“The ideal sunscreen” – the optimal UV protection profile

The festive season around the turn of the year is the time of wishes and resolutions. So, this seems to be the right time to ask the question “what is the ideal sun protection”, in other words “what is the optimal protection profile”. I have been asking myself this question since I started working with UV filters in personal care more than 20 years ago. Coming from a fabric care background and having even earlier experience with textile dyes, it was easy for me to come to terms with the idea that the best sun protection is to cover up. As I have expressed in previous issues of this column, the closer the protective profile of the sunscreen is to that of the clothing, i.e., the more uniform the sun protection, the better the sun protection. Brian Diffey suggested uniform sun protection 30 years ago¹ and we adopted this strategy in the development of UV filters². Later, Diffey elaborated on this idea of ideal sun protection and predicted: “Sunscreens and melanoma: the future looks bright”^{3, 4}. He also designed a new metric called “Spectral uniformity: a new index of broad spectrum (UVA) protection”⁵, but the time was apparently not yet ripe for industry to adopt it.

Perhaps the industry needed some “mathematical” evidence to support the advocacy of this ideal uniform protection, also known as spectral homeostasis protection? Five years ago, Diffey wrote again about optimizing the spectral absorption profile of sunscreens⁶. This time, he not only argued for uniform protection, but also debunked the myth that we need more UVB protection than UVA protection.

An often-heard argument against uniform protection is that more UVA protection only comes at the expense of less UVB protection. People would say, “I don’t want to compromise on UVB protection” … because “skin cancer is caused by UVB” …
While it may be tempting to believe that higher UVB than higher UVA protection is beneficial, this is a fallacy for several reasons):

1. Sunscreens with the same SPF provide the same protection against erythema regardless of whether UVB protection is higher or lower than UVA protection.
2. The total UV exposure of sunscreens with a 3:1 ratio between UVB and UVA protection, as expressed in the EU recommendation⁷, is up to 5 times higher than that of sunscreens with a flat spectral profile
3. A 3:1 profile provides lower protection against other action spectra than erythema, e.g. one with higher activity in the UVA range.

The latter statement is particularly true for melanoma prevention, where it now seems clear that UVA plays an important role. Figure 1 shows the monochromatic protection factor (mPF) of a sunscreen with SPF 30 over the entire UV range. We see that the maximum protection at mPF 40 is in the UVB range, but in the long UVA1 range, the protection is only about mPF 10 or less. This is the range where large amounts of UVA radiation can penetrate the sunscreen.

In summary, the spectral protection profile should ideally be flat at all wavelengths throughout the UV spectrum. This philosophy is gaining more and more acceptance, and for several years manufacturers have been working to develop sunscreens with spectral profiles that approach uniform absorption across the entire UV spectrum. Today, there are also already some products that practically meet this criterion of spectral uniformity^{8, 9}. Passeron T et al. recently pointed out that the observed adverse biological effects of UVA1 leading to clinical consequences, combined with the predominance of UVA1 rays in solar UV radiation, provide a solid rationale for the need for broad-spectrum photoprotection that includes UVA1 up to 400 nm¹⁰.

An mPF 30 (Figure 1) is actually good enough to protect against UVB in most circumstances, whereas an mPF 10 or less in the long UVA1 range can be harmful, e.g., if a so-called “UVA erythema” develops during a longer exposure. Anecdotal evidence from dermatologists suggests that such UVA erythema caused by UVBskewed sunscreens is much deeper and therefore more severe than ordinary erythema due mainly to UVB¹¹. I am not aware of any studies on this topic, but the recommendation is clear, i.e., choose sunscreens with adequate UVA protection, ideally the “ideal sunscreen with uniform protection”.

The question remains, how can you assess the protection profile? How can I check what my sun protection profile looks like? SPF alone does not tell us enough¹². Emerging alternative SPF methods provide this information across the spectrum¹³.

The various UVA metrics give us a good indication of how close we are to the ideal sunscreen. Figure 2 shows the highest UVA protection requirements in different countries/regions. The simplest and also most instructive approach is to use the available in-silico tools^{14, 15}. Both BASF Sunscreen Simulator and DSM Sunscreen Optimizer provide all UVA metrics commonly used anywhere in the world.

