Damaging and protective properties of inorganic components of sunscreens applied to cultured human skin cells

Titanium dioxide particles are used in sunscreens to reflect UV radiation and are chemically modified to inhibit their natural photocatalytic ability. Titanium dioxide extracted from eight randomly selected commercial sunscreens and three titanium dioxide powders obtained from their manufacturers were investigated and crystal form and modification type of each identified. Under 3.5 mW/cm2 UVA illumination, the type of particle modification and crystal form determined whether the particles were inert or rapidly photodegraded an aqueous dispersion of methylene blue dye (MB), or whether the particles killed or protected cultured human skin cells. Mixed anatase and rutile crystal forms of titanium dioxide coated with organosilane photodegraded MB at a similar rate (11.5 min MB half-life) to Degussa P25, a standard uncoated titanium dioxide powder (11.0 min MB half-life) and generated a 2–4.9-fold increase in cell death. In contrast, pure rutile particles with an alumina coat or manganese doped protected cells from UVA irradiation. This research concludes that mixed anatase and rutile crystal forms of titanium dioxide coated with organosilane or dimethicone may not be appropriate to use in sunscreen lotions.