Dose-effect relationships and general mechanisms of combined exposures

Organisms are exposed to a multitude of physical, chemical and biological agents. Many data are available on the risks associated with these agents when applied individually. However, an assessment of the overall risk has to take into consideration the possibility of unexpected risks after combined exposures. There is no way of testing all conceivable combinations of agents. Thus, we have to look for general mechanisms that permit extrapolations to situations not yet tested. The study of mechanisms requires the knowledge of the shape of the dose-response relations for all agents participating in the combined exposure. Possibilities for the analysis of effects after combined exposures are described. It emerges that very specific processes in the chain of events must be affected in order to influence radiation risk by another agent. Two such specific processes of importance are recovery phenomena and cell proliferation.