The single number on the front of a sunscreen carries less information than most people think, and the small mark next to it carries more. Understanding the relationship between them is the fastest way to read a pack properly, and it applies identically to mineral products.
The two bands, and why they are treated separately
Ultraviolet reaching the ground is divided into two bands that behave very differently. The shorter wavelength band, ultraviolet B, carries more energy per photon and is absorbed strongly by DNA. It is the principal cause of sunburn and of the characteristic mutations associated with sun exposure. It varies enormously with time of day, season and latitude, because the shorter the wavelength the more it is absorbed by atmospheric ozone.
The longer wavelength band, ultraviolet A, is far more abundant at ground level, varies much less through the day and the year, penetrates deeper into the dermis and passes through window glass. It causes far less immediate reddening, which is exactly the problem: a person can accumulate a substantial ultraviolet A dose without any signal that it is happening. It drives oxidative damage, degradation of dermal collagen and elastin, pigmentary change, and contributes to carcinogenesis.
Because the two bands do different things and are attenuated by different filters to different degrees, a single number cannot express protection against both.
What sun protection factor actually measures
Sun protection factor is defined as a ratio: the ultraviolet dose required to produce a just perceptible reddening on protected skin, divided by the dose required to produce the same reddening on unprotected skin. It is determined on human volunteers under a standardised method, with a specified quantity of product applied per unit area. Because the reddening endpoint is dominated by ultraviolet B, the number is dominated by ultraviolet B protection.
Two consequences follow, and both are widely misunderstood. First, the factor is a ratio and not a time. It does not mean a number of hours. Second, it is measured at an application density that is considerably heavier than most people apply, so the protection actually delivered is normally lower than the label figure. That gap is a formulation and behaviour problem rather than a labelling deception, and in the mineral category it is made worse by the tendency to apply thinly to avoid whiteness. That is examined in the quantity problem.
The UVA mark in a circle
The European Commission issued a recommendation on the efficacy of sunscreen products and the claims made about them, which established the convention that the UK market still follows. It set out the labelling categories, discouraged claims implying total protection, and defined the basis for the ultraviolet A mark: a product may show the letters UVA inside a circle when its ultraviolet A protection factor is at least one third of its labelled sun protection factor, assessed alongside a critical wavelength criterion.
Critical wavelength is a measure of how far into the long ultraviolet A a product's absorbance extends. It is the wavelength below which a defined proportion of the total absorbance falls, and it captures something a single ratio cannot: whether protection tails off before the end of the ultraviolet A range.
| Element | What it records | What it does not record |
|---|---|---|
| Sun protection factor | A ratio measured against reddening, dominated by ultraviolet B, at a standardised application density | Ultraviolet A protection; hours of protection; the protection you will get from a thin application |
| Labelling category | A band such as low, medium, high or very high, corresponding to the factor | Any statement about ultraviolet A |
| UVA in a circle | That ultraviolet A protection reaches at least one third of the labelled factor, on the recommended basis | How far above that threshold the product sits |
| Water resistance wording | That the product met a defined immersion protocol | That the film survives towelling, sand or several hours of activity |
| Ingredient list | Every filter present, by its standardised name, and any nanomaterial marked | The concentration of each filter, which is not required to be stated |
Mineral filters and ultraviolet A
There is a widespread assumption that a mineral product is automatically broad spectrum. It is not automatic, though the underlying chemistry is favourable. Zinc oxide's absorption edge extends further into the long ultraviolet A than titanium dioxide's, so a zinc oxide dominant formula has good intrinsic long wave coverage. Titanium dioxide is relatively stronger in the ultraviolet B and short ultraviolet A and weaker at the long end.
But intrinsic absorption is not the same as delivered protection. A metal oxide film protects in proportion to how completely it covers, and particle agglomeration produces gaps. A titanium dioxide dominant formula can achieve a high sun protection factor while falling short of the one third relationship, exactly as an organic filter formula can. The mark on the pack is the record of whether it did.
Mineral sunscreens are naturally broad spectrum
- What would have to be true
- That the metal oxides intrinsically absorb across the whole ultraviolet range in a way that guarantees a compliant ultraviolet A to sun protection factor relationship in any formulation.
- That formulation and application do not affect measured ultraviolet A protection.
- What is established
- Zinc oxide absorbs further into the long ultraviolet A than titanium dioxide, giving formulations built on it a favourable intrinsic spectrum.
- Ultraviolet A protection is measured on the finished product, and the mark on the pack records the result of that measurement.
- What is not established
- That any mineral formulation meets the ultraviolet A criterion by virtue of its filter class.
- That titanium dioxide dominant formulations have long wave coverage equivalent to zinc oxide dominant ones.
Reading a pack in thirty seconds
- Find the factor and the labelling category. Note that it is a ratio measured at a heavier application than you will use.
- Look for UVA inside a circle. If it is absent, the product has not made that claim.
- Read the filter names in the ingredient list. Zinc oxide and titanium dioxide will be there by name, with a nanomaterial marker where applicable.
- Ignore the front of pack adjectives. Reef safe, chemical free and natural are not defined terms and do not correspond to any measurement.
Boundaries
The detail of water resistance protocols, and the labelling law that governs what may be printed on a pack, are large subjects in their own right and are outside the scope of this publication, which is about the chemistry of the mineral and natural category and about its claims. What is in scope is the point that the mark is a measurement, that measurements are made on products rather than on ingredient classes, and that no amount of preference for mineral filters substitutes for the measurement having been made.
If you have had a skin cancer, are taking a medicine that increases sensitivity to light, or have a condition that makes you photosensitive, the protection you need is a clinical question and not a shopping one. Speak to your GP, your pharmacist or your dermatology team. The British Association of Dermatologists and the NHS both publish general guidance, and neither is a substitute for advice about your own case.
Why one third, and not something else
The ratio is a policy choice rather than a biological constant. It was set to ensure that a product claiming high protection could not achieve it entirely in the ultraviolet B while leaving the ultraviolet A largely unfiltered, which was a real feature of some older products. It is a floor, not a target, and products vary in how far above it they sit. Since the ratio scales with the labelled factor, a very high factor product meeting the criterion is delivering more absolute ultraviolet A protection than a lower factor product that also meets it.