Exposure of Selected Sunscreens to Artificial Sunlight Generates Persistent Free Radicals

Abstract

Inorganic ultraviolet (UV) filters in mineral sunscreens (MSCs) are known to generate reactive oxygen species (ROS), including transient free radicals, under light exposure. Recent findings indicate that these filters (titanium dioxide [TiO2] and zinc oxide [ZnO]) also assist in generating long-lived free radicals. The photochemical formation of these radicals during routine sunscreen use and as they enter the environment remains unknown, highlighting the need for studies to inform safer sunscreen formulation, reduce adverse health risks, and protect aquatic ecosystems. Here, we provide the first evidence that all commercial sunscreen formulations we used in this study generated substantial amounts of persistent free radicals (PFRs), which remain long after light exposure ends. Both MSCs and organic chemical sunscreens (OSCs) yielded PFRs, though MSCs generated higher levels overall. In most formulations, water exposure significantly reduced PFRs, except in MSC with ZnO-only content, where PFR yields increased. ZnO-only MSCs formed substantial levels of PFRs even when irradiated underwater, producing twice the radical signal observed under ambient air. Among OSCs, UV filters with phenolic groups produced more PFRs, though bulky substituents suppress their formation. Under typical application, we estimate 1017PFRs may form. These results raise concerns about potential environmental and health risks associated with MSC use that persist beyond exposure and may lead to prolonged oxidative stress in human skin and aquatic environments.