Glycol alkyl ethers represent a class of high-production-volume chemicals with widespread industrial applications as solvents and chemical intermediates. Comparative toxicity studies with three glycol ethers, 2-methoxyethanol, 2-ethoxyethanol, and 2-butoxyethanol, were conducted in F344/N rats and B6C3F1 mice in both 2-week and 13-week drinking water studies. Toxicologic endpoints evaluated in animals included histopathology, hematology, clinical chemistry, urinalysis, and reproductive system parameters. Genetic toxicity was also evaluated for each glycol ether in several in vitro and in vivo assays.
In the 2-week studies, groups of five male and five female rats and mice received 2-methoxyethanol, 2-ethoxyethanol, or 2-butoxyethanol in the drinking water. Estimates of compound consumption based on water consumption by male and female rats ranged from 100 to 400 mg/kg for 2-methoxyethanol, 200 to 1600 mg/kg for 2-ethoxyethanol, and 70 to 300 mg/kg for 2-butoxyethanol. For mice, consumption values ranged from 200 to 1300 mg/kg for 2-methoxyethanol, 400 to 2800 mg/kg for 2-ethoxyethanol, and 90 to 1400 mg/kg for 2-butoxyethanol. There were no chemical-related effects on survival for rats or mice in the 2-week studies. Decreased body weight gains were noted for both male and female rats treated with 2-methoxyethanol or 2-ethoxyethanol for 2 weeks, and there were dose-related decreases in water consumption for rats of each sex treated with the ethylene glycol ethers. Most of the changes in organ weights for rats and mice treated with the glycol ethers were sporadic (mice) or related to low final mean body weights (rats), except for thymicatrophy in male and female rats and testicular atrophy in males of both species receiving 2-methoxyethanol or 2-ethoxyethanol.
In the 13-week studies in rats, groups of 10 males and 10 females received 2-methoxyethanol, 2-ethoxyethanol, or 2-butoxyethanol in the drinking water at concentrations ranging from 750 to 6000 ppm, 1250 to 20,000 ppm, or 750 to 6000 ppm, respectively. In the 13-week studies in mice, groups of 10 males and 10 females received 2-methoxyethanol, 2-ethoxyethanol, or 2-butoxyethanol in the drinking water at concentrations ranging from 2000 to 10,000 ppm, 2500 to 40,000 ppm, or 750 to 6000 ppm, respectively. Estimates of compound consumption based on water consumption by male and female rats ranged from 70 to 800 mg/kg for 2-methoxyethanol, 100 to 2200-mg/kg for 2-ethoxyethanol, and 70 to 500 mg/kg for 2-butoxyethanol. For-mice, consumption values ranged from 300 to 1800 mg/kg for 2-methoxyethanol, 600 to 11,000 mg/kg for 2-ethoxyethanol, and 100 to 1300 mg/kg for 2-butoxyethanol.
Chemical-related mortality occurred in male and female rats administered 4500 or 6000 ppm 2-methoxyethanol and in male and female rats administered 20,000 ppm 2-ethoxyethanol. No deaths occurred in rats administered 2-butoxyethanol or in mice administered 2-methoxyethanol, 2-ethoxyethanol,or 2-butoxyethanol. Decreased body weight gains occurred in dosed rats and mice in all three studies; the greatest reductions in body weight gain were seen with 2-methoxyethanol.
In rats administered 2-methoxyethanol or 2-ethoxyethanol, treatment-related histopathologic changes were observed in the testes, thymus, and hematopoietic tissues (spleen, bone marrow, and liver). A dose-related degeneration of the germinal epithelium in the seminiferous tubules of the testes was more severe in 2-methoxyethanol-treated rats than in rats treated with 2-ethoxyethanol. In special stop-exposure studies in male rats in which administration of the glycol ethers was stopped after 60 days, marked degeneration of the seminiferous tubules was present in rats treated with 3000 ppm 2-methoxyethanol, and mild to moderate degeneration was observed in rats treated with 1500 ppm. Moderate to marked testicular degeneration was present in rats treated with 10,000 or 20,000 ppm 2-ethoxyethanol but not in rats treated with 5000 ppm. After 30 and 56 days of recovery from treatment with these chemicals, only partial recovery from testicular degeneration was observed. There was no testicular degeneration after 60 days of treatment with 1500 to 6000 ppm 2-butoxyethanol.
