ABSTRACT
In the natural environment, organisms are frequently exposed to complex mixtures of pollutants and it is relatively uncommon to find any one pollutant dominant over all others. Yet, because of 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 that exist in terrestrial, marine or freshwater ecosystems or that may arise because of the release of new chemicals into the environment. The complexity of the situation is illustrated in figure 9.1, which gives 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 the case of the mollusc than in the harbour seal. When effluents or contaminated environmental samples are subjected to testing, toxicity is often due to more than one chemical component of a mixture, and questions arise concerning possible potentiation. This issue has already been discussed in section 6.2.
