Titanium Dioxide and Zinc Oxide Nanoparticles in Sunscreens: A Review of Toxicological Data

The positive effects of sunlight have been known for many years, and the negative ones, too. Sunscreens are physical and chemical UV absorbers. Nanotechnology has developed nanoparticles of physical blockers: titanium dioxide (TiO₂) and zinc oxide (ZnO). Their smaller diameter and increased bioreactivity are the focus of many toxicological studies. The usage of sunscreens has increased around the world, so all toxicological aspects should be carefully considered. There are in vitro and in vivo studies: studies on animal and human skin; investigations of potential genotoxicity and cytotoxicity; generation of reactive oxygen species; penetration; skin irritation; acute, subchronic, and chronic toxicity; and carcinogenesis. The experimental conditions of these studies differ from study to study, but most authors agree that there is no penetration of nanoparticles into viable skin layers. Risk-benefit analysis of TiO₂ and ZnO nanoparticles (NPs) usage in sunscreens strongly indicates that potential risks are vastly outweighed over the benefits. Because of the results of some authors indicating possible penetration through damaged skin, further studies should be conducted, primarily addressed on skin penetration mechanisms.