Monte Carlo simulations of light transport in sunscreen formulations.

Sunscreens are used for the protection of human skin against the harmful effects of solar UV radiation. Due to the low thickness of sunscreen films typically applied to the skin, it can be challenging to achieve the strong absorbance needed for good UV-protection, and most efficient sunscreen compositions are desirable. The presence of scattering particles can increase the efficacy of dissolved UV-absorbers in the oil or water phases of the formulation. As many sunscreens contain UV-absorbing particles, it is of interest how much the scattering effect of such materials contribute to the protection of the respective sunscreen. The currently available software programs for simulating sunscreen performance are based on a Beer-Lambert law approach and do not take into account such scattering effects of particles. However, Monte Carlo simulations of the UV-light transport through sunscreen films are capable to take scattering from particles into consideration. Using Monte Carlo simulations, this work shows that the efficacy of absorbance is indeed increased in the presence of scattering particles. However, this is of limited significance when the particles are UV-absorbers themselves.