The Formula for Best Sunscreen Performance: Beer-Lambert's Law Under the Microscope.

Sunscreens used for the protection of human skin work by attenuating the potentially harmful solar UV radiation. In recent years, the quantitative understanding of this attenuating effect has grown tremendously, enabling model calculations of sunscreen performance. Such calculations are based on the simulation of the UV transmission of the sunscreen film applied on human skin. However, there are 2 prerequisites assumed to hold. The first prerequisite is the applicability of the Beer-Lambert law for sunscreen films, and the second is that the thickness variation of the sunscreen film can be described with a gamma distribution of film heights. There is strong evidence from recent experimental work that both assumptions are correct. For several applications, calculations of sunscreen performance have been shown to be useful, for instance, in the design of new sunscreen formulations aiming for a certain sun protection factor or other characteristics, prediction of pre-vitamin D production in the skin in the presence of sunscreen, in vitro measurement of water resistance, and assessment of the ecotoxicological profile of a sunscreen formulation or the influence of oil polarity on UV-filter absorbance and the consequence for sunscreen performance.

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