Synergy effects between organic and inorganic UV filters in sunscreens.

The influence of the synergy effects between organic and inorganic UV filter substances on the sun protection factor (SPF) of topically applied sunscreen formulations is investigated. The medium is considered to have reflection, absorption, and scattering properties. The distribution of photons in this medium is investigated by Monte Carlo calculation. Typical optical parameters of the skin and substances are used to characterize the synergy effect. The results of the model calculation are checked by in vitro and in vivo measurements investigating the influence of different types of scattering microparticles on the absorption efficacy of topically applied formulations. It is found that the inorganic filter substances act as scattering microparticles in the upper skin layers. They increase the optical pathway of the photons in the topically applied absorbing formulation also localized there. In this way, more photons are absorbed, increasing the SPF. The results obtained are important for the optimization of the SPF of sunscreen formulation containing organic and inorganic UV-filter components.