Interaction between human carcinogens.

: In the absence of direct information on the carcinogenicity of a complex mixture, assessment of its risk requires not only knowledge of the risks due to the separate components, but also assumptions about the interaction between the components. A formal definition of interaction is given, followed by a discussion of the theoretical basis for different kinds of interactions. Epidemiological studies which have considered the simultaneous effect of two chemical carcinogens are reviewed, and shown to provide examples of additivity, multiplicativity and interaction both intermediate between the two and greater than multiplicative. Finally, implications for the risk assessment of mixtures are discussed.