[UV, visible and infrared light. Which wavelengths produce oxidative stress in human skin?]

Experimental evidence suggests that the creation of free radicals--mainly reactive oxygen species (ROS)--is the common photobiological answer to the skin-sunlight interaction. The free radical action spectrum (wavelength dependency) for ultraviolet and visible light (280-700 nm) has been determined by quantitative ESR spectroscopy. Visible light produces around 50% of the total oxidative stress caused by sunlight. Reactive species like *O(-)(2), *OH and *CHR are generated by visible light. The amount of ROS correlates with the visible light intensity (illuminance). We demonstrated the creation of excess free radicals by near-infrared light (NIR, 700-1600 nm). Free radical generation does not depend exclusively on the NIR irradiance, but also on the NIR initiated skin temperature increase. The temperature dependence follows the physiological fever curve. Our results indicate that the complex biological system skin creates the same type of free radicals over the entire active solar spectrum. This general response will make it possible to define the beneficial or deleterious action of sunlight on human skin by introduction of a free radical threshold value.