Two Additions to the SPF Toolbox!

EURO COSMETICS Magazine • Two Additions to the SPF Toolbox! • Uli Osterwalder • Uli Osterwalder
ULI OSTERWALDER
Principal and Owner of Sun Protection Facilitator GmbH

In late 2024, two long-awaited alternative SPF test methods were finally published: ISO 23675:2024 and ISO 23698:2024 [1]. What does this mean? How soon will these methods be adopted? Will their SPF always match the in vivo gold standard ISO 24444? And which of the three available official ISO methods will apply in case of discrepancies between them? This column will provide some answers.

1. Overview SPF-Methods

Here is a brief summary of the three current ISO methods for determining the SPF, taken directly from the respective international standard (available at iso.org):

  1. ISO 24444:2019 (Gold standard, in vivo)
  2. ISO 23675:2024 (in vitro, transmittance)
  3. ISO 23698:2024 (in vivo/in vitro, hybrid, non-invasive)

1.1 ISO 24444:2019(E)

Cosmetics — Sun protection test methods — In vivo determination of the sun protection factor (SPF)

Cosmétiques — Méthodes d’essai de protection solaire — Détermination in vivo du facteur de protection solaire (FPS)

Scope

This document specifies a method for the in vivo determination of the sun protection factor (SPF) of sunscreen products. It is applicable to products that contain any component able to absorb, reflect or scatter ultraviolet (UV) rays and which are intended to be placed in contact with human skin.

This document provides a basis for the evaluation of sunscreen products for the protection of human skin against erythema induced by solar ultraviolet rays.

1.2 ISO 23675:2024(E)

Cosmetics — Sun protection test methods — In vitro determination of sun protection factor (SPF) Cosmétiques — Méthodes d’essai de protection solaire — Détermination in vitro du facteur de protection solaire (FPS)

Scope

This document specifies a method for the in vitro determination of sun protection factor (SPF). This method is applicable to sunscreen products in form of an emulsion or alcoholic one-phase formulation, excluding in form of a loose or compressed powder or stick. Specifications are given to enable determination of the spectral absorbance characteristics of SPF protection in a reproducible manner.

Use of this method is strictly for the determination of a static sun protection factor. It is not applicable for the determination of water-resistance properties of a sun protection product.

1.3 ISO 23698:2024(E)

Cosmetics — Measurement of the sunscreen efficacy by diffuse reflectance spectroscopy

Cosmétiques — Mesurage de l’efficacité des produits de protection solaire par spectroscopie de réflectance diffuse

Scope

This document provides a procedure to characterize the sun protection factor (SPF), UVA protection factor (UVA-PF) and critical wavelength (CW) protection of sunscreen products without requiring biological responses. The test method is applicable for emulsions and single-phase products. The method has not been evaluated for use with powder forms sunscreen products.

This document gives specifications to enable determination of the absolute spectral absorbance characteristics of a sunscreen product on skin to estimate sunburn and UVA protection. It is applicable to products that contain any component able to absorb, reflect or scatter ultraviolet (UV) rays and which are intended to be placed in contact with human skin.

2. Thank you Consortium ALT-SPF!

The idea and desire to replace the invasive in vivo SPF standard (now ISO 24444) is not new, in fact at least 50 years old. Almost 20 years ago the European Commission recommended the use of in vitro SPF methods [2]:

17)   While these testing methods should be used as reference methods, preference should be given to in vitro testing methods delivering equivalent results, as in vivo methods raise ethical concerns. Industry should increase efforts to develop in vitro testing methods for the protection against both UVB and UVA radiation.

Why did it take so long? The simple answer is that it is not easy to find alternative SPF methods that deliver results equivalent to the gold standard. The first in vitro project failed after nine years of development (“ISO 24445”, 2006-2014). Only the Consortium ALT-SPF delivered results that led to the two new standards [3]. We would therefore like to thank the Thank You Consortium ALT-SPF for providing the scientific and statistical basis for two alternative methods (participants in Figure 1). We are now looking forward to the upcoming publications with the detailed results of the largest SPF round robin test ever conducted.

