Titanium dioxide (TiO2) is a common ingredient in cosmetics and sunscreens. It is generally considered safe because it does not penetrate the stratum corneum, the outermost layer of the skin that protects the underlying tissue. But when TiO2 reacts to ultraviolet light exposure, reactive oxygen species are generated that can affect the skin's permeability and susceptibility to harmful substances. Because the ability of TiO2 to adhere to the skin and its reactivity to light largely depend on the crystal form it takes, choosing the proper form of TiO2 is key in designing safer products for human use.
Francesco Turci (University of Torino, Italy) led a study to investigate the properties of two crystal forms of TiO2 when applied to the ear skin of pigs under low levels of ultraviolet light. A form called anatase was not easily removed with washing and reacted with light to disrupt the natural arrangement of lipids in the stratum corneum, making it more permeable. A form called rutile, on the other hand, was efficiently removed by washing and was less reactive when exposed to light (Chem. Res. Toxicol. doi:10.1021/tx400285j; published online 12 September 2013). The findings suggest rutile should be used to produce safer TiO2-based products.
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