Sunscreens: the end of skin testing?

The sun protection factor doesn't depend solely on a sunscreen's chemical formula, but also on how it spreads and adheres to the skin.


Until now, this factor was obtained through in vivo tests conducted on human volunteers, which raises ethical and practical issues (cancer risks, cost...). Scientists have recently developed promising in vitro experiments that could improve the reliability and understanding of this type of testing.

A sunscreen's protection factor was previously determined by exposing volunteers' skin to increasing doses of UV radiation until the first redness appeared. Although these in vivo tests have been used for decades to establish the protection factor of commercial products, they present numerous problems.

Ethical issues first, as exposing people to potentially carcinogenic radiation is questionable, but also because these tests are costly and time-consuming. From a regulatory standpoint, increasing restrictions on in vivo experimentation jeopardize their future in the medium term.

To overcome these challenges, scientists from the Paul Pascal Research Center (CNRS/University of Bordeaux) are working on improving tests - this time in vitro - conducted on plastic substrates. This is an inexpensive method providing quick results, but scientists still struggle to reproduce the values obtained in vivo.

By using substrates with varying roughness levels that approximate those of skin, researchers studied the correlation between in vivo and in vitro results. Analyzing sunscreen behavior on plates with different roughness levels through UV photography and spectroscopy showed that the sunscreen forms a thin film on the substrate's surface whose texture, thickness and homogeneity strongly influence effectiveness.

For the first time, by applying the Beer-Lambert law (which describes how light is absorbed as it passes through a substance) to these inhomogeneous films, they established an analytical link between absorbance, thickness distribution, film homogeneity and protection effectiveness.

This model, validated on 3 sunscreens, explains the poor correlation between in vitro and in vivo results for high SPF values. It also represents a major step forward in understanding variability depending on sunscreen product types (stick, cream, lotion, spray).

Published in the journal ACS Applied Materials & Interfaces, this research demonstrates how significantly the properties of films formed on skin by sunscreens influence their effectiveness.

These findings align with the new in vitro standard published in December 2024, which now permits using only in vitro tests for SPF claims. This approach should enable generalization to all types of formulations, ultimately providing increasingly reliable protection factors that match what's actually observed on humans.