Oxidative Potential and Binding Specificity of Sunscreen-Derived Zinc Oxide in Different Swimming Pool Environments

Sunscreens containing metal oxide particles as active ingredients are currently classified by the FDA as the only safe topical agents for protection of the skin against UV rays. Throughout the century-long history of sunscreens, many of their active ingredients have been proven unsafe after the period of commercial success, making the rigorous assessment of adverse side effects of sunscreens per se , including those based on metal oxides, imperative. In this study, zinc concentrations measured in water samples from nearly a dozen public swimming pools in the southern Californian city of Irvine were found to be directly proportional to the number of swimmers and peaked around midday, coinciding with the highest facility usage. Next, the zinc oxide phase was chemically isolated from a commercial sunscreen to study its interaction with the accompanying organic components using a series of microscopic and spectroscopic techniques, including SEM, EDX, NMR, FTIR and Raman spectroscopy. The aromatic and highly hydrophilic organic moieties, such as carboxylates, esters and silanes, were found to be most closely associated with the ZnO surface. Moreover, despite the sunscreen being neither soluble nor dispersible in water, the ZnO phase underwent evident morphological changes upon exposure to chlorinated swimming pool water. Photocatalytic assays demonstrated substantial production of reactive oxygen species (ROS) when the intact sunscreen and its isolated ZnO extract were exposed to short-wave or long-wave UV light, whereas the isolated organic phase produced no detectable ROS under the same conditions. ROS scavenging experiments identified superoxide anion radicals and hydrogen peroxide as the primary mediators of the observed photocatalytic activity. Curiously, the organic components of the sunscreen intensified the ROS generation by ZnO, and the same oxidative stress augmentation was observed in electrochemically chlorinated pool water, where chlorine is generated from dissolved sodium chloride, while traditionally chlorinated water produced no such effect. Reduced water column height separating the sunscreen layer from the UV light source also contributed to greater ROS production. The degradation kinetics were pseudo-first order for isolated ZnO but defied standard kinetic modeling upon introduction of the organic sunscreen components. Findings reported here cast a shadow on the universal safety attributed to today’s sunscreens utilizing metal oxide particles as active ingredients and suggest that research on amending the gold standards and state-of-the-art sunblock ointments must continue.