Water resistance is a key performance attribute of sunscreen products, yet its assessment has historically relied on in vivo testing, raising ethical, practical and reproducibility challenges.

This presentation introduces a new physiologically relevant in vitro approach for the determination of sunscreen water resistance. The method was validated through a blinded multi‑laboratory ring test, demonstrating robustness, reproducibility and good alignment with in vivo outcomes.

Beyond the methodology itself, the work illustrates how scientifically sound in vitro approaches can support innovation, standardisation and ethical testing while maintaining meaningful prediction of real‑life sunscreen performance