Introduction and rationale

When toxic substances cause or appear to cause problems in the environment or for human health, the issues are rarely simple, and the chemical and other potential causes of harm rarely occur in isolation. Typically, there is a great deal of complexity in the system of concern and uncertainty concerning the relationship between cause and effect. The reasons for this include the facts that (a) one typically encounters combinations of toxic substances, rather than single pollutants; (b) the system of concern may not in itself be clearly understood, even in the absence of chemical perturbation; and (c) data, particularly from field studies, are often incomplete. Yet, the environmental toxicologist is frequently challenged to address the effects of natural or anthropogenic perturbations, either after the fact or, ideally, in the planning stage of any human endeavour that might have an environmental impact.

Up to this stage in the text, although examples and case studies have been presented, we have dealt mainly with definitions, methods, and approaches and the environmental toxicology of individual substances or classes of substance. In the present chapter, we address a number of complex issues, exemplifying the impact of certain processes and also providing vehicles for integrating some of the basic concepts and information that have been covered so far.