Methyl bromide is widely used as a fumigant and pesticide. Toxicology and carcinogenesis studies were conducted by exposing groups of male and female B6C3F1 mice to methyl bromide (99.8% pure) by inhalation 6 hours per day, 5 days per week, for 14 days, 6 weeks, 13 weeks, or 2 years. Six-week and 13-week inhalation toxicity studies in F344/N rats were conducted concurrently with the mouse studies. Hematology parameters were measured during the 6-week, 13-week, and 2-year studies. Quantitative neurobehavioral testing was performed during the 14-day, 13-week and 2-year studies. Genetic toxicology studies were conducted for gene mutation induction in Salmonella typhimurium and for induction of sister chromatid exchanges in mouse bone marrow cells and of micronuclei from peripheral blood erythrocytes.
Fourteen-day studies
Groups of five B6C3F1 mice of each sex were exposed to 0, 12, 25, 50, 100, or 200 ppm methyl bromide by inhalation 6 hours per day, 5 days per week for 2 weeks. Only four female mice and one male mouse survived 10 exposures at 200 ppm. No deaths occurred at the lower doses. Neurobehavioral effects including trembling and paralysis were noted in all groups, but were most pronounced in the three highest dose groups. Red urine was noted in the mice exposed to 200 ppm.
Thirteen-week studies
Groups of 10 mice of each sex were exposed to 0, 10, 20, 40, 80, or 120 ppm methyl bromide by inhalation 6 hours per day, 5 days per week for 13 weeks. Additional groups of eight to 17 mice were concurrently exposed for neurobehavioral and genetic toxicology studies. The final mean body weight of males exposed to 120 ppm was significantly (12%) lower than that of the controls. Four of 24 males exposed to 120 ppm died during the study.
Groups of 10 rats of each sex were exposed to 0, 30, 60, or 120 ppm methyl bromide by inhalation 6 hours per day, 5 days per week for 13 weeks. Additional groups of eight rats were concurrently exposed for neurobehavioral studies. Final mean body weights of rats exposed to 120 ppm were 12% lower than those of the controls for males and 13% lower for females. No rats died as a result of methyl bromide exposure during the studies.
Special six-week target organ toxicity studies
Neither the 14-day nor the 13-week studies provided strong evidence for specific organ toxicity. Six-week studies were therefore conducted to identify target organs for the 2-year studies. Groups of 20 rats and mice of each sex were exposed to methyl bromide by inhalation for 6 hours per day, 5 days per week for 6 weeks at a dose of 160 ppm. Mortality rates exceeded 50% in the male mice after eight exposures, in female mice after six exposures, and in male rats after 14 exposures. Only the female rat group survived 30 exposures with less than 50% mortality. The study identified the brain, kidney, nasal cavity, heart, adrenal gland, liver, and testis as the primary organs to examine for toxicity in the 2-year methyl bromide inhalation studies.
Two-year studies
Groups of 70 B6C3F1 mice of each sex were exposed to methyl bromide by inhalation at 0, 10, 33, or 100 ppm for 6 hours per day, 5 days per week for up to 103 weeks. Additional groups of 16 mice were included for neurobehavioral evaluations throughout the 2-year studies. By 20 weeks (139 days), 27 males and 7 females exposed to 100 ppm had died and methyl bromide exposure was discontinued for the remaining mice in this dose group. Ten female mice from the 100 ppm group predesignated for the 15-month interim evaluation were killed on schedule and all other high-dose animals were allowed to live to term (24 months) for evaluation of chronic toxicity and carcinogenicity. Clinical signs indicative of neurotoxicity, including tremors, abnormal posture, tachypnea, and hind leg paralysis, persisted in these high-dose mice until the end of the studies.
Final mean body weights of surviving 100 ppm males and females were markedly lower (33% and 31%) than those of the controls. Neurobehavioral changes occurred in male and female mice initially exposed to 100 ppm methyl bromide, with more pronounced changes observed in males. In general, these animals were less active and manifested a heightened sensitivity in the startle response than mice in other dose groups.
Exposure to methyl bromide was not carcinogenic under the conditions of these studies. However, there was an increase in the incidence of several nonneoplastic lesions in the brain, heart, bone (sternum), and nose. Degenerative changes in the cerebellum and cerebrum occurred in males and females exposed to 100 ppm. Myocardial degeneration and cardiomyopathy were observed in the hearts of mice exposed to 100 ppm. An increased incidence of sternal dysplasia was seen in treated animals, particularly in those exposed to 100 ppm. An increased incidence of olfactory epithelial necrosis and metaplasia within the nasal cavity was seen in the mice exposed to 100 ppm, particularly males.
Genetic toxicology
Methyl bromide was positive for induction of gene mutations in Salmonella typhimuriumstrain TA100, with and without exogenous metabolic activation; negative results were obtained with TA98 in this assay. In vivo, methyl bromide induced sister chromatid exchanges in bone marrow cells and micronuclei in peripheral erythrocytes of female mice exposed by inhalation for 14 days. No significant increase in either sister chromatid exchanges or micronuclei was observed in male or female mice exposed to methyl bromide by inhalation for 4, 8, or 12 weeks.
Conclusions
Under the conditions of these 2-year inhalation studies, methyl bromide caused degenerative changes in the cerebellum and cerebrum, myocardial degeneration and cardiomyopathy, sternal dysplasia, and olfactory epithelial necrosis and metaplasia. Toxic effects persisted although exposure to methyl bromide in the 100 ppm group terminated after 20 weeks. There was no evidence of carcinogenic activity of methyl bromide in male or female B6C3F1 mice exposed to 10, 33, or 100 ppm.