The Toxic Equivalents Approach for Fish and Wildlife

The use of industrial, agricultural, and commercial products, as well as in­ cineration of the wastes of our society has lead to the release of polychlorinat­ ed biphenyls (PCBs), polychlorinated dibenzo-pdioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs) into the environments around us. PCBs, PCDDs, and PCDFs, as well as other planar halogenated hydrocarbons (PHHs), can persist in the environment, bioaccumulate in top predators of the food web, and result in deleterious effects in exposed fish and wildlife. Manda­ tory restrictions in use (viz., PCBs) and increased efforts to reduce the unin­ tentional production of other PHHs, such as PCDDs and PCDFs, have largely been successful in controlling the release of PHHs into the environment. De­ clining concentrations of PHHs in fish and wildlife species have been observed in many areas around the world. For example, the concentrations of PCBs, PCDDs, and PCDFs in most food webs in the Great Lakes have declined pre­ cipitously over the past three decades (Baumann and Whittle, 1988; Herbert et aZ., 1994). Likewise, the concentrations of many of the persistent organochlo­ rine chemicals found in biota of the Baltic Sea have decreased (Bignert et al., 1995). Even with many successful stories of significant reductions in environ­ mental concentrations of persistent PHHs, we still have the need and the re­ quirement to understand if these chemicals are above or below a threshold for adverse, toxic effects in fish and wildlife species. The complex nature of chem­ ical mixtures do not allow simple chemical by chemical regulatory procedures to work as accurately as is needed. The concept of toxic equivalency factors (TEFs) and toxic equivalents (TEQs) for PHHs was introduced some time ago as the cytosolic receptor, the aryl hydrocarbon receptor (Ah-R) was discovered (Poland and Glover, 19’75). The