The missing link--light-induced (280-1,600 nm) free radical formation in human skin

The recent European recommendation on the efficacy of sunscreen products requests now a minimum ratio of UVA/UVB protection. However, the visible and the infrared (IR) parts of the sun spectrum have received little attention concerning their possible contribution to skin damage. A common biophysical answer for the different wavelengths of the sun spectrum can be found in the creation of excess free radicals - mainly reactive oxygen species (ROS). Thanks to electron spin resonance spectroscopy applied to skin biopsies, we determined for the first time the free radical action spectrum covering UV and visible light (280-700 nm). Convolution of the action spectrum with sunlight spectral irradiance showed that 50% of the total skin oxidative burden was generated by visible light. Creation of ROS by visible light was experimentally confirmed by varying the illuminance of a spotlight. We also evidenced the creation of excess free radicals by near-IR radiation. In that case, free radical generation does not depend exclusively on the dose, but also on the skin temperature increase initiated by near-IR light. Some phenomena which are still unclear, such as the question about the deleterious or beneficial role of sunlight, are reviewed, implying the research on new protection strategies for the prevention of skin cancer.