Interaction of light and chemicals in carcinogenesis.

Interactions between chemicals and UV light, as they may affect the carcinogenic process, are examined in terms of the three-component system: chemical, light, and biologic target. Various two-component interactions, in addition to unique three-component interactions, are considered. Available information is incomplete concerning such possibilities as promotion by noncarcinogenic light, the contribution of acute or chronic phototoxic events to chemical or physical carcinogenesis, and the relationship between either photochemical carcinogenesis or chemically enhanced photocarcinogenesis and demonstrable phototoxic activity. Interactions such as optical absorption by, or photochemical alteration of, chemicals are considered primarily as confounding variables in experimental situations. It is argued that realities of human exposure may reduce the complexity of these problems in practical safety or regulatory considerations.