The quantity written into the method
The test method that produces a sun protection factor specifies the quantity of product applied: two milligrams per square centimetre of skin. It is not a suggestion or a typical value. It is a controlled variable, and the figure that comes out of the test is a property of the product at that quantity.
Two milligrams per square centimetre is a thicker film than it sounds. Applied to an adult body it amounts to a quantity most people would consider generous, and applied to a face it is more than most people use. That is the whole of the issue: the number on the pack is true, and it is true about a situation that rarely occurs.
Why less does not mean proportionally less
The intuitive assumption is that half the product gives half the protection. The mechanism says otherwise, and it is worth setting out because it explains a great deal that is usually asserted rather than explained.
A sunscreen film works by absorbing and scattering radiation as it passes through. For a uniform absorbing layer, the proportion of radiation transmitted falls exponentially as the layer gets thicker. Turned around, this means the protection factor of a uniform film does not fall in step with thickness. Under that simple model, halving the thickness of the film would take the protection factor to roughly its square root rather than to half its value.
That is a statement about a model, not a measurement of a product. Real films are not uniform, skin is not flat, and formulations differ in how they spread and settle. But the direction is not in doubt: the loss from under-application is steeper than proportional, and it is steeper at higher factors than at lower ones, because the reciprocal relationship compresses at that end of the scale.
The relationship between factor and effect is set out separately in why SPF and protection are not proportional, and it is the reason the two subjects have to be read together.
Why we do not print a percentage
Almost every article on this subject reaches for a single figure: a specific proportion of the labelled factor that people actually achieve in use. Those figures are widely repeated, they are not consistent with each other, and we have not been able to trace a version of the figure to a source we are prepared to cite. So we do not print one.
That is a rule of this publication rather than a hedge. The editorial standards set it out in full: no percentage, protection figure or statistic that cannot be attributed to a named public source. The mechanism is real, well understood and describable without a number attached. The number attached to it in most retellings is the part we cannot stand behind.
The practical consequence for a reader is the same either way. The protection delivered in ordinary use is lower than the printed figure, sometimes considerably, and the shortfall grows as the quantity applied falls.
Unevenness, which is separate from quantity
There is a second effect that compounds the first, and it is not about how much is applied but about how evenly.
A film applied by hand is not uniform. It is thicker in some places and thinner in others, and it thins fastest over the raised parts of a surface. Because transmission through the thin patches is disproportionately high, the protection of an uneven film is not the average of its parts. It is pulled down towards the performance of the thinnest areas.
This is why laboratory application is done by a trained operator over a defined area, and why the standard is specific about the method of spreading. It is also why the parts of the body that people miss, and the parts where the film is thinnest, matter more than the overall quantity used.
| Condition | In the test | In ordinary use |
|---|---|---|
| Quantity | Two milligrams per square centimetre, weighed | Not measured, generally less |
| Spreading | Trained operator, specified technique | By hand, uneven, some areas missed |
| Surface | Defined flat test area | Curved, hairy, mobile, variable |
| Settling time | Allowed for by the method | Often none |
| Removal | None | Water, towelling, clothing, sand, sweat, touch |
| Applications | One | Depends entirely on behaviour |
Framework of this publication. It describes how published rules and guidance fit together and is not a measurement, a survey or a study.
Reapplication is about the film, not the clock
Guidance from national health bodies consistently recommends reapplying sun protection products, including after swimming, towel drying and sweating, and applying generously.
Read through the mechanism, that guidance is not arbitrary and it is not about the product wearing out chemically. It is about the film. Water, towelling, clothing, sand and touch remove product from the surface, and a thinner film transmits disproportionately more. Reapplication is a way of restoring the thickness the number assumed. It is also the reason a claim such as all day protection is treated as one that should not be made: it invites the reader to stop restoring the film.
For what to do in practice, the source to read is the NHS guidance on sunscreen and sun safety, and the sun safety information published by Cancer Research UK and the British Association of Dermatologists. This publication describes what the label means and does not issue protection advice.
The areas where the film is thinnest
Because an uneven film performs worse than its average thickness suggests, the places where the film is habitually thinnest matter disproportionately. Those are not secrets: they are the edges and the awkward geometry. Hairlines, ears, the sides and back of the neck, the tops of feet, the backs of hands, the area around the eyes and any parting in the hair are all places where application is either skipped or applied thinly because the surface is difficult.
The reason to raise this in a labelling publication rather than leave it to health guidance is that it explains something about the printed figure. The figure was determined on a prepared, flat test site chosen precisely because it does not have any of those difficulties. Nothing about the number is wrong, and nothing about it accounts for geometry. A person who applies the test quantity in total but distributes it across a body will not achieve the tested film thickness everywhere, and the places where they fall short are predictable.
What to do about that is a matter for the NHS, Cancer Research UK and the British Association of Dermatologists, all of whom publish plain guidance on application and reapplication. This publication stops at explaining why the printed figure and the delivered protection are different quantities.
What this means for labelling
The test quantity is the most significant condition attached to the most prominent number on the pack, and it is not printed on the pack. There is no requirement that it should be, and it is hard to see how it could be expressed usefully in the space available.
What labelling and advertising can be held to is the surrounding presentation. A claim that implies the printed figure describes ordinary use, or that a product performs at its labelled factor regardless of how much is applied, is a claim about performance that the test does not support. Under the common criteria for cosmetic claims, a claim must be supported by the evidence held, and the evidence held is a determination at a fixed quantity.
What to take from this
The number assumes two milligrams per square centimetre, evenly spread, undisturbed. Ordinary use meets none of those conditions perfectly, the loss from under-application is steeper than proportional, and the correct response to that is not a figure but an understanding of the mechanism.