Multichemical exposure is the rule rather than an exception.
Chemical mixtures in the environment can be and generally are complex, consisting of parent compounds, reaction or transformation products along with other residues and potentially inert materials. Exposure of biota to combinations of pollutants may sometimes result in unexpected consequences, i.e., significantly lower or greater toxic response than a simple summation of the response induced by the mixture components taken individually. The toxicological interactions are frequently described using terms like antagonism, potentiation and synergism. Antagonism results when one component of the mixture interferes with another in such a way that the total toxic response is less than the sum of the individual responses. The protection against mercury toxicity afforded by selenium is a typical example of this category.