Every sunscreen carries an SPF number, and it is the figure most consumers rely on. Yet SPF answers a deliberately narrow question, measured under idealised laboratory conditions. Whether a sunscreen protects skin effectively — across the full ultraviolet range, throughout prolonged exposure, in water, and at the quantity actually applied — depends on several properties the SPF value was never designed to capture. This article sets out the six that matter, explains what the SPF label does and does not measure, and clarifies why a product can pass its SPF test yet still perform poorly on some of them.

 

What the SPF number is, and how it is measured

 

SPF (sun protection factor) is measured in vivo — on human volunteers, under laboratory conditions. The product is applied at a fixed density of 2 milligrams per square centimetre, and the skin is then exposed to a controlled, graded series of ultraviolet doses. The smallest dose that produces just-perceptible redness — the threshold of sunburn — is identified on protected skin and compared with the equivalent dose on unprotected skin; the ratio of the two is the SPF (ISO 24444:2019). An SPF of 30 therefore indicates that protected skin required thirty times the ultraviolet dose to reach that same threshold, under those specific conditions.

 

Two features of the method matter for what follows. First, the endpoint is sunburn (erythema), which is driven predominantly by the shorter-wavelength UVB rays, so the value is essentially a UVB-weighted measure. Second, it is determined on a thick, uniform, freshly applied film. In the European Union the resulting value is grouped into label categories — low, medium, high, and very high protection (European Commission, 2006).

 

What Aelon IQ analyses

 

Broad-spectrum coverage

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Ultraviolet radiation reaching the skin spans UVB (approximately 290–320 nm) and the longer-wavelength UVA (320–400 nm, conventionally divided into UVA2 and UVA1). Because SPF tracks sunburn, a high SPF principally certifies UVB protection; on its own it does not guarantee proportionate UVA protection. This matters because UVA penetrates more deeply and drives the changes in the deeper skin that underlie premature ageing — loss of firmness, wrinkling and uneven tone (Battie, C., et al., 2014). The EU recognises this gap explicitly: it recommends that, to carry a UVA claim, a product's UVA protection be at least one-third of its SPF, with a critical wavelength of at least 370 nm (European Commission, 2006). Effective protection therefore requires coverage across the entire range, not solely against burning.

 

Organic filter solvency

 

Several modern organic (chemical) UV filters are crystalline solids that must be dissolved to become effective; a small number are instead engineered to function as finely dispersed particles. For the dissolved type, the filter must pass fully into the product's oil phase, which requires its solubility in that oil to exceed the concentration used (Osterwalder, U., et al., 2014). Where a formulation is over-loaded (more filter than the oils can hold) the excess may slowly recrystallise into microscopic crystals. Laboratory work has confirmed "the negative impact of crystal formation on the delivered photoprotection of a sunscreen" (Sohn, M., et al., 2019). Bringing the filters into true solution, and holding them there, is a precondition for the protection stated on the label.

 

Organic filter photostability

 

A UV filter functions by absorbing ultraviolet energy, and some filters are chemically altered by that energy and progressively lose their capacity to absorb. The best-known example is the widely used UVA filter avobenzone, which is photounstable in isolation and loses UV-absorbing capacity under irradiation unless it is combined with stabilising ingredients (Chatelain, E. and Gabard, B., 2001). This degradation is easily overlooked, because its effect falls largely on UVA protection, which the sunburn-weighted SPF value, reflecting chiefly UVB, may not fully capture. A product may therefore retain its nominal SPF while its UVA protection declines over hours of exposure. Photostable filter systems are what maintain protection, across the whole range, throughout real sun exposure.

 

Mineral filter stability

 

The mineral (inorganic) filters titanium dioxide and zinc oxide are photocatalysts: under ultraviolet light their uncoated particle surfaces generate reactive oxygen species such as superoxide and hydroxyl radicals (Brezová, V., et al., 2005). Left unchecked, this surface reactivity is undesirable within a formulation, which is why cosmetic-grade mineral filters are supplied with an inert surface coating — such as silica, alumina or a silicone — that markedly reduces, though does not entirely eliminate, the photocatalytic activity (Dransfield, G., et al., 2000). Whether the mineral particles in a formulation are well coated is therefore a genuine difference in quality — one the ingredient list cannot reliably convey, yet central to the product's stability.

 

A durable, uniform film

 

SPF is measured on an intact, uniform layer. In use, that layer must withstand perspiration, water and abrasion against clothing. Formulators incorporate film-forming ingredients to render the protective layer more substantive and water-resistant, and the effect is quantified by standardised water-immersion tests that measure how much of a product's SPF is retained after repeated immersion (ISO 16217:2020; ISO 18861:2020). A product retaining enough of its SPF after immersion may be labelled "water resistant" or "very water resistant"; the distinction is the duration of the immersion protocol, not a higher standard of protection. A film that migrates or is unevenly removed leaves areas of skin under-protected, irrespective of the original SPF value.

