||Springborn Labs., Inc., Wareham, MA.; Elf Atochem North America, Philadelphia, PA.; Environmental Protection Agency, Washington, DC. Office of Toxic Substances.
Methyl bromide (CAS No. 74-83- 9) was evaluated for chronic inhalation toxicity and carcinogenicity in Wistar rats (90/sex/treatment level) administered exposures (protocol unspecified) to 0, 3, 30, or 90 ppm for 6 hours/day, 5 days/week, for up to 29 months. Ten of each sex/treatment group were sacrificed for interim assessments at 13, 52 and 104 weeks. This report by Dr. Jerry F. Hardisty of Experimental Pathology Laboratories, Inc. primarily constitutes confirmation of the original study pathologist's conclusions. Upon specific reexamination of nasal cavity specimens in groups sacrificed at 52 and 104 weeks, incidence of basal cell hyperplasia of olfactory epithelium in the dorsi-medial nasal cavity in low dose groups did not reach statistical significance (Fischer's exact probability test) relative to controls. Such changes in mid- and high-dose rats were significantly related to exposure level; however, a low level exposure established the inhalation NOEL of 3 ppm for this study, contrary to original study conclusions. The reviewer also found the incidence, but not the severity, of these lesions to increase with time.