Interactions of different wavelengths in effects of UV radiation on skin

Literature data on photoaddition, photorecovery and photoaugmentation are discussed. The practical conclusion is that for estimates of the main risks of ultraviolet radiation to human skin, sunburn and skin cancer, any deviations from photoaddition are small. Risk estimates may be made on the basis of addition, and the errors possibly made by doing so are small compared to other uncertainties in such calculations. Deviations from photoaddition reported are interesting from the viewpoint of studying mechanisms. Apparent controversies cannot be resolved on the basis of presently available data. Confusion can certainly be reduced, and there are indications that more subtle techniques of investigation are needed to resolve the remaining problems.