A ratio, not a rating
The sun protection factor is defined as a ratio. Take an area of skin, protect part of it with a measured quantity of product, expose all of it to a controlled ultraviolet source, and find the dose that produces a just perceptible reddening on each part. Divide the dose required on the protected area by the dose required on the unprotected area. That quotient is the sun protection factor.
Everything else follows from that sentence, including several things the number is regularly assumed to mean and does not.
The quantity is fixed by the test method, not chosen by the tester. The light source is a solar simulator with a specified spectral output, not sunlight. The endpoint is a visible reddening of skin, assessed by a trained observer, not a biological measurement of damage. And the panel is a group of human volunteers, which is why the figure carries a statistical uncertainty that is calculated, reported and then, on the pack, discarded in favour of a single tidy integer.
The effect it is a measure of
The endpoint of the test is erythema: the reddening that follows an ultraviolet dose large enough to cause it. That is a deliberate choice, because erythema is visible, reproducible and dose dependent, which makes it usable as a laboratory endpoint. It is also, importantly, a proxy.
Reddening is caused predominantly by the shorter wavelengths in the ultraviolet band reaching skin, which is why SPF is often described as a UVB measure. Longer wavelength ultraviolet contributes far less to reddening while still reaching the skin, so a product can hold a high SPF and still allow a substantial amount of that longer wavelength radiation through. That is precisely the problem the UVA circle and the star rating were introduced to address, and it is the reason a sun protection label carries more than one mark.
It is not a percentage
The most common misreading treats SPF as a percentage. It is not, although a proportion can be derived from it, and the derivation is worth doing carefully because it is the source of a great deal of confusion.
If a product doubles the dose required to produce reddening, its factor is 2, and under the test conditions one half of the erythemally effective radiation is reaching the skin. A factor of 30 means one thirtieth reaches the skin, a factor of 50 means one fiftieth. Those fractions are arithmetic consequences of the definition, not separate measurements, and they hold only under the conditions the test imposed.
Converting those fractions into percentages produces the familiar figures that circulate in sun protection writing. We treat them with care, because a percentage invites a comparison the definition does not support, and because the gap between one thirtieth and one fiftieth is smaller in effect than the gap between the numerals 30 and 50 suggests. That is set out in full in why SPF and protection are not proportional.
It is not a length of time
The second misreading turns the factor into a clock. If skin reddens in ten minutes unprotected, the reasoning runs, a factor of 30 buys three hundred minutes.
The arithmetic is not wrong about the ratio. What is wrong is the assumption that the inputs are stable. The dose reaching skin is not constant through a day: it varies with the sun's elevation, cloud, altitude, surface reflection and latitude. The film on the skin is not stable either, because it is removed by water, towelling, clothing, sweat and touch. And the starting figure, the time to redden unprotected, is not something a person knows about themselves with any precision.
A ratio multiplied by an unknown, applied to a changing quantity, over a film that is degrading, is not a schedule. This is why guidance from national bodies is expressed in terms of reapplication intervals and behaviour rather than in hours calculated from the factor, and why the phrase all day protection is treated as a claim that should not be made.
It is not a measure of performance in use
The test applies a specified quantity of product to a defined area, evenly, on a flat site, on a person sitting still, with no water, no towel and no interval. The number that results describes the product under those conditions.
People do not use products under those conditions. They apply less than the test quantity, unevenly, over uneven surfaces, and then swim, dry themselves and forget. None of that is a criticism of anybody. It is simply the reason a laboratory figure is a property of a product rather than a prediction about a day, and the reason the application quantity the number assumes is the single most important condition on the whole label.
| The question | Does SPF answer it | Where the answer sits |
|---|---|---|
| How much ultraviolet is stopped | Only as a derived fraction under test conditions | The definition, read as a ratio |
| How long can I stay outside | No | Not a labelling question at all |
| Am I protected against UVA | No | The UVA circle and the star rating |
| Will it perform like this on me | No | The application quantity the number assumes |
| Is one product higher than another | Yes, at the same quantity, in the same kind of test | The test method and its uncertainty |
Framework of this publication. It describes how published rules and guidance fit together and is not a measurement, a survey or a study.
Why the measure looks the way it does
The choice of reddening as the endpoint is a historical decision with consequences that are still being managed. When the measure was devised, reddening was the only effect of ultraviolet exposure that could be produced reliably, graded consistently and related to dose without instruments that did not yet exist. It was the practical option, and it became the international convention.
Everything that has been added to sun protection labelling since has been an attempt to compensate for that original narrowness. The ultraviolet A mark exists because the factor does not describe the longer wavelengths. The category words exist because the numeral is a reciprocal that people read as a linear scale. The cap at the top of the scale exists because the numeral kept rising while the effect stopped changing much. Read in that order, a sun protection label is a record of successive attempts to stop a single narrow measurement from being read as a broad promise.
That is the reason this publication treats labelling as a subject in its own right rather than as an afterthought to formulation. The measurement is not in dispute. What a reader takes from the way it is presented is where the whole difficulty sits.
Who fixes the definition
The definition is not a manufacturer's choice. The test method is an international standard, and the labelling conventions built on top of it come from regulatory guidance rather than from marketing departments.
In the United Kingdom a sun protection product is a cosmetic, governed by the retained cosmetics regulation and enforced under the Cosmetic Products Enforcement Regulations 2013. The way the factor is expressed on the pack, the protection categories, and the recommended relationship between the ultraviolet A protection and the labelled factor come from a European Commission recommendation on the efficacy of sunscreen products and the claims made relating to them. That recommendation is not itself a binding rule in the way a regulation is, which surprises people, and it is nevertheless the reference point that industry, enforcement bodies and testing houses work to. The distinction is set out in the European recommendation behind the label.
How to read the figure once you know what it is
Read it as a comparative property of a product measured under fixed conditions. It tells you that one product required a larger ultraviolet dose to redden protected skin than another did, at the same application quantity, in the same kind of test.
It does not tell you how long you can stay out. It does not tell you what proportion of ultraviolet reaches your skin in your circumstances. It does not, by itself, tell you anything about the longer ultraviolet wavelengths. And it does not describe the product as you will actually use it.
That is not a failure of the number. A measurement that is precise about a narrow question is more useful than a vague one about a broad question. The failure, where there is one, is in labelling and advertising that lets a precise narrow figure be read as a broad reassurance.
What to take from this
SPF is a laboratory ratio about reddening, at a fixed application quantity, under a solar simulator. Everything printed near it on a pack exists because that ratio, on its own, does not answer the question most people are asking when they pick the product up.