Potential risks of TiO₂ and ZnO nanoparticles released from sunscreens into outdoor swimming pools

The potential risks of nanoparticles (NPs) in sunscreens being released into swimming water were evaluated by a series of laboratory experiments simulating the fate and transport of NPs in outdoor swimming pools. NPs released from sunscreen-applied skin were estimated using pig skins covered with five different commercial sunscreens containing TiO₂, ZnO, or both at various concentrations. Assuming that the swimming water treatment processes consisted of filtration, UV irradiation, heating, and chlorination, possible removal of the released NPs by each process was estimated. Generation of hydrogen peroxide (H₂O₂) by the NPs under sunlight and after UV photochemical treatment were measured, and the H₂O₂ concentration possibly present in the swimming pool was calculated based on some specific scenarios of operating an outdoor swimming pool. It was found that a significant amount of the NPs in sunscreens could be released into the swimming water, and accumulate during circulation through the treatment system. However, the concentration of H₂O₂ possibly present in the swimming pool should be below the level at which an adverse effect to bathers is concerned.