SPF-Testing methods – Scientific and statistical basis

At the end of 2024 two alternative SPF methods have been published, ISO 23675:2024 an in vitro method and ISO 23698:2024 a non-invasive in vivo/in vitro hybrid method [1,2]. Over the years, I have often written about these methods. Just last year the two columns appeared “Two additions to the SPF toolbox!” and “Which SPF-Testing Method to use?” [3-6].

Just recently, in September 2025, the scientific and statistical basis was published in a supplement to the International Journal of Cosmetic Science (IJCS) entitled “Are there alternatives to the traditional in-vivo SPF test (ISO 24444)? Comparison and statistical analysis of 5 proposed methods” [7-13].

This is an important and unique milestone (Figure 1). The seven publications not only form the basis for the two new ISO methods, but also shed light on the gold standard ISO 24444 and three other alternative SPF methods that can helps us progress further.

The IJCS editors Marc Pissavini, and Majella Lane along with guest editor John Chave (Director General, Cosmetics Europe) emphasize the importance of this work for the future [14]:

This edition features seven groundbreaking publications authored by leading experts. These papers detail the protocols, findings, and conclusions of the ALT-SPF initiative. Their contributions have been instrumental in advancing the science of SPF testing and shaping a future where innovative, reliable, and ethical methodologies take centre stage. We hope this special edition provides you with profound insights into the future of SPF testing and underscores the importance of collaboration in driving scientific progress.

To get a quick overview of the 7 publications, I asked ChatGPT to summarize the 135 pages describing the SPF method ISO 24444 and the 5 alternative methods [7-13].

This little exercise was also meant to test the quality of the ChatGPT’s answer. The AI generated Table 1 provides a useful overview of the 5 alternative methods that were compared in the Consortium ALT-SPF. The rest of the approximately 7-page ChatGPT summary was also useful. I encourage you to try it out yourself. The advantage of using of generative AI in this way is that the answers are limited to the content of the input and do not cover the “entire Internet”. This means that there should be no hallucinations.

The goal and work of the Consortium ALT-SPF were described in an article by Surber et al. entitled Past, Present, and Future of Sun Protection Metrics [15].

Let us now turn to a question that cannot be answered by reading the 135 pages of this scientific basis for alternative SPF methods, as none of these works make any recommendations as to which method or methods should be used, since there is no direct comparison between the individual methods. However, there is always a comparison with the gold standard ISO 24444, which also contains the definition of SPF.

We will discuss the Gold standard ISO 24444 and the three different types of alternative methods ALT-1, ALT-2 and ALT-3, along accuracy and costs. The accuracy includes trueness and precision. The original validation criteria are explained in the first of the 7 publications Test design and results of a method performance characterization study for SPF and UVA-PF testing [7]

The costs vary from test institute to test institute. The cost to apply each method is not given, but estimations are sufficient, because the cost can differ by orders of magnitudes between methods, especially compared to the in-silico method, as shown in Figure 2.

Gold standard ISO 24444 [8].

On the “Accuracy versus Cost” chart (Figure 2), the in vivo method ISO 24444 is located on the far right, indicating relatively high costs. In fact, it is not only one point, but an entire cloud that is ranging from low to high accuracy. This reflects the accuracy gained by using up to 4 different testing institutes, as already described by Miksa et al. [16]. A single in vivo test alone can be quite inaccurate, but measuring four independent measurements involve relatively high costs. Such measurements remain an exception, as was the case, for example, in the ring test conducted by the ALT-SPF Consortium ring test [7-13].

We can roughly estimate the costs of measuring the SPF value per sample. Assuming that the ALT-SPF consortium, which tested 32 samples, spent € 800’000, this amounts to an average of € 25’000 per SPF sample. This is, of course completely beyond the scope of any routine SPF testing. Therefore, we need a different, i.e. smarter approach that involves the various alternative SPF methods that are now available.

ALT-1: Hybrid in-vivo/in-vitro Diffuse Reflectance Spectroscopy [10,12]

The accuracy of the ALT-1 in-vitro/in-vivo hybrid type of methods is comparable to in vivo measurement, as indicated in Figure 2. There are 3 sub methods, Mono, Poly and Multi, referring to the type measurement wavelengths. The determination of the UVA-PF is included, the water resistance (WR) is currently under development.

ALT-2: in-vitro Transmission Spectroscopy (ISO 23675 and Fused method) [9,13]

The accuracy of the ALT-2 in-vitro type of methods is comparable to in vivo measurement, as indicated in Figure 2. Further developments are needed, e.g. the combination/integration of UVA-PF measurement and also water resistance. Features of the fused method may be added. In general, ISO requires that ISO standards be reviewed for revision at least every five years.

ALT-3: in-silico Calculation, in vivo calibrated (BASF, DSM-F sunscreen simulation tools) [11]

The third type of alternative methods, ALT-3 is the in-silico type of method. The accuracy of is comparable to in vivo measurement, as indicated in Figure 2. In fact, the in-silico methods proved to be the best predictor of the SPF value of the 32 samples tested. Given the decades of development of this method, this is not surprising [17-21]. However, despite the fairly accurate prediction of the SPF value, in silico methods alone cannot guarantee that a particular product will actually act as a sunscreen. Therefore, the simple advice is not to rely on a single method.