EURO COSMETICS Magazine • Two Additions to the SPF Toolbox! • Uli Osterwalder • Uli Osterwalder
Figure 1  ALT-SPF Consortium, 29 Participants plus QuoData (Statistics) The specific explanations 1)-5) can be found above Table 1

In a past issue of Sweep-the-Sun entitled “Alternative SPF Methods on the Horizon”, I mentioned all the alternative SPF methods that are currently being investigated and can be used for internal and unofficial purposes [4]. Figure 2 shows the current state of the SPF toolbox. With the help of the upcoming publications of the “ALT-SPF” results, it will be possible to confirm or correct the current positions of the tools (methods) in terms of accuracy/cost. The goal, of course, is to obtain SPF results that are as accurate and reliable as possible, at a reasonable cost.

EURO COSMETICS Magazine • Two Additions to the SPF Toolbox! • Uli Osterwalder • Uli Osterwalder
Figure 2 SPF–TOOLBOX: Cost/Accuracy Position of SPF Methods (tentative) More information regarding the SPF methods can be found below and in Ref 4 (SPF Methods on the Doorsteps)

Table 1 shows the merits and limitations of the current SPF methods. The following reference numbers refer to the numbers in table 1.

1)      With ISO 24444, the SPF of practically every product that can be applied on the human skin can be determined. However, it only determines an integral endpoint, which is an SPF value. There is no granular information about the protection at each wavelength

2)      HDRS/ISO 23698, can be used to determine the SPF of any product that can be measured using ISO 24444 and ISO 24443. The results obtained go far beyond ISO 24444, as the in vivo and the in vitro results provide granular information over the UVA range from 320 to 400 nm and over the entire UV range from 280 to 400nm, respectively.

3)      The scope of products that can be tested with ISO 23675 is restricted, e.g. no single-phase oil formulations. The results obtained go far beyond ISO 24444, the in vitro results provide granular information over the entire UV range, 280 to 400nm. However, they do not yet include the UVA-PF and the water-resistance SPF.

4)      In silico results go far beyond ISO 24444 because both in vivo as well as the in vitro results provide granular information over the entire UV range from 280 to 400nm. Every metric imaginable can be determined. However, it is necessary to check wheather the chosen UV-Filter combination can be formulated as sunscreen product. If the UV-filter combination is unknown, it can be determined by analytical means (EN 17156:2018 or EN 16344:2013).

5)      UVA-photography is an excellent tool for raising awareness of sun protection, but it lacks the accuracy for quantitative determination of the sun protection factor.

EURO COSMETICS Magazine • Two Additions to the SPF Toolbox! • Uli Osterwalder • Uli Osterwalder
Table 1 SPF-TOOLBOX: Every Method has its merits and limitations

3. What’s next?

Let’s do a little exercise by looking into the past and check how much has been implemented and what needs to be discarded or revised. In 2018, the article SPF Assessment Revisited – Status and Outlook, triggered the founding of the ALT-SPF consortium [8]. The last paragraph «The Way Forward» reads as follows:

The Way Forward [7] (2018).

Several alternative methods to replace ISO 24444 SPF in vivo have been developed and are currently still optimized. In particular in silico and in vitro method appear attractive as they can be run without involving human subjects and in fact these methods proved to be valuable prediction tools already during the planning and development phase for new sunscreens. The ISO/TC 217 Workgroup 7 is in the last steps of finalizing the validation methodology for the new methods and is eager to put some of the methods to a solid validation test. Will a non-human method make it or is the “human component” due to the complexity the interaction of sunscreen with skin an unavoidable condition? Or are all those efforts wiped away one day through “Big Data” calculation of the SPF? Time will tell.

Now, seven years later, I expect it will take some time for the new ISO SPF methods to be proven. Both new alternative methods have their merits, but it will depend on the country/region how quickly these new methods are adopted. Australia, for example, is only just introducing the revised version of the gold standard ISO 24444 from 2019, and when the US FDA (Food and Drug Administration) will ever adopt an ISO standard is anyone’s guess. The first tests with the new methods are being carried out in Europe. It will take a transitional period for the new methods to become established. The aim is to obtain the most reliable SPF values at a reasonable cost. To do this, it is best to use at least two methods to gain experience with your own formulations. The ALT-SPF consortium has characterized the methods with a ring test on 32 products, divided into 8 different product groups [3]. These 32 products cannot, of course, be representative of every product on the market, so we have to expect cases where the new alternative SPF may not correspond to the in vivo SPF gold standard ISO 24444.

