Sunscreen nanoparticles: study on localization in human skin, UV protection, phototoxicity

Ultrafine particlesof titanium dioxide (TiO2) and zinc oxide (ZnO) are extensively used in sunscreens as UV protective compounds. In this paper, we investigate how deep they penetrate into human skin, their UV screening ability in comparison to silicon (Si) particles, and also discuss generation of free radicals in presence of the nanoparticles under UV irradiation. We show that: a) even after multiple applications, TiO2 and ZnO particles are localized within the stratum corneum(uppermost skin layer); b) optimal diameters are found to be less or about hundred nanometers for TiO2 and Si particles for attenuation of the 310-400-nm UV light; c) when applied onto glass, small TiO2 particles (25 nm in diameter) are more phototoxicthan large (400 nm in diameter) particles.