Conclusion

The long-awaited non-invasive SPF testing methods are now available, as is the scientific and statistical basis. Looking at the SPF toolbox (Figure 2), it is clear that it makes most sense to start with the in-silico method. This is free of charge and already provides a reliable SPF value. We know that these values are based on more than two decades of development, but we need at least a second method to verify and confirm that a particular product actually works, i.e., filters UV radiation. So far, there are no published experiences that go beyond the results of the ALT-SPF consortium.

Therefore, I recommend trying out different methods to find out which one provides reliable results for your products. This means investing in testing, both in terms of expertise and financing. In the medium to long term, this will most likely pay off once you have found a reliable and cost-effective method for SPF testing of your products. It would be great if you could also make such findings available to the public, e.g., in the form of a scientific publication. This would help to achieve further progress in this area.

Stay tuned!

References

1. ISO 23675:2024, Cosmetics — Sun protection test methods — In vitro determination of sun protection factor (SPF), www.iso.org, published Dec 2024
2. ISO 23698:2024, Cosmetics — Measurement of the sunscreen efficacy by diffuse reflectance spectroscopy, www.iso.org, published Dec 2024
3. Osterwalder U, Two Additions to the SPF Toolbox, EURO Cosmetics News Letter, https://www.eurocosmetics-mag.com/two-additions-to-the-spf-toolbox/, 2025
4. Osterwalder U, Which SPF test method to use? EURO Cosmetics News Letter, https://www.eurocosmetics-mag.com/which-spf-test-method-to-use/, 2025
5. Osterwalder U, Alternative SPF-Methods on the Doorsteps, EURO Cosmetics News Letter, https://www.eurocosmetics-mag.com/alternative-spf-methods-on-the-doorsteps/, 2024
6. Osterwalder U, AIternative SPF Methods, EURO Cosmetics News Letter, https://www.eurocosmetics-mag.com/alternative-spf-methods/, 2023
7. Colson B, Vollhardt J, Cole C, Kolbe L, Matts P, Muller B, Nogueira L, Oliveira S, Pissavini M, Pouradier F, Renner G, Rohr M, Ruvolo E, Tricaud C; ALT-SPF Consortium; Uhlig S. Test design and results of a method performance characterization study for SPF and UVA-PF testing. Int J Cosmet Sci. 2025 Sep;47 Suppl 1(Suppl 1):2-24. doi: 10.1111/ics.70019.
8. Cole C, Colson B, Uhlig S. The variability of in vivo sunscreen sun protection factor values. Int J Cosmet Sci. 2025 Sep;47 Suppl 1(Suppl 1):25-36. doi: 10.1111/ics.70000. Epub 2025 Jul 10.
9. Pouradier F, Miksa S, Batzer J, Mangassi M, Ludwikowska A, Nocairi H, Colson B, Renner G, Matts PJ, Pissavini M. Performance assessment of the Double Plate method (ISO23675) in ALT-SPF Consortium: A highly reproducible and accurate in vitro method to determine SPF. Int J Cosmet Sci. 2025 Sep;47 Suppl 1:37-52. doi: 10.1111/ics.13088.
10. Ruvolo E, Cole C, Rohr M, Silverman J, Yousefian O, Batzer J, Lange N, Touti R, Pouradier F, Nogueira L, Colson B. Performance of hybrid diffuse reflectance spectroscopy (HDRS-ISO 23698) methodology for assessment of sunscreen protection in the ALT-SPF Consortium validation study. Int J Cosmet Sci. 2025 Sep;47 Suppl 1(Suppl 1):53-77. doi: 10.1111/ics.13089.
11. Kunze G, Sohn M, Giesinger J, Quass K, Prigl Y, Schleiger C, Schlifke A, Ens W, Wilfert F, Vollhardt J, Colson B, Herzog B. The ALT-SPF ring study-Correlation in silico versus in vivo SPF ISO24444 and in vitro UVA-PF ISO24443. Int J Cosmet Sci. 2025 Sep;47 Suppl 1(Suppl 1):78-95. doi: 10.1111/ics.13086.
12. Reble C, Bayer M, Colson B, Emmler T, Hubaud JC, Seise M, Perroux-David E, Wiora G, Zamudio Diaz DF; ALT-SPF Consortium; Meinke MC, Bielfeldt S. Characterization of LED-based hybrid diffuse reflectance spectroscopy method for determination of SPF and UVA-PF in blinded multi-centre study (ALT-SPF). Int J Cosmet Sci. 2025 Sep;47 Suppl 1(Suppl 1):96-114. doi: 10.1111/ics.70007.
13. Acker S, Büchse A, Nogueira L, Zur Lage J, Perroux-David E, Drulhon F, Moraes J, Peters S, Janssen A, Komjani NM, Colson B, Herzog B. The ALT-SPF ring study – in vitro determination of the SPF & UVA-PF by the fused method. Int J Cosmet Sci. 2025 Sep;47 Suppl 1(Suppl 1):115-132. doi: 10.1111/ics.70015.
14. Pissavini M, Chave J, Lane M. Are there alternatives to the traditional in-vivo SPF test (ISO 24444)? Comparison and statistical analysis of 5 proposed methods. Int J Cosmet Sci. 2025 Sep;47 Suppl 1:1. doi: 10.1111/ics.70026.
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