Sun Protection: Two new SPF testing methods approved by ISO.

Photoprotection is a major public health issue. Indeed, let's remember that prolonged and/or repeated exposure to the sun's UV rays is responsible for immediate skin damage, such as sunburn, but also for long-term consequences, such as premature skin ageing and an increased risk of skin cancers, particularly melanomas. To prevent these dangers, the use of sun protection is essential. Their action is based on the presence of chemical or mineral filters in their formulas, which absorb and reflect UV rays, thus protecting the skin. Their effectiveness is evaluated by the sun protection factor (SPF), characterising the ability of a sunscreen to block erythemal UV rays. There are now four levels of SPF.

For several decades, the in vivo test ISO 24444:2019 has been regarded as the benchmark for measuring the SPF of a sunscreen product. It involves applying the sunscreen to the skin of volunteers, then exposing them to an artificial UV source to observe the onset of erythema. The SPF is then calculated by comparing the UV dose required to cause sunburn with and without protection. This method therefore relies on a direct approach, where the skin's reaction to UV rays is used to quantify the effectiveness of a sunscreen. Although the ISO 24444:2019 test has proven its worth, it presents several ethical, scientific and practical problems, which justify the development of alternative methods.

Indeed, the main drawback of the in vivo ISO 24444:2019 method is its impact on the participants and the necessity for them to develop a sunburn. However, even in the short term, the damage caused by UV rays is significant. As has been done for other cosmetic evaluations, there is therefore a real need to minimise human testing as much as possible. Furthermore, the ISO 24444:2019 standard suffers from a high degree of inter-individual variability as the reaction to UV rays depends on many factors, such as the phototype and the effectiveness of the participant's skin barrier. Finally, the cost of this test is not insignificant for manufacturers, which can be a barrier to innovation.

For several years, the European Union has been encouraging the development of alternative methods to in vivo tests, particularly with Recommendation 2006/647/EC. The aim is to develop in vitro or ex vivo tests capable of providing results as reliable as the ISO 24444:2019 method, without the need for human exposure to UV.

Note : There are also the ISO 24442:2011 and ISO 24443:2021 standards that allow the determination of the UVA protection factor, respectively in vivo and in vitro. However, they do not allow for the evaluation of the protection provided by a sunscreen against erythemal UV, that is, the SPF.

In response to the limitations of in vivo tests, the cosmetics industry has been working for several years on alternatives capable of assessing SPF without resorting to exposing human volunteers to UV rays. The challenge for these methods is to be more ethical, but also to provide statistically correlated results with the values obtained with the ISO 24444:2019 method. Among the various approaches studied, two methods were validated by the International Organisation for Standardisation in December 2024 and could become the first reference methods in addition to in vivo tests. These are the ISO 23675 standards (SPF in vitro "double plate" test) and ISO 23698 (HDRS sun protection test).

The method in vitro "double plate" (ISO 23675).

Among the alternatives currently being validated, the in vitro method known as the "double plate" is based on the use of two superimposed quartz plates, between which the sunscreen product to be tested is applied. This protocol aims to simulate the diffusion and absorption of the product on the skin, while ensuring rigorous standardisation. In practice, the sunscreen product is first applied evenly on a quartz plate, then subjected to a drying phase. This step replicates the stabilisation time of a sunscreen after application on the skin, a key factor influencing the effectiveness of the UV filter. A second plate is then placed on top, forming a multilayer system that mimics the distribution of sunscreen filters within the stratum corneum, the outermost layer of the epidermis.

The evaluation of the sun protection factor is then based on measuring the transmission of UV through this system, using a calibrated light source and specific optical sensors. By comparing the amount of light absorbed by the preparation with that of a control sample, it is possible to determine the sun protection factor. The main challenge of this approach lies in its ability to accurately replicate the interactions between sunscreens and human skin. Indeed, the lipid and protein structure of the epidermis influences the distribution of filters, a parameter that is difficult to replicate on an inert substrate such as quartz.

Despite this complexity, the results obtained so far with the ISO 23675 method show a good correlation with the values measured in in vivo tests and allows the elimination of the need for human volunteers. However, the use of this technique is limited to the determination of a static sun protection factor, and is not applicable to the characterisation of a sunscreen's water resistance properties.

More information about the standard ISO 23675 can be found on the International Organisation for Standardisation's website.

The hybrid diffuse reflectance spectroscopy (HDRS - ISO 23698).

Hybrid Diffuse Reflectance Spectroscopy (HDRS) is based on the analysis of interactions between light and sunscreens applied to a standardised surface. Unlike the so-called "double plate" method, which assesses UV transmission through a multilayer system, HDRS relies on a double spectroscopic analysis. Firstly, an in vitro measurement is carried out on a standardised surface to evaluate the transmission and reflection of light through the tested formulation across the entire UV spectrum. Subsequently, a second analysis is conducted directly on human skin, focusing specifically on the transmission of UVA. This hybrid approach allows for the generation of a combined spectrum, incorporating both UVB and UVA protection performances.

The major advantage of this technique lies in its ability to provide a comprehensive evaluation of sun protection, without requiring prolonged exposure to UV rays. It thus allows the determination of the SPF, but also the UVA protection factor (UVA-PF) and the critical wavelength, which characterises the extent of protection across the entire UV spectrum. Thanks to the combination of in vitro and in vivo non-invasive measurements, the results obtained show a close correlation with the ISO 24444:2019 method. This technique is also presented as an alternative to the ISO 24442:2011 and ISO 24443 standards.

More information about the standard ISO 23698 can be found on the website of the International Organisation for Standardisation.

The "double plate" and HDRS methods are now integrated into ISO standards and mark a key step in the evolution of sun protection tests. While they will not immediately replace in vivo tests, they will help to limit their use and offer reliable and more ethical alternatives for evaluating SPF.

• Regulation (EC) No 1223/2009 of the European Parliament and Council.

• LIM H. & al. Photoprotection of the Future: Challenges and Opportunities. Journal of the European Academy of Dermatology and Venereology (2020).

• PASSERON T. & al. Photoprotection of the skin against pigmentation induced by visible light: Current testing methods and suggested harmonisation. Journal of Investigative Dermatology (2021).

• BANSAL V. & al. Sunscreen testing: A critical perspective and future roadmap. TrAC Trends in Analytical Chemistry (2022).