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. 2025 Oct 28;12(11):1554–1560. doi: 10.1021/acs.estlett.5c00861

Exposure of Selected Sunscreens to Artificial Sunlight Generates Persistent Free Radicals

Eric P Vejerano ‡,*, Khushboo Khushboo ‡, Juan Vejerano ‡, Santosh Kiran Ballejipalli †
PMCID: PMC12613795  PMID: 41246181

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 1017 PFRs 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.

Keywords: persistent free radicals, reactive oxygen species, titanium dioxide sunscreen, zinc oxide sunscreen, UV filters

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Introduction

Mineral sunscreens (MSCs) containing nano- to submicrometer TiO2 and ZnO particles as inorganic ultraviolet (UV) filters are widely used and approved for topical application. Under UV exposure, these metal oxides generate reactive oxygen species (ROS), including the transient free radicalssuperoxide (O2 •–) and hydroxyl radicals (•OH). Most studies have focused on these short-lived intermediates, leaving it unclear whether metal oxide particles also facilitate the formation of persistent free radicals (PFRs). Understanding this knowledge gap is important because PFRs, unlike transient ROS, can linger for days postirradiation and continue generating ROS even without light.

Recent work has shown that at near-ambient conditions, TiO2 and ZnO aid in forming PFRs. While TiO2 readily converts aromatic organics to PFRs, ZnO requires pretreatment of smaller molecules. Thermal energy at 40 °C (∼26.0 kJ/mol) is typically insufficient for significant electronic transitions in metal oxides. However, the nanoparticle’s size and associated organic compound can lower the energy barrier, − allowing electron movement, and ultimately forming radicals. In contrast, UV light provides enough energy (∼400 kJ/mol at 300 nm) for bond cleavage and electronic excitation. Here, we test our hypothesis that sunscreens containing metal oxides and the undisclosed organic compounds in the formulations should yield more PFRs under light than under heat alone. We also evaluate PFR formation in organic chemical sunscreens (OSCs), which do not list metal oxides. We also examine whether PFRs persist or quench when wet.

Materials and Methods

Sunscreen Samples and Organic Filters

Seven commercial sunscreens were selected, including four mineral-based formulations (MSC1–MSC4) containing various ratios of TiO2 and ZnO, and three organic chemical sunscreens (OSC1–OSC3). In addition, five organic UV filters, namely, oxybenzone, avobenzone, homosalate, octisalate, and octocrylene (≥98% purity, Sigma-Aldrich) were used without further purification. Pristine TiO2 and ZnO nanoparticles (≥99% purity, NanoAmor) were used as controls.

Sample Preparation

We prepared a slurry of each commercial sunscreen in silica. The dried materials were stored in the dark before use. Six 100 mg portions of the dried material were placed in 20 mL glass vials for exposure to light. See the Supporting Information (SI) for details. We used silica as an inert solid support because amorphous silica is widely employed as a coating and filler in commercial sunscreen formulations.

Exposure Conditions

For irradiating samples, we used a 100 W small-area solar simulator (Newport/Oriel LCS-100), which produces a spectral irradiance distribution characteristic of natural sunlight. See the SI for details. The lamp was positioned approximately 20 cm above the samples. The vials were uncapped to ensure maximum direct light contact, as borosilicate glass can partially attenuate UV. Samples were irradiated for 0, 1, 2, 4, 6, 8, or 10 h, and electron spin resonance (ESR) spectra were acquired immediately after each exposure interval to track PFR formation.

Formation of PFRs from Organic UV Filters

To assess PFR generation by individual organic UV filters, samples were prepared on silica, irradiated, and the PFR yields monitored over time. See the SI for details.

Water Exposure and Quenching Experiments

To investigate how water contact impacts PFR stability, we added deionized water to previously irradiated sunscreen–silica samples. In addition, we also irradiated partially or fully submerged sunscreen–silica mixtures of MSC3, MSC4, or OSC2 samples at ∼1 cm depth within a beaker. These samples were allowed to dry for 3 days and then analyzed by ESR. See the SI for details.

Kinetic Studies

Kinetic stability of irradiation-generated radicals was evaluated by tracking ESR signal intensities over 10 days and fitted to a pseudo–first-order decay model to obtain 1/e-fold times (τ1/e). See the SI for details.

Materials Characterization

The solid mineral matrix from a representative sunscreen (MSC3) was isolated by sequential solvent extraction to remove as much organic compounds as possible, and the cleaned TiO2 and ZnO solids were analyzed using multiple surface characterization and imaging techniques. See the SI for details.

ESR Measurements, Data Analysis, and Reproducibility

All ESR spectra were recorded at room temperature using a Bruker EMXplus spectrometer. Experiments were conducted in duplicate, and the intensity of ESR signals was reported as the relative standard error. See the SI for details.

