Nanotechnology, cosmetics and the skin: is there a health risk?

Cosmetic formulations may contain nano-emulsions and microscopic vesicles consisting of traditional cosmetic materials, although it is uncertain whether they should be qualified as actual nanomaterials. Vesicle materials do not penetrate into living human skin. Vesicle formulations may enhance or reduce skin absorption of ingredients, albeit at a limited scale. Sunscreens contain TiO2 or ZnO nanoparticles (NP), which are efficient UV filters. A number of studies suggest that insoluble NP do not penetrate into or through human skin. The results of in vivo toxicity tests showed that TiO2 and ZnO NP are non-toxic. In vitro and in vivo cytotoxicity, genotoxicity, photogenotoxicity, acute toxicity, sensitisation and ecotoxicology studies on TiO2 NP found no difference in the safety profile of micro- or nano-sized materials, all of which were non-toxic. Although some in vitro investigations on TiO2 particles reported cell uptake, oxidative cell damage or genotoxicity, these results may be secondary to phagocytosis of cells exposed to excessive particle concentrations. Studies on wear debris nano- and microparticles support the traditional view that toxicity of small particles is related to their chemistry, rather than their particle size. There is little evidence supporting a general rule that adverse effects of particles on the skin or other tissues increase with smaller particle size, or produce novel toxicities relative to those of larger particles. Overall, the current evidence suggests that nano-sized cosmetic or sunscreen ingredients pose no potential risk to human health, whereas their use in sunscreens has large benefits, such as the protection of human skin against skin cancer.