Interactions in occupational and environmental toxicology were described and discussed. The importance of evaluating the whole person when health professionals diagnose potential toxicity from occupational and/or environmental exposure was noted, and examples were provided. Combined exposures to toxic agents with subsequent interactions may result from the personal lifestyle of an individual, including drugs which the person may be taking. Chemical exposures may occur at home, and chemicals may enter the home by way of workplace contaminated clothing or other items brought home from work. Toxic exposures to chemical and physical agents and other modifying factors (for example, diurnal variations due to shift work or stress) must be taken into consideration. Combined agent interactions have not been as well studied in occupational/environmental medicine as in clinical medicine. Drug interactions are known to occur, and a similar effect could be seen with chemical exposures. Interactions may be antagonistic, additive, or synergistic, and the type of interaction may depend on the dose. Age, sex, and general health condition (coexisting diseases, immune status, or obesity) can affect the toxicity of an occupational exposure. Environmental conditions such as circadian cycles, seasonal changes, temperature, humidity, noise and stress can all affect chemical toxicity. The author concludes that various kinds of interactions must be evaluated by occupational/environmental health professionals as they diagnose potential toxicity, and that many diseases may be the result of complex interactions among a wide variety of exposures and modifying conditions.