Toxicity equivalents and EPA's risk assessment of 2,3,7,8-TCDD.

Toxicity equivalent factors (TEFs) have proved useful in estimating the toxicity of complex mixtures of chlorinated dibenzo-p-dioxins and dibenzofurans (CDDs/CDFs). An international consensus has formed around a specific set of TEF values as interim solution for addressing environmental contamination by CDDs/CDFs. This procedure capitalizes on the congener-specific analytical results that are more routinely available in recent years. The TEF approach should be updated as necessary and replaced by more definitive bioassay approaches as soon as practicable. In an independent activity, the USEPA considered a proposal to change (reduce) the cancer potency ascribed to 2,3,7,8-TCDD by a factor of 16. The recommendation was based upon an analysis of the literature and the Agency's earlier risk assessment. The proposal was reviewed by the Science Advisory Board, a group of outside scientific advisors. Subsequently, the Agency decided against making any changes in its assessment at this time. However, it is likely that a reassessment will be conducted shortly that will incorporate new data and a new approach to estimating of cancer risks posed by 2,3,7,8-TCDD.