These simulation tools also calculate the extra UV exposure resulting from the fact that most sunscreens do not have ideal protection profiles. The metric used is NTUV dose (normalized transmitted UV). To give an example: The difference in NTUV dose of a SPF 10 sunscreen with the broad-spectrum Filter Zinc Oxide (NTUV = 1.8) and with the UVB filter EthylHexylMethoxyCinnamate (NTUV = 7.2) is 4-fold. This means that 4 times more UV radiation, mainly more UVA, reaches the skin with the latter sunscreen. An ideal sunscreen profile would result in an NTUV dose = 1.0.

Conclusion: do NOT just pay attention to the SPF, a balanced profile is just as important. Just aim for as much UVA protection as possible, and even if you don’t reach the optimal protection profile of the ideal sunscreen right away, it’s still good to know you’re on the right track.

References:

• 1 Diffey, B.L. The need for sunscreens with broad spectrum protection. In: Biological Responses to Ultraviolet A Radiation (Urbach, F., ed.), pp. 321–328. Valdenmar Publishing Company, Overland Park KS (1992).
• 2 Osterwalder U, Herzog B. The long way towards the ideal sunscreen–where we stand and what still needs to be done. Photochem Photobiol Sci. 2010 Apr;9(4):470-81. doi: 10.1039/b9pp00178f. PMID: 20354640
• 3 Diffey BL. Sunscreens and melanoma: the future looks bright. Br J Dermatol. 2005 Aug;153(2):378-81. doi: 10.1111/j.1365-2133.2005.06729.x. PMID: 16086753.
• 4 Diffey BL, Brown MW. The ideal spectral profile of topical sunscreens. Photochem Photobiol. 2012 May-Jun;88(3):744-7. doi: 10.1111/j.1751-1097.2012.01084.x. Epub 2012 Jan 31. PMID: 22233183.
• 5 Diffey B. Spectral uniformity: a new index of broad spectrum (UVA) protection. Int J Cosmet Sci. 2009 Feb;31(1):63-8. doi: 10.1111/j.1468-2494.2008.00471.x. PMID: 19134129.
• 6 Diffey BL. Optimizing the spectral absorption profile of sunscreens. Int J Cosmet Sci. 2017 Feb;39(1):90-doi: 10.1111/ics.12353. Epub 2016 Jul 25. PMID: 27388284.
• 7 COMMISSION RECOMMENDATION of 22 September 2006 on the efficacy of sunscreen products and the claims made relating thereto. Official Journal of the European Union. L 265/39. Available from: https://eur-lex.europa.eu/legalcontent/EN/TXT/PDF/?uri=CELEX:32006H0647&from=CS, viewed October 12,2020
• 8 Osterwalder, U. and Hareng, L. Global UV filters: current technologies and future innovations. In: Principles and Practice of Photoprotection (Wang, S.Q. and Lim, H.W. eds.), pp. 179–198. Springer International Publishing, Switzerland (2016).
• 9 Dudley DK, Laughlin SA, Osterwalder U. Spectral Homeostasis – The Fundamental Requirement for an Ideal Sunscreen. Curr Probl Dermatol. 2021;55:72-92. doi: 10.1159/000517593. Epub 2021 Oct 25. PMID: 34698022
• 10 Bernerd F, Passeron T, Castiel I, Marionnet C. The Damaging Effects of Long UVA (UVA1) Rays: A Major Challenge to Preserve Skin Health and Integrity. Int J Mol Sci. 2022 Jul 26;23(15):8243. doi: 10.3390/ijms23158243. PMID: 35897826; PMCID: PMC9368482
• 11 Personal Communication, Sharyn A Laughlin, MD, Ottawa, CAN, 2019
• 12 ISO 24444:2019: Cosmetics – Sun protection test methods – In vivo determination of the sun protectionfactor (SPF). Available from: https://www.iso.org/standard/72250.html, viewed October 12, 2020.
• 13 Surber C, Uhlig S, Bertrand C, Vollhardt J, Osterwalder U. Past, Present, and Future of Sun Protection Metrics. Curr Probl Dermatol. 2021;55:170-187. doi: 10.1159/000517667
• 14 BASF Sunscreen-Simulator. Available from: https://www.sunscreensimulator. basf.com/Sunscreen_Simulator/login, viewed September 30, 2020
• 15 DSM SUNSCREEN OPTIMIZER. Available from: https://www.sunscreenoptimizer.com/index.html?sid=9bdaee5622fecd3909de24ea4236f74c, viewed September 30, 2020