Wash-off behaviour of seven different sunscreens assessed with a novel skin application method

As global sunscreen use rises, UV filters are increasingly entering marine environments, particularly in touristic coastal coral reef areas, where they can harm marine organisms. Despite "water-resistant" claims, sunscreens vary widely in wash-off behaviour, affecting both user protection and environmental impact. This study investigated the wash-off behaviour of seven commercial sunscreens (ARUUN™, Cien™, Ladival™, Nivea™, Sundance™, V.Sun™ Body, V.Sun™ Face) selected based on water-resistance and ecotoxicological claims. We applied a novel, reproducible hand-application protocol simulating real-world usage, with defined skin absorption periods and immersion in artificial seawater. Initial tests with V.Sun Face revealed reduced wash-off after 2-5 min absorption, stabilising at ∼50%, leading to standard 5-min absorption for product comparison. In this comparative framework, the four quantifiable products showed broadly similar wash-off (38 - 58%, no significant difference) despite "water-resistant" labelling, all containing the water-soluble UV filter PBSA, which likely drives release. Ladival and ARUUN, which lack PBSA and rely on lipophilic or mineral filters, gave signals largely below the limit of quantification and are reported qualitatively only. Our findings generally recommend 5-min absorption duration before water exposure and highlight the need for standardised testing under realistic conditions. The proposed method provides a reproducible tool for quantifying UV filter release, thereby informing laboratory ecotoxicology assessments and supporting the development of evidence-based, lower-impact sunscreens. From a regulatory perspective, updated test protocols and improved labelling reflecting both human protection and environmental behaviour are needed, aligning with Reef Protection Factor (RPF) recommendations.