2-Methoxyethanol treatment for 13 weeks resulted in a progressive anemia associated with a cellular depletion of bone marrow and fibrosis of the splenic capsule. Anemia was also seen with 2-ethoxyethanol, but evidence of an adaptive response was indicated by increased hematopoiesis in the bone marrow, spleen, and liver. Toxicity with 2-butoxyethanol was limited to the liver and hematopoietic system. Cytoplasmic alteration and a minimal hepatocellular degeneration were present in the liver of male and female rats. A minimal anemia was present, and a hematopoietic response was evident in the bone marrow and spleen.
In mice, 2-methoxyethanol and 2-ethoxyethanol had similar effects on the testes, spleen, and adrenal gland (females only). A dose-related degeneration of the germinal epithelium in seminiferous tubules of the testes was more severe with 2-methoxyethanol than with 2-ethoxyethanol. A dose-related increase in splenic hematopoiesis was also more prominent with 2-methoxyethanol. Both 2-methoxyethanol and 2-ethoxyethanol caused a prominent lipid vacuolization of the X-zone of the adrenal gland in female mice. There were no chemical-related lesions attributed to 2-butoxyethanol administration in mice.
All three of the glycol ethers were negative in Salmonella typhimurium mutation tests conducted with and without induced hamster and rat liver S9. In the mouse lymphoma L5178Y cell mutation assay, 2-ethoxyethanol was negative without S9 but was weakly positive in the presence of induced rat liver S9; 2-methoxyethanol and 2-butoxyethanol were not tested in this assay. At high concentrations, 2-ethoxyethanol induced sister chromatid exchanges (SCEs) in Chinese hamster ovary cells with and without S9. Chromosomal aberrations (Abs) were also induced by 2-ethoxyethanol, but only in the absence of S9 and without a delay in cell cycle. In contrast, 2-butoxyethanol induced cell cycle delay but did not induce SCEs or Abs with or without S9. 2-Ethoxyethanol was the only glycol ether tested for induction of sex-linked recessive lethal mutations in germ cells of Drosophila melanogaster; both feeding and injection trials were negative.
In summary, based on survival, decreased body weight gains, and histopathologic effects, the rank order of toxicity for the three glycol alkyl ethers was 2-methoxyethanol>2-ethoxyethanol>2-butoxyethanol; the toxic effects were more severe in rats than in mice. In the 13-week study of 2-methoxyethanol in rats, a no-observed-adverse-effect level (NOAEL) was not reached, since testicular degeneration in males and decreased thymus weights in males and females occurred at the lowest concentration administered (750 ppm). In the 13-week study of 2-ethoxyethanol in rats, the NOAEL for decreased thymus weights in males was 1250 ppm; for female rats treated with 2-ethoxyethanol for 13 weeks, the NOAEL for all histopathologic and hematologic effects was 5000 ppm. In rats treated with 2-butoxyethanol for 13 weeks, the NOAEL for liver degeneration was 1500 ppm in males and females.
For male mice treated with 2-methoxyethanol for 13 weeks, the NOAEL for testicular degeneration and increased hematopoiesis in the spleen was 2000 ppm. A NOAEL was not reached for female mice treated with 2-methoxyethanol, since adrenal gland hypertrophy and increased hematopoiesis in the spleen occurred at the lowest concentration administered (2000 ppm). For male mice treated with 2-ethoxyethanol for 13 weeks, the NOAEL for testicular degeneration and increased hematopoiesis in the spleen was 20,000 ppm. For female mice in the 13-week study of 2-ethoxyethanol, the NOAEL for adrenal gland hypertrophy and increased hematopoiesis in the spleen was 5000 ppm. No clear chemical-related effects were seen in male or female mice administered 2-butoxyethanol for 13 weeks at concentrations as high as 6000 ppm.