Executive Summary

The National Toxicology Program (NTP) chronic bioassay of rats and mice exposed to sodium dichromate dihydrate in drinking water is the first study that provides data on the carcinogenicity of hexavalent chromium (Cr+6) by ingestion that is appropriate for quantitative risk assessment. Sodium dichromate dihydrate is readily soluble, yielding the dichromate ion that exists in equilibrium in solution with the chromate ion. The results of the NTP study are, therefore, applicable to the cancer risk assessment of Cr+6 by ingestion in general. NTP concluded that the study provides “clear evidence of carcinogenicity ” in male and female mice and rats, based on benign and malignant tumors in rat oral mucosa and mouse small intestine. Consistent with the criteria for carcinogen characterization in the USEPA Guidelines for Carcinogen Risk Assessment, Cr+6 by ingestion is determined to be “likely to be carcinogenic to humans. ” The mouse was selected as the most sensitive species and the human cancer slope factor was developed based on assumptions and approaches that are consistent with the 2005 USEPA Guidelines for Carcinogen Risk Assessment. The human cancer slope factor was estimated to be 0.5 (mg/kg/day)-1 based on the tumor incidence in male mice. Results from the combined data sets of male an