploidy inducing activity of Trichloroethylene (TCE, CAS No. 79-01-6) an industrial chemical was investigated by employing three cytogenetic end points i.e., C-mitotic effects, Micronuclei (MN) and parallel chromosome structural aberration (CA) analysis in vivo. The experiments were conducted in mouse bone marrow cells. The animals were treated with TCE in the dose of 500, 1000, 2000 and 4000 mg/kg for 6, 12, 24, 48 hr. Colchicine (COL) was taken as positive control for its known aneuploidy-inducing effects and Cyclophosphamide as a model mutagen. TCE showed positive CM effects accompanied with increases of Mitotic Index and decreased frequencies of anaphases in higher doses. The chemical showed a positive MN response in bone marrow polychromatic erythrocytes but was negative in CA analysis. The preliminary results indicated that TCE is capable of inducing C-mitotic effects in mouse bone marrow cells which is suggestive of its possible aneuploidy induction potential.

Increased frequency of micronucleated kidney cells in rats exposed to halogenated anaesthetics

Six halogenated anaesthetics were tested for their ability to induce micronuclei formation in the rat kidney. A statistically significant increase in the frequency of micronucleated cells was detected in rats given a single p.o. dose of 4 mmol/kg of halothane (3.48 x baseline), chloroform (3.32 x baseline), trichloroethylene (3.24 x baseline), sevoflurane (2.98 x baseline), and isoflurane (2.95 x baseline). In contrast, the response was substantially negative in rats given the same dose of enflurane. As compared to controls, rats treated with halothane and trichloroethylene displayed a reduction in the frequency of binucleated cells presumably due to a toxicity-induced inhibition of cellular proliferation. These findings suggest a potential genotoxic activity of halogenated anaesthetics for the rat kidney.

Carcinogens induce reversion of the mouse pink-eyed unstable mutation

Deletions and other genome rearrangements are associated with carcinogenesis and inheritable diseases. The pink-eyed unstable (pun) mutation in the mouse is caused by duplication of a 70-kb internal fragment of the p gene. Spontaneous reversion events in homozygous pun/pun mice occur through deletion of a duplicated sequence. Reversion events in premelanocytes in the mouse embryo detected as black spots on the gray fur of the offspring were inducible by the carcinogen x-rays, ethyl methanesulfonate, methyl methanesulfonate, ethyl nitrosourea, benzo[a]pyrene, trichloroethylene, benzene, and sodium arsenate. The latter three carcinogens are not detectable with several in vitro or in vivo mutagenesis assays. We studied the molecular mechanism of the carcinogen-induced reversion events by cDNA analysis using reverse transcriptase-PCR method and identified the induced reversion events as deletions. DNA deletion assays may be sensitive indicators for carcinogen exposure.

Trichloroethylene cancer risk: simplified calculation of PBPK-based MCLs for cytotoxic end points

Cancer risk assessments for trichloroethylene (TCE) based on linear extrapolation from bioassay results are questionable in light of new data on TCE's likely mechanism of action involving induced cytotoxicity, for which a threshold-type dose-response model may be more appropriate. Previous studies have shown that if a genotoxic mechanism for TCE is assumed, algebraic methods can considerably simplify the use of physiologically based pharmacokinetic (PBPK) models to estimate virtually safe environmental concentrations for humans based on rodent cancer-bioassay data. We show here how such methods can be extended to the case in which TCE is assumed to induce cancer via cytotoxicity, to estimate environmentally safe concentrations based on rodent toxicity data. These methods can be substituted for the numerical methods typically used to calculate PBPK-effective doses when these are defined as peak concentrations. We selected liver and kidney as plausible target tissues, based on an analysis of rodent TCE-bioassay data and on a review of related data bearing on mechanism. Tumor patterns in rodent bioassays are shown to be consistent with our estimates of PBPK-based, effective cytotoxic doses to mice and rats used in these studies. When used with a margin of exposure of 1000, our method yielded maximum concentration levels for TCE of 16 ppb (87 micrograms/m3) for TCE in air respired 24 hr/day, 700 ppb (3.8 mg/m3) for TCE in air respired for relatively brief daily periods (e.g., 0.5 hr while showering/bathing), and 210 micrograms/liter for TCE in drinking water assuming a daily 2-liter ingestion. Cytotoxic effective doses were also estimated for occupational respiratory exposures. These estimates indicate that the current OSHA permissible exposure limit for TCE would produce metabolite concentrations that exceed an acute no observed adverse effect level for hepatotoxicity in mice. On this basis, the OSHA TCE limit is not expected to be protective.