Homeostasis in Topical Photoprotection: Getting the Spectral Balance Right

The solar radiation range has harmful and beneficial effects. Sunscreens, which selectively block specific spectral regions, may potentially interfere with skin homeostasis. For instance, the ultraviolet (UV) B waveband produces erythema and DNA damage; simultaneously, it induces pre-vitamin D₃ synthesis. UVA1 and visible light can both induce pigmentation in skin phototypes IV–VI, and act in synergy to induce erythema and persistent pigment darkening. In contrast, UVA may contribute to blood pressure control and cardioprotection by inducing release of nitric oxide from intracutaneous photolabile nitric oxide derivatives. Finally, infrared A radiation alters the collagen equilibrium of the dermal extracellular matrix but is involved in the regulation of body temperature and in nitric oxide release, with a potential beneficial impact on blood pressure regulation. Ideally, photoprotection should thus be performed with a neutral density filter, mitigating all radiation ranges homogeneously, to maintain solar spectrum homeostasis. Natural compounds such as mycosporine-like amino acids are promising natural UV radiation-filtering compounds for an improved homeostasis with our environment. Lastly, we should not forget individual characteristics and behavior, as homeostasis differs according to individual phototypes and skin exposure behaviors.