Evaluation of the carcinogenicity of 1,1-dichloroethylene (vinylidene chloride)

The U.S. Environmental Protection Agency has classified 1,1-dichloroethylene (vinylidene chloride; VDC) as a "C" carcinogen and has developed an inhalation unit risk value and an oral cancer slope factor for this chemical. The development and use of these cancer potency estimates for risk assessment purposes are questionable. The inhalation unit risk value is based on increased kidney adenocarcinomas in Swiss mice from one study. This type of cancer was not increased in female mice or in rats or hamsters in the same study nor in male mice of a similar strain in another study with higher VDC exposures. The VDC oral cancer slope factor is based on a non-statistically significant increase in adrenal pheochromocytomas in male rats following oral exposure in a standard National Toxicology Program chronic bioassay. Both human and animal literature relevant to VDC carcinogenicity was reviewed according to the USEPA draft Guidelines for Carcinogen Risk Assessment with the objective of determining the weight-of-evidence for VDC carcinogenicity. We conclude that information currently available for VDC is most appropriately characterized in a weight-of-evidence narrative by the descriptor "inadequate for an assessment of human carcinogenic potential." For chemicals with this descriptor, dose-response assessment is not indicated. Under this guidance, quantitative estimates of cancer risks associated with VDC exposure are inappropriate until additional, more definitive evidence for human carcinogenicity becomes available.