ABSTRACT
In a natural environment, organisms are frequently exposed to complex mixtures of pollutants, and it is relatively uncommon to –nd any single pollutant dominant over all others. Due to limitations of time and resources, nearly all regulatory toxicity testing is carried out using single compounds. It is not feasible to test the toxicity of more than a very small proportion of the chemical combinations present in terrestrial, marine, or freshwater ecosystems or that may arise from the release of new chemicals into the environment. Figure 9.1 illustrates the complexity of the situation and shows analytical data for residues of PCBs in tissues of organisms from a polluted area. A number of different PCB congeners are found in both species, with a wider selection in mollusks than in harbor seals. When ef¨uents or contaminated environmental samples are subjected to testing, toxicity is often caused by more than one chemical component of a mixture, and questions arise concerning possible potentiation. This issue was discussed in Section 6.2.
