Human risk assessment and TEFs

The concept of toxic equivalency factors (TEFs) has been developed to facil itate risk assessment and regulatory control of exposure to complex PCDD, P CDF and PCB mixtures. Recently the European Centre for Environment and Heal th of the World Health Organization (WHO-ECEH) and the International Progra mme on Chemical Safety (IPCS) jointly reevaluated the TEFs of PCDDs, PCDFs and dioxin-like PCBs for mammals and derived consensus TEFs for birds and f ish (Stockholm, 1997). From a mechanistic point of view it can be concluded that, although the quantitative response will vary depending on the congen er involved, the occurrence of a common mechanism (binding to the Ah recept or) legitimates the use of the TEF concept across species. But there also i s criticism regarding the TEF concept, Pharmacokinetic differences between species can significantly, influence the TEF value, and uncertainties due t o additive ol nonadditive interactions, to differences in species responsiv eness and to differences in the shape of the dose-response curve might hamp er the derivation of consensus TEF values. In this context it should be not ed however, that using TCDD alone, as the only measure of exposure to dioxi n-like PCDDs, PCDFs and PCBs, would severely, underestimate the risk from e xposure to these compounds. Therefore, it can be concluded that, for pragma tic reasons, the TEF concept remains the most feasible approach for risk as sessment purposes, in spite of the uncertainties associated with its use.