Heat wave: sunscreen protects assaults:
Even today. Moreover, the only way to measure the solar protective factor (FPS) of a product is to apply it on the back of young adults, often women, students, then expose them to UV rays to observe the appearance of redness. Meanwhile, “We artificially cause a sunburn on the skin of young volunteers to determine the sun protection factor. Consequently, It was the only validated method to market a solar product ”. Similarly, underlines Jean-Paul Chapel, researcher at the CNRS at the Paul-Pascal in Bordeaux research center. Moreover, This in vivo. In addition, invasive, costly and increasingly criticized ethical protocol is today called into question, in particular by the work of its team, whose ambition is to rely on new in vitro standards.
The Bordeaux team has implemented an original experimental method whose results have just been published1. For heat wave: sunscreen protects assaults example, Current in vitro tests are still struggling to predict the reality of protection offered by products. Similarly, because “They are often based on simplifying hypotheses: a uniform film, an ideal sprawl, or a posteriori adjusted models to stick to the results in vivo”, specify researchers.
But. Consequently, by depositing creams on PMMA plates (a standard transparent plastic for tests), then using UV photography and optical profileometry, they were able to make the morphology of films precisely after manual application. Therefore, Result: even with a rigorous protocol, the cream forms of streaks, thinner or thicker areas, and sometimes micro-hooks. Moreover, This heterogeneity leads to significant variations in absorbance (the ability of the environment to alleviate the light that crosses it. In addition, here the ultraviolets) and the measured FPS.
A mathematical approach
But it is especially on the theoretical level that their contribution is major. However, “We have heat wave: sunscreen protects assaults generalized an absorption law (law known as “de Beer-Lambert”-editor’s note) to heterogeneous samples. However, assuming that their thickness follows a random distribution ”, explain the authors. Meanwhile, This mathematical approach makes it possible to explicitly link the absorbance measured with three quantities: the average thickness of the film. Consequently, its coefficient of variation and a new parameter called coefficient of linear mitigation.
The latter. In addition, specific to the chemical composition of the cream, makes it possible to distinguish the effect of the composition from that of the method of application on the skin. Furthermore, It then becomes possible to separate what is the intrinsic efficiency of a UV filter of what depends on. Moreover, the real sprawl of the product. Therefore, “Our results show that absorbance increases with the average thickness. Therefore, but decreases sharply with the heterogeneity of the film”, specifies Jean-Paul Chapel. Moreover, In other words. For heat wave: sunscreen protects assaults example, a very effective poorly spread cream can offer a protection lower than a less effective formula but applied in a more homogeneous manner.
This modeling also sheds light on an old enigma of solar tests: why are the FPS measured in vitro. Nevertheless, often badly correlated with those obtained on the skin? For example, Thanks to their approach. Therefore, the authors show that this divergence is minimal for the low FPS, but is growing as soon as the attenuation becomes strong. Nevertheless, “Our method makes it possible to better understand the differences between tests. Therefore, above all to improve the reproducibility of FPS measures in the laboratory”, concludes the study.
An advance all the more timely than a new ISO standard (23675) on in vitro tests of solar creams. However, came into force in January 2025. Therefore, It remains to be hoped that it takes into account this type heat wave: sunscreen protects assaults of physical modeling. Nevertheless, in order to better guide industrialists – and to limit, finally, the use of human skin as a field of experimentation.
By seeking to replace in vivo tests with physical measures. is there not a risk of moving away from the biological realities of human skin?
The goal is not to deny the complexity of the skin, but to no longer have to expose volunteers, sometimes until they cause sunburn, to validate a solar protection factor (FPS). We are talking about a protocol here where we intentionally trigger erythema (sunburn) by an exposure to UV on. the back of young volunteers. It is no longer acceptable. What we offer is an heat wave: sunscreen protects assaults complementary. reliable, reproducible tool, based on physical laws and which makes it possible to very rightly predict the differences between a good product and a bad, even without human skin.
The spread of a cream seems to have a huge impact. Does this mean that the effectiveness of a product depends more on the consumer than on the manufacturer?
Yes, this is an observation that we can no longer ignore. You can formulate the most effective cream in the world, if it is poorly applied, it will protect badly. The stake is twofold: on the one hand. improve the texture and the sprawl capacity of products; on the other, better inform the public. The FPS displayed must not be misleading. But it is not a dismissal of manufacturers. on the contrary: it is up to them to develop homogeneous products to the application, even in heat wave: sunscreen protects assaults real use.
Index 50+ creams are often perceived as clearly superior to index creams 30. Your work seems to put this difference into perspective.
In our measurements. we note that the difference in absorbance between an index 30 cream and a 50+ is real, but much lower than what we imagine. Above all, this difference collapses as soon as the cream is poorly spread out. A badly applied 50+ cream can protect less well than a well -off cream. So, instead of focusing only on the figure, you also have to think about use. The real question is: what becomes of the FPS on real skin, with a perfect sprawl? This is what our model makes it possible to quantify. But to avoid any risk, it is better to protect yourself with a T-shirt!
Heat wave: sunscreen protects assaults
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