Combined, Simultaneous Exposure to Radiation Within and Beyond the UV Spectrum: A Novel Approach to Better Understand Skin Damage by Natural Sunlight

The effects of UVB and UVA radiation on human skin have been extensively studied, and as one consequence, sunscreen products usually provide protection against both radiation types. There is now also compelling evidence that nonionizing radiation which is present in natural sunlight but extends beyond the UV spectrum can exert biological effects on human skin. For example, the blue light part of visible light can cause skin pigmentation in darker individuals and radiation in the near-infrared range, i.e., Infrared A radiation can influence the homeostasis of the dermal extracellular matrix and thereby contribute to the formation of wrinkles. Accordingly, more and more sunscreen products now also claim to protect against radiation beyond UV. Of note, almost all of these studies have used artificial irradiation devices, which exclusively emit in the spectral region of interest. Here we would like to emphasize that such exposure scenarios are in sharp contrast to irradiation with natural sunlight, where UVB, UVA, visible light, and infrared radiation are simultaneously delivered to human skin. Studies employing sequential irradiation protocols with, e.g., UVB plus UVA, UVA plus UVB, or IRA plus UVB, however, indicate that such additive regimens give biological responses which differ from those obtained with single wavelength areas. We therefore here hypothesize that signaling responses elicited in human skin by distinct wavelengths may influence each other and that because of this reason, the net response caused by natural sunlight is different from that induced by artificial UVB or UVA or visible light or infrared irradiation alone. If case our hypothesis turns out to be true, then studies to define the relative contribution of each wavelength area to the overall effect of natural sunlight are urgently needed, because such knowledge will directly impact on the design of photoprotection strategies for human skin.