Sun protection is often discussed as though the film were inert: you put it on, it sits there, it either stays or comes off. In fact a working sunscreen film is a busy place. It is absorbing energy continuously, it is being heated, it is being lifted by sweat from below, it is being redistributed by clothing and hands, and it is being physically removed by everything you touch.
This note separates the four processes, because they are frequently confused and only one of them is addressed by anything on the label.
The four processes
1. Physical removal
Water, sweat, towels, clothing, straps, hands, sand. This is the dominant process for most people on most days, and it is the one our conditions files are largely about. It is also the only one partly addressed by a claim, and only for immersion. See water, sweat and rub resistance.
2. Migration and unevenness
Sweat is produced under the film and floats it along the drainage lines of the body. Clothing drags it. Hands move it. The result is that evenness degrades before total quantity does, and evenness is what protection depends on: a film with channels in it has no protection in the channels.
3. Heat
Skin in strong sun gets hot, and a film on hot skin behaves differently from a film on cool skin: it is less viscous, it moves more easily, and it interacts more readily with sweat.
4. Photochemistry
This is the one people mean when they say sunscreen stops working. Ultraviolet filters work by absorbing ultraviolet energy. Some filter chemistries handle that absorption while returning to their original state very reliably. Others degrade to some degree under prolonged exposure. This is a well studied area of formulation science, and modern products are commonly built as filter systems in which combinations are chosen partly for how they stabilise one another.
Reapplication clock: A filter system across a long day
The four processes below run in parallel rather than in sequence. This rail describes mechanism only and implies no measured decay for any formulation.
Which process dominates
For almost everyone, physical loss dominates. A film that has been swum in, towelled, sweated through and rubbed by a rucksack strap has been reduced by mechanical processes long before photochemistry becomes the limiting factor.
That matters because it changes the advice. If you believed photochemistry was the main issue, you would shop for a specific filter system. Since physical loss dominates, you reapply. Reapplication addresses all four processes at once, because it replaces the film entirely: fresh chemistry, even coverage, full quantity, cool product. That is why our note on reapplication treats it as the dominant variable rather than as a maintenance task.
Mineral and organic filters, briefly and neutrally
Filters are commonly divided into mineral, or inorganic, and organic. The two families work differently, they are permitted under the same UK and EU framework within their own limits, and they have different properties in terms of feel, appearance on the skin, and behaviour under prolonged exposure.
We are not going to declare one family superior, because that argument is usually conducted with more confidence than evidence and because we test nothing. What is useful to know is that both are regulated, both appear in the permitted filter annexes of the cosmetics framework, and the practical differences most people notice, how it looks and how it feels, are the ones that determine whether they apply enough of it. Which brings you back to quantity. See how much to apply.
What the pack says about time
Photostable
What it certifies
That the manufacturer holds evidence relating to the stability of the filter system under ultraviolet exposure. Photostability is a genuine and studied property of ultraviolet filters, and formulators work hard on it, including by combining filters that stabilise one another.
What it does not
There is no standardised photostability claim on a UK or EU pack equivalent to the water resistance procedure, and no consumer visible rating. You cannot compare two products on it from the shelf, and the word tells you nothing about how the film behaves once it has been sweated on, rubbed and partially removed.
Notice what is not on any UK pack: a wear time, an expiry within a day, an indicator, a colour change, anything at all that tells you the film has stopped working. That absence is the reason this publication exists. The pack cannot tell you, so you have to use a rule instead, and the rule is reapplication.
Storage and the bottle itself
Separate from what happens on your skin, there is what happens in the bottle. Sunscreen has a shelf life, indicated by an expiry date or a period after opening symbol, and it is a formulated product that can separate or change if it spends a summer in a hot car. The practical test is whether it looks and smells as it did when you bought it. If it has separated and will not recombine, replace it.
Bringing last year's half bottle on holiday is common and usually fine within its dates. Bringing the one that has lived in a boot for two summers is a false economy.
Layering products, and what it actually does
A common question is whether applying one sun protection product over another, or over a moisturiser or a foundation carrying a factor, adds the two figures together. It does not, and the assumption that it might is worth dismantling.
Sun protection factors are not additive. Applying a product with one factor over a product with another does not produce the sum, and the resulting protection is generally governed by whichever film is doing the work at that point on the skin, along with how evenly each one was applied. Layering also introduces a practical problem: rubbing a second product over a first can disturb and redistribute the first, particularly if the first has not settled.
The reliable version is simpler. Apply one product that you are willing to use in quantity, apply it generously, let it settle, and reapply the same product later. If you are wearing a cosmetic carrying a factor over it, treat that as a small bonus rather than as part of the calculation.
What is not verifiable
We do not publish photostability data, we do not compare filter systems, and we do not tell you how long any product lasts, because none of that is available to us and much of it is not available to anyone outside a laboratory. Where a number would normally go, we have described the mechanism instead. That is a deliberate policy set out in our editorial standards.
What to take away
- Four processes act on a film. Only immersion is addressed by a claim.
- Physical loss dominates for almost everyone, almost always.
- Evenness fails before quantity does, and you cannot see either.
- Reapplication resets all four at once, which is why it beats product selection.
- Nothing on any pack will ever tell you the film has gone.