Derivation of an Ingestion-Based Soil Remediation Criterion for Cr+6 Based on the NTP Chronic Bioassay Data for Sodium Dichromate Dihydrate

Abstract

Although the carcinogenicity of hexavalent chromium (Cr+6, Cr(VI)) by inhalation has been known for a long time, there has been little evidence regarding the potential for the carcinogenicity and no ability to estimate cancer potency of Cr+6 by ingestion until recently. The release in 2008 of the National Toxicology Program’s (NTP) chronic bioassay of rats and mice exposed to Cr+6 in drinking water provided clear evidence of cancer risk by ingestion and permits the estimation of the cancer potency and the associated soil remediation criterion. Dose-related increases in oral cavity tumors were observed in both sexes of rats and small intestine tumors were observed in both sexes of mice. Following USEPA guidance, NJDEP calculated a value for the human-equivalent cancer potency of 0.5 (mg Cr+6/kg body weight/day)-1 based on the most sensitive species and sex (male mice). For a one-in-a-million (1x10-6) lifetime cancer risk, this is equivalent to a daily dose of 1x10-6 mg Cr+6/kg body weight/day. Based on NJDEP soil remediation standards guidance, this corresponds to a soil concentration of 1 ppm (part per million). The NTP study was scientifically sound in its design and execution. Taking into account the ability of the stomach to metabolize Cr+6 to the less toxic Cr+3 form, the NTP animal data are judged to be relevant to human exposure. As per the USEPA scheme for characterization of carcinogenic potential, it is concluded that Cr+6 is “likely to be carcinogenic to humans” by ingestion.