Sunscreen products with high ZnO concentrations appear to be a product group for which in vivo and in vitro do not correlate well: SPF in vitro too low compared to SPF in vivo – a brief overview [9]. Seven years ago, we mentioned “big data” as a possible future solution. Today, we call this artificial intelligence (AI), e.g. à la Chat GPT. I already expressed my opinion in one of my columns titled “AI to predict Sunscreen Performance?” [10]. We don’t have enough high-quality sunscreen data to take the “big data” approach, but AI could still help with less but higher-quality data.

Stay tuned!

References

  1. Grabenhofer R, Will See Faster, More Ethical and Accurate SPF Testing: ISO Approves Two New Methods, C&T magazine, Jan 3rd, 2025, https://www.cosmeticsandtoiletries.com/testing/sun-protection/news/22929537/2025-will-see-faster-more-ethical-and-accurate-spf-testing-iso-approves-two-new-methods
  2. European Commission. Recommendation on the efficacy of sunscreen products and the claims made relating thereto. J Eur Union. 2006;245:39–43
  3. Consortium ALT-SPF, CALL FOR INTEREST– TO EVALUATE ALTERNATIVE SPF METHODS. Available from: https://www.alt-spf.com/home/, viewed January 23, 2025.
  4. Osterwalder U, Alternative SPF Methods on the Doorsteps, EURO Cosmetics News Letter, https://www.eurocosmetics-mag.com/alternative-spf-methods-on-the-doorsteps/, viewed 2025-01-23
  5. Herzog B, Osterwalder U. Simulation of sunscreen performance. Pure Appl Chem. 2015;87(9–10):937–51.doi: 10.1515/pac-2015-0401
  6. Osterwalder U, Sohn M, Herzog B. Global state of sunscreens. Photodermatol Photoimmunol Photomed.2014;30(2–3):62–80. doi: 10.1111/phpp.12112. PMID: 24734281.
  7. Crowther JM, Schütz R, Vollhardt JH. Ultraviolet transmission microscopy for the imaging of topical sunscreen emulsions. Int J Cosmet Sci. 2022 Dec;44(6):663-671. doi: 10.1111/ics.12809. Epub 2022 Sep 21. PMID: 35924327.
  8. Osterwalder U, Schütz R, Vollhardt J, SPF Assessment Revisited – Status and Outlook, SOFW Journal, 144, 38-42 (2018)
  9. Osterwalder U, Hubaud JC, Perroux-David E, Moraine T, van den Bosch J. Sun-protection factor of zinc-oxide sunscreens: SPF in vitro too low compared to SPF in vivo-a brief review. Photochem Photobiol Sci. 2024 Oct;23(10):1999-2009. doi: 10.1007/s43630-024-00644-0. Epub 2024 Oct 21.
  10. Osterwalder U, AI to predict Sunscreen Performance? EURO Cosmetics News Letter, https://www.eurocosmetics-mag.com/ai-to-predict-sunscreen-performance/ , viewed 2025-01-23

EURO COSMETICS Magazine • Uli Osterwalder • Uli Osterwalder • Uli Osterwalder
Uli Osterwalder

Uli Osterwalder studied Chemical Engineering at ETH Zurich, Switzerland and at the University of Houston in Houston, Texas. He joined Ciba-Geigy in Basel in 1979 where he first developed a Phosgene Generator in central process development. Later he developed his leadership skills in Project Management and Process Analytics. At Ciba Specialty Chemicals Uli Osterwalder helped establish new business development in Fabric Care and Personal Care. After the acquisition by BASF SE he became Senior Marketing Manager and Scientific Adviser in Sun Care in Ludwigshafen and Duesseldorf.
2016 he came back to Basel, working for DSM as senior Senior Scientific Adviser suncare for two years. 2018 he started his own company, Sun Protection Facilitator GmbH and is committed to contribute to further improvements in sun protection. Uli Osterwalder works for ISO on the development of new UV protection assessment methods and is now chairing the technical committee ISO TC/217 (Cosmetics). He is author and co-author of numerous scientific articles and book chapters on sun protection.

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