The sun protection factor (SPF) does not depend solely on the chemical formula of a sunscreen, but also on how it spreads and adheres to the skin. Until now, this factor was determined through in vivo tests conducted on human volunteers—an approach that raises ethical and practical concerns, such as cancer risk and high costs. Recently, scientists have developed promising in vitro experiments that could improve the reliability and understanding of such testing methods.
Traditionally, a sunscreen’s SPF was determined by exposing the skin of volunteers to increasing doses of UV radiation until the first signs of redness appeared. Although these in vivo tests have been used for decades to establish the SPF of commercial products, they raise several issues. Ethically, exposing individuals to potentially carcinogenic radiation is questionable. These tests are also expensive and time-consuming. Furthermore, from a regulatory perspective, the growing restrictions on in vivo testing are putting its medium-term future in jeopardy.
To overcome these challenges, researchers at the Paul Pascal Research Center (CNRS/University of Bordeaux) are working on improving SPF assessment methods—this time using in vitro testing on plastic substrates. This low-cost method provides rapid results, but scientists still struggle to replicate the values obtained through in vivo testing.
By using substrates with varying roughness that closely mimic human skin, the researchers studied the correlation between in vivo and in vitro results. Using UV photography and spectroscopy to observe the behavior of sunscreen on plates with different textures, they demonstrated that the cream forms a thin film on the substrate’s surface. The texture, thickness, and homogeneity of this film significantly impact the sunscreen’s effectiveness. For the first time, by adapting the Beer–Lambert law—which describes how light is absorbed as it passes through a substance—to these inhomogeneous films, the researchers established an analytical link between absorbance, thickness distribution, film homogeneity, and protective performance.
This model, validated on three different sunscreens, helps explain the poor correlation between in vitro and in vivo results for high-SPF products. It also marks a major step forward in understanding the variability depending on the type of sunscreen (stick, cream, lotion, spray).
These findings, published in the journal ACS Applied Materials & Interfaces, highlight how crucial the film-forming properties of sunscreens are to their effectiveness. The results align with the new in vitro standard published in December 2024, which now allows the use of in vitro tests alone to substantiate SPF claims. This approach is expected to enable broader application across all types of sunscreen formulations and yield SPF values that more accurately reflect real-world performance on human skin.
Reference
Etienne Lepoivre, Sylvain Auge, Frederic Nunzi, Philippe Cluzeau & Jean-Paul Chapel
Generalized Beer−Lambert Law for Evaluating the Absorption of Heterogeneous Sunscreen Films
ACS Applied Materials & Interfaces 2025
https://pubs.acs.org/doi/10.1021/acsami.5c02458