NTP Toxicology and Carcinogenesis Studies of Crocidolite Asbestos (CAS No. 12001-28-4) In F344/N Rats (Feed Studies).
Carcinogenesis studies of crocidolite asbestos were conducted with male and female F344/N rats. This form of asbestos was administered at a concentration of 1% in pelleted diet for the lifetime of the rats, starting with the dams of the study animals. The studies were started in January 1978 and ended in December 1980. Group sizes were 118 for male and female controls and 250 for male and female crocidolite asbestos-exposed rats. The offspring from mothers exposed to crocidolite asbestos and the controls were similar in size at birth but were slightly smaller at weaning and remained so throughout their life. Feed consumption and survival were comparable in the exposed and control groups. No overt toxicity was observed in the crocidolite asbestos-exposed animals. There was an elevated (P<0.05) incidence of thyroid gland C-cell adenomas (control, 4/117, 3%, vs. exposed, 23/250, 9%) and of thyroid gland C-cell carcinomas (12/117, 10%, vs. 46/250, 18%) in crocidolite asbestos-exposed female rats relative to concurrent controls. Because these control incidences were low relative to control incidences observed in other contemporary studies at this laboratory (21% for thyroid gland C-cell tumors), this slight increase was not regarded as being biologically important. The data, documents, and pathology materials from the lifetime studies of crocidolite asbestos have been audited. The audit findings show that the conduct of these studies is documented adequately and support the data and results presented in this Technical Report. Conclusions: Under the conditions of these feed studies, crocidolite asbestos was not overtly toxic and did not cause a carcinogenic response when ingested at a concentration of 1% in the diet by male and female F344/N rats for their lifetime. Levels of Evidence of Carcinogenicity: Male Rats: Negative Female Rats: Negative