Atmospheric pollutants that reduce the amount of ozone in the stratosphere may markedly increase the flux of intermediate-wavelength solar ultraviolet (UV) radiation that reaches the Earth's surface. Like short-wavelength germicidal UV radiation (less than 280 nm), these intermediate UV wavelengths (280-315 nm) can promote photochemical reactions in nucleic acids, leading to the appearance of such products as cyclobutadipyrimidines and single- and double-strand breaks. These photochemical reactions strongly affect the biological activities of the nucleic acids. Computer techniques are now available for predicting the chemical and biological effects of increased in vitro irradiation of purified nucleic acids. However, the effect of increased UV irradiation in vivo is complicated by the presence of sensitizing agents in cells and by the action of nucleic acid repair processes. There is strong evidence that in vivo damage to nucleic acids injures irradiated cells and tissues, but further research is needed to predict quantitatively the physiological consequences of increases in solar UV.