National Toxicology Program

TR-326
Toxicology and Carcinogenesis Studies of Ethylene Oxide (CAS No. 75-21-8) in B6C3F₁ Mice (Inhalation Studies)

Chemical Formula:  C2H4O	-	3D Structure*
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Ethylene oxide is a major industrial chemical used primarily as an intermediate in the manufacture of other chemicals; e.g., ethylene glycol, a major component of automotive and other antifreeze products. Exposure to ethylene oxide is greatest in the health care industry, where an estimated 75,000 workers are potentially exposed. Ethylene oxide was nominated for toxicology and carcinogenesis studies in B6C3F₁ mice because of its extensive production; the potential for human exposure in the workplace, from medical devices, or from food; the positive results of genetic toxicology assays; and the previous use of only F344/N rats in inhalation carcinogenicity studies.

Two inhalation studies reported in 1984 by Snellings et al. and by Lynch et al. demonstrated carcinogenic responses in F344/N rats. Results were similar in both studies and consisted of increased incidences of mononuclear cell leukemia, peritoneal mesotheliomas, and primary brain tumors.

Experimental Design:

Toxicology and carcinogenesis studies of ethylene oxide (greater than 99% pure) were conducted by exposing groups of 50 B6C3F₁ mice of each sex to air containing 0, 50, or 100 ppm ethylene oxide, 6 hours per day, 5 days per week for 102 weeks. These doses were selected because, in 14-week studies, all mice exposed at 600 ppm died within 1 week, and all mice exposed at 400 ppm died by week 4. Rhinitis was observed in both sexes exposed at 200, 400, and 600 ppm as was renal tubular degeneration in both sexes at 100, 200, and 400 ppm. The latter effects observed at 100 ppm were slight and deemed not to be life threatening in 2-year studies.

Two-Year Studies:

Survival of exposed and control mice was comparable in the 2-year studies (male: control, 28/50; low dose, 31/50; high dose, 34/50; female: 25/50; 24/50;31/50). Final mean body weights in exposed mice were 95%-102% of those of the controls. No compound-related clinical signs were observed.

Those neoplastic lesions that occurred at elevated incidences in mice exposed to ethylene oxide are reported in the following table (see page 6 of the Technical Report). In male mice, alveolar/bronchiolar carcinomas, alveolar/bronchiolar adenomas, and papillary cystadenomas of the harderian gland occurred with positive trends. In female mice, alveolar/bronchiolar adenomas, alveolar/bronchiolar carcinomas, papillary cystadenomas of the harderian gland, malignant lymphomas, and uterine adenocarcinomas occurred with positive trends. Mammary gland tumors also were increased in exposed female mice.

Data Audit:

An audit of the experimental data was conducted for the 2-year studies of ethylene oxide. No data discrepancies were found that influenced the final interpretations.

Conclusions:

Under the conditions of these 2-year inhalation studies, there was clear evidence of carcinogenic activity for B6C3F₁ mice as indicated by dose-related increased incidences of benign or malignant neoplasms of the lung and benign neoplasms of the harderian gland in both male and female B6C3F₁ mice following exposure to ethylene oxide vapors at 50 and 100 ppm. In female mice, ethylene oxide caused additional malignant neoplasms of the uterus, mammary gland, and hematopoietic system (lymphoma).

Synonyms: oxirane; EO; ETO; dihydrooxirene; dimethylene oxide; 1,2-epoxyethane; oxane; a,ß-oxidoethane

Report Date: November 1987

Target Organs & Incidences from 2-year Studies

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