Results and Discussion

Under artificial sunlight, all tested sunscreens generated PFRs, with MSCs producing higher yields (Figure A–D) than OSCs (Figure E–G) and those exposed to heat alone. The formulation with 18.6 wt % ZnO and 6.6 wt % TiO2 showed the strongest signal, highlighting the photoreactive influence of these metal oxide particles. We observed no clear trend between metal oxide loading and PFR yields in MSCs. The nonactive organic components in MSCs, which likely influence yields, react with these particles, generating PFRs. The organic precursors for these radicals are likely undisclosed inactive phenolic ingredients based on the g-factors (∼2.0040–2.0055). Among OSCs, which shared similar ingredients, OSC1, which contains additional oxybenzone, produced the highest ESR signal that generally increased with exposure time. The radicals persisted for days, showing a slight decrease in concentration a week after formation for MSCs (Figure ). OSCs showed slightly less persistent radicals, as there are no transition metal oxides to stabilize them (Figure ). The estimated average τ1/e values are ∼4.3 ± 0.3 days for MSCs and ∼2.5 ± 0.5 days for OSCs.

1.

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Representative first-derivative ESR spectra (panels A–G) of seven sunscreen formulations after exposure to artificial sunlight for 0, 1, 2, 4, 6, 8, 10 h, and at 40 °C. Panels A-i to G-i are ESR spectra after wetting and drying the exposed samples. MSCs contained ZnO and/or TiO2 as UV filters. OSCs contained organic UV filters. The declared UV filters and concentrations (w/w%) are listed in panels A–G. Panels A/A-i to G/G-i are on the same scale. Only the x-axis (magnetic field, gauss) label for panel A is shown for reference. SiO2 control was exposed to artificial sunlight for 10 h. For clarity of presentation, error bars as relative standard deviation (RSD) for duplicate measurements are omitted in panels H and H-i. RSD ranged between 5 and 10%. AU is arbitrary unit.

2.

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Decay of PFR signals for seven sunscreen formulations.

Control ESR of pristine TiO2 and ZnO for two average particle sizes (∼15 and 30 nm) showed no radical signals under irradiation (Figure ). In both air and water, traces were slanted baselines without derivative-like resonances at g ≈ 2 (the window expected for PFRs), confirming the slopes reflect background rather than true signals attributed to PFRs. The signals for ZnO irradiated in water (Figure B) were similar to the SiO2 control (Figure ). These controls demonstrate that ESR activity in sunscreen arises from surface-bound photoproducts or the organic matrix, not the oxide cores.

3.

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ESR spectra for the nanoparticle controls of varying sizes irradiated in (A) air and (B) water. The size indicated on the label is the average particle size of the nanoparticles.

In the control experiment, we did not use the nanoparticles extracted from the formulations, as complete removal of the organic compounds is not guaranteed. However, the size of particles in the MSC3 formulation, which we used as a representative for the other MSCs, contained NPs that are ∼ 30 nm, based on transmission electron microscopy (TEM) analyses (Figure S1). This particle size is within the size range used for the control ESR measurement (Figure ). Energy-dispersive X-ray (EDX) analysis confirmed the presence of Ti and Zn in MSC3 (Figure S2). High-angle annular dark-field (HAADF) imaging and X-ray photoelectron spectroscopy (XPS) analysis revealed the absence of mineral shell coatings (e.g., silica) in the ZnO and TiO2 particles in MSC3 (Figure S3).

While all seven sunscreens examined generated PFRs, the magnitude of formation is likely formulation-dependent, due to differences in active and inactive components. Mineral sunscreens, often coated with organic or inorganic layers, may further vary in radical-forming potential depending on the chemistry and stability of these surface treatments. In some formulations that utilize organic passivation layers, our usage of dichloromethane as the dispersing solvent during sample preparation may solubilize or strip these coatings, exposing the underlying oxide surface. Loss of this passivation may increase the availability of reactive surface sites, and under irradiation, it might have enhanced PFR formation.

We probed the role of molecular structure on the PFR yield of each organic UV filter in the OSCs (Figure A). The trend oxybenzone > homosalate > octisalate > avobenzone (Figure A) links phenoxy groups to PFR generation. Oxybenzone, with an exposed OH and minimal steric hindrance (Figure C-i), efficiently forms triplet states that drive electron transfer, yielding the highest PFRs (Figure A). Oxybenzone also likely drives the strong ESR signal in OSC1 (Figure E). Steric hindrance from cyclohexyl (homosalate) and ethylhexyl (octisalate) groups suppresses radical formation (Figure C-ii and -iii). Despite lacking an OH (Figure C-iv), avobenzone produced weak signals (Figure A), possibly from cleavage of the methoxy group. Octocrylene, lacking an OH and no cleavable bond on both phenyl rings (Figure C-V) showed no ESR signals above the silica control (Figure A).

