The weight of the evidence among group C carcinogens.

Pesticide chemicals (more than 40) with limited evidence for carcinogenicity in animal experiments, and assigned to Group C according to the EPA Guidelines for Carcinogen Risk Assessment, were further analyzed and ranked by refining the weight of the evidence evaluation. For some of these chemicals there were considerable concern with respect to their potential as human carcinogens and for others there was minimal concern. A clear tumor response at several doses, malignancy of the tumors, the tumor type (rare versus common), the timing of the tumor, and the observance of tumors in both sexes raised the concern for potential human carcinogenicity, as did ancillary information such as the structure-activity relationship and genotoxicity. The design and quality of the studies used in the evaluation were significant factors in reaching a conclusion concerning the carcinogenic potential of the chemicals. Potency factors (Q1*) calculated for these chemicals indicated no correlation with the level of biological evidence for carcinogenicity.