We have studied the generation of reactive intermediates of two widely used sunscreen agents, zinc oxide and titanium dioxide, upon illumination with visible and UV light. We also studied the effects on reactivity of adding coatings. Reactivity was studied by the use of the spin trapping method. We found that in the presence of light both zinc oxide and titanium oxide can generate reactive species which can be spin trapped and the resulting spin adduct is similar to that observed when hydroxyl radicals are present. Coating the particles with a silicone surface treatment eliminated the generation of detectable spin adducts. The relative yield for light-induced spin adducts was higher in zinc oxide than in titanium dioxide. The exact wavelength band which drives the photochemistry of these materials cannot be derived from the current study due to limited experiments; however, for zinc oxide it is less than 435 nm, and less than 620 nm for titanium dioxide. We conclude that there are at least two potential scenarios in which these reactivities might be biomedically important: A. when the materials reach live cells through transport or a lack of the usual dead layer of skin (e.g. in an abraded area). B. if the materials generate secondary reactive species which can penetrate through the dead layers of the skin. This study indicates that the methodology used for these studies could (and probably should) be applied to study the potential generation of reactive intermediates by sunscreens and other cosmetic preparations which may receive substantial exposure to light.
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