Using a particular type of titanium dioxide -- a common ingredient in cosmetics（化妆品）, food products, toothpaste and sunscreen -- could reduce the potential health risks associated with the widely used compound. The report on the substance, produced by the millions of tons every year for the global market, appears in the ACS journal Chemical Research in Toxicology. Francesco Turci and colleagues explain that titanium dioxide (TiO2) is generally considered a safe ingredient in commercially available skin products because it doesn't penetrate healthy skin. But there's a catch. Research has shown that TiO2 can cause potentially toxic effects when exposed to ultraviolet light, which is in the sun's rays and is the same kind of light that the compound is supposed to offer protection against. To design a safer TiO2 for human use, the researchers set out to test different forms of the compound, each with its own architecture.

They tested titanium dioxide powders on pig skin (which often substitutes for human skin in these kinds of tests) with indoor lighting, which has very little ultraviolet light in it. They discovered that one of the two most commonly used crystalline forms of TiO2, called rutile（金红石）, easily washes off and has little effect on skin. Anatase（锐钛矿）, the other commonly used form, however, was difficult to wash off and damaged the outermost layer of skin -- even in low ultraviolet light. It appears to do so via "free radicals," which are associated with skin aging. "The present findings strongly encourage the use of the less reactive, negatively charged rutile to produce safer TiO2-based cosmetic and pharmaceutical products," the researchers conclude.