Photoprotection of human skin

UV radiation can cause damage to human skin. The ambient UV radiation which humans are exposed to is largely controlled by the amount of stratospheric ozone, which absorbs the UVC and short-wave UVB radiation of the sun. There has been a certain depletion of the ozone layer due to emission of chlorofluorocarbons in the last century. As these substances have been banned, the ozone layer is already slowly recovering. In the epidermis of human skin melanin can be produced as an own protection system against UV radiation. The amount of this protection depends strongly on skin type. Sunscreens can help to increase the protection against the damaging effects of solar irradiation in the UVB and UVA range. The active ingredients of sunscreens are UV filters, which have to be specifically approved for this application. Quite high concentrations of these filters have to be incorporated into the sunscreen formulations to achieve good protection, and in most cases a cocktail of filters is used, which also allows various designs of the UV protection spectra. The most important quantity for the assessment of a sunscreen is the sun protection factor (SPF), which is measured in vivo on human volunteers. As the same sun protection factor can be obtained with different shapes of the absorbance curve, the protection in the UVA range has to be assessed with another method. Great progress has been achieved in the development of in vitro methodology for the assessment of UVA protection in the last decade. Not all of the available UV filters are photostable, and therefore irradiation steps are included in most of these methods. The performance of sunscreens can be simulated with model calculations. For the calculation the quantitative UV spectra of all UV absorbers must be known as well as the kinetics of a possible photodegradation. In addition, action spectra, irradiance profiles, and the irregularity of the sunscreen film forming on the skin have to be taken into consideration. With that realistic simulations are possible only with the input of the filter amounts to be used. The simulations can help to understand specific properties of sunscreens such as the dependence of the protection at different application amounts or variations of the solar spectrum.