4.

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Representative first-derivative ESR spectra. Graphs A and B are on the same scale and twice those of panels A/A-i to G/G-i in Figure . We used ∼100 mg of sunscreen-silica mixture for ESR measurement similar to the amount used to obtain the ESR spectra in Figure . Panel B is the PFRs generated from MSC4, MSC3, and OSC2 irradiated under water at 1 cm depth. Panel C depicts the molecular structures of the active organic UV filter in OSCs, showing the trend in PFR yields. Heteroatoms are colored.

Water typically quenches radicals. Subsequent exposure to water of irradiated sunscreen decreased PFR levels (Figure A-i to G-i), likely through radical quenching or changes to the ZnO and TiO2 particle surfaces. However, MSC4 (ZnO-only formulations) exhibited higher PFR yields postwater treatment. We performed a comparative underwater irradiation test involving MSC4. Subsequent testing confirmed higher ESR signals of nearly 5-fold in MSC4 than compared with MSC3 (ZnO–TiO2) and OSC2 (Figure B). Notably, the ESR signal for MSC4 irradiated underwater was twice that in ambient air. For the PFRs generated underwater, we observed substantial shifts in the resonance field (Figure B), suggesting different types of radicals. We do not yet fully understand the mechanism that drives the amplification of PFRs in the ZnO-only formulation or why it promotes radicals to form in aqueous environments. However, we conjecture that in this formulation, UV/water hydroxylates the surface of ZnO, creating more active sites for PFR generation. Indeed, there are reports that ZnO is prone to surface hydroxylation under UV and water compared to TiO2. Such surface changes can lower the energy barriers for PFR formation.

We performed XPS analyses to validate our hypothesis of surface hydroxylation in ZnO during underwater irradiation. We used the extracted solid matrix of MSC3 as well as the pristine TiO2 and ZnO nanoparticles for comparison. High-resolution Zn 2p and O 1s spectra confirmed hydroxylation signatures, while TiO2 remained unchanged. For ZnO, the Zn 2p region showed components attributable to Zn/ZnO (∼1021 eV), Zn­(OH)2 (∼1022–1023 eV), , and ZnO–H2O adducts (∼1025–1026 eV), consistent across all samples (Tables S2 and S3, Figures S6–S8). The O 1s spectra further resolved contributions from lattice oxygen (∼530 eV), hydroxyls (∼532–533 eV), and adsorbed water (∼534–535 eV). In MSC3 particles, we observed a similar trend. Zn species shifted slightly to lower binding energies after irradiation, consistent with partial reduction under light exposure, while Ti remained in the TiO2 state (∼458.8 eV). These results support the assignment of surface hydroxylation on ZnO, but not TiO2 (Table S3, Figures S6 and S11), under aqueous conditions.

In contact with liquid water or at high humidity, ZnO surfaces hydroxylate readily. Water adsorbs, often dissociatively, to form terminal/bridging −OH groups, especially at steps/defects, increasing the density of surface hydroxyls and changing local acidity and electronic structure. , On hydroxylated ZnO, phenols and presumably compounds containing phenoxy moiety, such as those declared as inactive ingredients in the sunscreen formulations (Table S1), bind more strongly as phenolate complexes. , Deprotonation is promoted at Zn2+ sites and by the high-dielectric interfacial environment. Computational work on phenol/catechol–ZnO shows chemisorbed geometries with interfacial charge transfer, which are the precursor state for PFRs. , Additionally, our previous computational studies suggest that acidic or basic small molecules (water in this case) can remove the activation barrier required for the adsorption of organic molecules. Once adsorbed as phenolate, charge transfer between the organic and the oxide yields surface-stabilized phenoxy/semiquinone radicals. Spectroscopy on phenol/ZnO single crystals and powders directly observes these charge-transfer states and long-lived surface radicals. Moreover, hydrogen (or OH) on ZnO can induce a charge-accumulation layer and band-bending near the surface, , which helps trap charge carriers and stabilize the radical adlayer rather than extinguish it. This mode serves as an alternative route for hydroxylation, which can promote PFR persistence in aqueous environments. Selected area electron diffraction (SAED) reveals no discernible lattice change after illumination (Figures S4 and S5), supporting an electronic rather than structural origin , Taken together, band bending, rather than the single-electron transfer mechanism, is the most likely mechanism that eventually generates the PFRs in these sunscreens.