 

Cosmetic elegance

 

The labelled SPF assumes 2 mg/cm² of product, whereas in practice consumers apply approximately a quarter of that amount, and delivered protection falls substantially as the applied layer thins (Faurschou, A. and Wulf, H.C., 2007). Any property that discourages generous, even application (e.g., a heavy feel, a greasy finish, or a white cast from mineral filters) therefore erodes real-world protection, because a product's sensory characteristics strongly influence how well it is used (Osterwalder, U., et al., 2014). A sunscreen pleasant enough to apply liberally and to reapply outperforms a technically superior product used sparingly. Aelon IQ assesses every mineral sunscreen for white cast potential (chemical sunscreens do not leave a white cast) and other cosmetic elegance features, described in more detail here.   

 

Why a product can pass its SPF test yet still perform poorly

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The SPF test answers a single, narrowly defined question under idealised conditions: how much greater an ultraviolet dose the skin can tolerate before it begins to burn when a thick, uniform, freshly applied 2 mg/cm² layer is exposed in the laboratory (ISO 24444:2019). It characterises the sunburn protection of the freshly applied product. It is not designed to indicate whether UVA is covered in proportion, whether the filters stay dissolved, coated and photostable under prolonged exposure, whether the film resists water and wear, or if the tested quantity is applied. Some of these are addressed by separate, claim-specific tests — a sunscreen carries a UVA or water-resistance claim only when it has been tested against the relevant standard (European Commission, 2006; ISO 16217:2020). But none of them is conveyed by the SPF number alone. These are exactly the properties described by the six factors above. A product can therefore legitimately earn its SPF value yet still underperform on breadth, durability or real-world use, not because the value is incorrect, but because it addresses a narrower question than whether the product will protect skin well under ordinary conditions. 

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References

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Battie, C., Jitsukawa, S., Bernerd, F., Del Bino, S., Marionnet, C. and Verschoore, M. (2014) 'New insights in photoaging, UVA induced damage and skin types', Experimental Dermatology, 23(Suppl. 1), pp. 7–12. 

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Brezová, V., Gabčová, S., Dvoranová, D. and Staško, A. (2005) 'Reactive oxygen species produced upon photoexcitation of sunscreens containing titanium dioxide (an EPR study)', Journal of Photochemistry and Photobiology B: Biology, 79(2), pp. 121–134.

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Chatelain, E. and Gabard, B. (2001) 'Photostabilization of butyl methoxydibenzoylmethane (avobenzone) and ethylhexyl methoxycinnamate by bis-ethylhexyloxyphenol methoxyphenyl triazine (Tinosorb S), a new UV broadband filter', Photochemistry and Photobiology, 74(3), pp. 401–406.

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Dransfield, G., Guest, P.J., Lyth, P.L., McGarvey, D.J. and Truscott, T.G. (2000) 'Photoactivity tests of TiO2-based inorganic sunscreens. Part 1: Non-aqueous dispersions', Journal of Photochemistry and Photobiology B: Biology, 59(1–3), pp. 147–151. 

 

European Commission (2006) Commission Recommendation 2006/647/EC of 22 September 2006 on the efficacy of sunscreen products and the claims made relating thereto. Official Journal of the European Union, L 265, 26 September 2006, pp. 39-43.

 

Faurschou, A. and Wulf, H.C. (2007) 'The relation between sun protection factor and amount of sunscreen applied in vivo', British Journal of Dermatology, 156(4), pp. 716–719.

 

International Organization for Standardization (2019) ISO 24444:2019 Cosmetics — Sun protection test methods — In vivo determination of the sun protection factor (SPF). Geneva: ISO.

 

International Organization for Standardization (2020) ISO 16217:2020 Cosmetics — Sun protection test methods — Water immersion procedure for determining water resistance. Geneva: ISO.

 

International Organization for Standardization (2020) ISO 18861:2020 Cosmetics — Sun protection test methods — Percentage of water resistance. Geneva: ISO.

 

Osterwalder, U., Sohn, M. and Herzog, B. (2014) 'Global state of sunscreens', Photodermatology, Photoimmunology and Photomedicine, 30(2–3), pp. 62–80.

 

Sohn, M., Prost-Dame, M., Bayraktar, M., Schäfer, A. and Herzog, B. (2019) 'Crystallization velocity and UV performance of formulations with oversaturated UV-filter content', Journal of Pharmaceutical Sciences, 108(5), pp. 1800–1807.