Formation of Persistent Free Radicals in Sunscreens During Light Exposure

Sunscreens protect skin from sun damage. However, their ingredients can enter coastal waters through swimming, bathing, and wastewater release. Some ingredients can pose increasing chemical stress on coral reefs and other marine life. Two of the most widely used active ingredients in mineral sunscreens, zinc oxide and titanium dioxide, are tiny particles that, when exposed to sunlight, can generate reactive chemicals. These reactive chemicals can damage living tissue and may contribute to coral stress and reef decline. However, how they form, how long they persist, and how seawater influences their behavior are not well understood. This project will determine the processes by which these particles produce reactive substances. The research will measure how these substances form and decay, identify how sunlight and seawater change their behavior, and explain how particle surfaces control these processes. Data from this research will help scientists, sunscreen manufacturers, and coastal managers better understand the environmental footprint of mineral sunscreens. They will also support the development of safer formulations that protect people and reef ecosystems. The project's educational and outreach activities will strengthen the future scientific workforce by engaging graduate, undergraduate, and high school students through hands-on research and mentoring. The PI will collaborate with the University of South Carolina summer research internships and regional high schools to increase participation in environmental science. The research project will help protect coastal ecosystems, sustain local economies, and provide the foundational scientific knowledge that will benefit public health and environmental stewardship. Persistent free radicals have been characterized extensively in combustion systems and atmospheric particles, but their formation and stabilization in aquatic conditions remain largely unresolved, limiting the ability to assess the environmental reactivity and ecological risk of sunscreen-derived particles. Recent studies indicate that mineral- and organic-based sunscreens generate long-lived radicals upon exposure to simulated solar irradiation. Sunscreens with zinc oxide particles produce the strongest signals and higher yields underwater than in air. This project will establish the mechanistic basis for radical formation, persistence, and reactive oxygen species production from sunscreen-derived nanoparticles and ingredients in air and aqueous environments. The research team will assess how surface hydroxylation, phenolate binding, and near-surface electronic structure in zinc oxide and titanium dioxide promote the formation of phenoxy- and semiquinone-type radicals and sustain reactive oxygen species cycling under dark conditions via interfacial charge transfer and band-structure effects. The activity will distinguish intrinsic particle behavior from formulation-driven effects by combining well-characterized benchmark nanoparticles with commercial products and targeted organic ingredients. By integrating electron spin resonance spectroscopy, simulated sunlight chamber experiments, and complementary surface and electronic probing techniques, the research will resolve charge-transfer pathways, hydroxylation dynamics, radical speciation, and decay kinetics under reef-relevant conditions. Outcomes will include quantitative rates, mechanistic pathways, and environmental lifetimes that yield predictive relationships linking particle composition, surface state, and water chemistry to radical persistence and the generation of reactive oxygen species. These insights will guide future mesocosm and field-scale investigations, inform the design of sunscreen formulations with reduced adverse environmental reactivity, and link nanoscale interfacial chemistry to ecosystem-scale processes that affect coral reef resilience. Workforce development activities will engage graduate, undergraduate, and high school students through structured mentorships. The project will advance foundational science in environmental nanochemistry and inform efforts to protect aquatic ecosystems and the communities that depend on them. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.