We estimate 1016 PFRs can form per gram of sunscreen. At a standard application of 2 mg/cm2, totaling around 20–40 g for full-body coverage, , 1017 PFRs may form upon sunlight exposure. These surface-bound radicals on TiO2 and ZnO can generate up to 1018 •OH without light. While studies reported that these metal oxide particles do not penetrate intact human skin beyond the stratum corneum, compromised skin may allow penetration, at least for ZnO, within viable epidermal layers. In real use, such radicals would arise within the sunscreen film on the stratum corneum. Evidence from human in vitro/in vivo studies and regulatory reviews indicates that, in healthy skin, TiO2 and ZnO remain on the surface or within the stratum corneum/upper epidermis with negligible systemic exposure; ,, but barrier disruption could alter this behavior. In UVB-damaged murine skin, increased dermal zinc and macrophage activation have been reported, , but mouse skin differs from human skin , and effects are often attributed to dissolved Zn2+ rather than intact particles. PFRs are less reactive than •OH and unlikely to cause direct injury unless near viable tissue; however, their persistence enables redox cycling that generates •OH in the absence of light that might sustain oxidative stress at the skin surface when the barrier is compromised. Note that our laboratory findings are mechanistic and point only to the need to study the potential of sunscreen to generate PFRs and their potential adverse impacts. Studies under realistic use conditions (application thickness, sweat/seawater, abrasion, reapplication, and long-term photodegradation) are needed to determine whether sunscreen-derived PFRs contribute to photo-oxidative stress or cutaneous impacts.

As active ingredients in sunscreens enter aquatic environments, their transformation into long-lived, photoreactive species raises potential concerns about ecological risks. Here, we show that all tested sunscreen formulations, including those marketed as “mineral-only”, generate PFRs. Notably, ZnO-only formulations exhibited enhanced radical formation in watercontrary to the typical quenching effect observed for radicals in aqueous environments. This finding raises new concerns about chronic exposure and potential harm in sensitive marine ecosystems, such as coral reefs.

As a preliminary order-of-magnitude assessment, we estimated PFR abundance and ROS generation in reef waters from reported environmental nanoparticle levels (TiO2 typically 0.021–10 μg L–1, , with tourist-beach hotspots ∼7–40 μg L–1; − ZnO generally lower. We modeled the particles as uniformly mixed within a ∼ 1 mm coral diffusive boundary layer , (∼1 L water per m2). PFRs are assumed to form under sunlight and persist for days. We used the results of our experiment and calculated spin densities on photoactive particles an order-of-magnitude lower (1015–1016 spins g–1), with a PFR-active fraction f = 1, and each PFR generates 10 •OH in the absence of light before deactivation. We assume steady inputs over the exposure window and no loss in PFRs due to quenching by O2/dissolved organic matter (DOM)/minerals. Under these assumptions, the implied event counts are ∼ 105–108 •OH cm–2 (see Supporting Information for details). Although the quantitative threshold of •OH events needed to induce lipid peroxidation in corals is lacking, these •OH levels may initiate lipid peroxidation in coral membranes. Perhaps, these fluxes may cause biologically meaningful microscale oxidative stress if particles concentrate at tissue interfaces ,, or under costress from elevated temperature and irradiance. − Note that these estimates are subject to key uncertainties; the fraction of PFR-active particles, PFR lifetimes, near-surface concentration factors, and other confounding variables present in natural aquatic environments will modulate the production of PFRs.

Systematic studies are needed under relevant aquatic conditions to refine this estimate by (i) quantifying field concentrations and hotspots across reef waters, sediments, and seasons; (ii) measuring PFR formation and persistence in natural seawater and reef mucus under realistic light and DOM conditions; (iii) resolving near-surface exposure by mapping particle enrichment, boundary layers, and ROS at tissue interfaces; (iv) modeling aggregation, settling, photochemistry, and quenching; (v) establishing dose–response relationships in mesocosm studies at environmentally relevant exposures; and (vi) distinguishing sunscreen-derived particles from other sources. These data and models are essential for site-specific risk assessments and for evaluating when sunscreen-derived PFRs could significantly affect coral reef health.

Our results highlight the need to reevaluate the continued use of TiO2 and ZnO in sunscreen formulations. Mitigation strategies may include employing larger metal oxide particles or substituting with organic UV filters that exclude phenolic structures prone to radical formation. These alternatives may preserve UV protection while reducing environmental risk. The detection of long-lived PFRs from both mineral and organic sunscreens reveals unknown risks that warrant urgent attention, particularly their formation under relevant environmental or use conditions.

Supplementary Material

ez5c00861_si_001.pdf (1.6MB, pdf)

Acknowledgments

This work was supported by the National Science Foundation Grant No. 1834638 and CAREER Award Grant No. 214282. XPS data were collected at the University of South Carolina XPS Facility (RRID: SCR_026176), which is financially supported by the Office of the Vice President for Research.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.estlett.5c00861.

  • Experimental details, list of inactive ingredients in sunscreens, XPS, TEM, HAADF, SAED, and EDX data for TiO2 and ZnO, and sample calculation for the formation of •OH (PDF)

The authors declare no competing financial interest.

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