In the Tittabawassee River basin, the major proportion of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-like exposure to mammals is from 2,3,7,8-tetrachlorodibenzofuran (TCDF) and 2,3,4,7,8-pentachlorodibenzofuran (PeCDF). Mink tissues collected from the Tittabawassee River had concentrations of TCDF and PeCDF that exceeded toxicity reference values (TRV), suggesting the potential for adverse effects. However, field evaluation of mink residing in the area indicated that the population was healthy. Two mink feeding studies were conducted to investigate the toxic potencies of TCDF and PeCDF in attempt to explain the unexpected lack of effects in the field. The first study was a toxicokinetic study that indicated hepatic cytochrome P450 activity can be used as an index of exposure to TCDF and PeCDF. The second study assessed the effects of feeding TCDD, TCDF or PeCDF at doses expected to cause adverse effects on reproduction and offspring viability and growth. The lack of significant effects on reproduction and offspring viability was unexpected based on TRVs established from other mammalian studies. Results suggest that the World Health Organization (WHO) Toxic Equivalency Factor (TEF) for TCDF requires further evaluation, and in the case of mink, the TEF for PCB 126 is underestimated or should be standardized outside the TCDD-centric TEF approach.
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