Three different physical processes remove sun protection from skin, and the sunscreen label addresses one of them properly, one of them loosely, and one of them not at all. Understanding which is which is the most practically useful piece of regulatory literacy available to anyone who spends time outdoors.
Water: the one with a procedure
Immersion removes a film by dissolution and by flow across the surface. It is the mechanism that has been standardised, because it is the easiest to control and reproduce. ISO 16217 defines the water immersion procedure and ISO 18861 defines how the percentage of water resistance is expressed. A claim of water resistant or very water resistant on a UK or EU pack points at that work.
Everything in our note on what water resistant certifies applies. It is a real test, it produces comparable results, and its boundaries are defined periods of still water.
Sweat: the one that behaves differently
Sweat is not water arriving from outside. It is produced continuously from beneath the film, which means it lifts and floats the film rather than washing it off. The result is migration: protection moves along the drainage lines of the face, pools at the eyebrows and jaw, and leaves channels of bare skin between ridges of remaining product. To the eye it looks intact.
It is also chemically different from tap water and physically different from immersion, being continuous, warm and produced under the film. There is no single public procedure in the UK and EU that a sweat resistance claim must pass, comparable to the immersion standards. That does not make the claim meaningless: under Commission Regulation (EU) No 655/2013, retained in UK law, cosmetic claims must be supported by adequate and verifiable evidence. But the evidence is held by the manufacturer rather than expressed through a standard you can look up, so two packs carrying the same phrase may rest on very different work.
This matters most in cycling and running, where sweat is the dominant mechanism. See our conditions files on road cycling and running.
Rub: the one with nothing behind it
Abrasion is mechanical removal by contact: a board against a chest, a rucksack strap across a shoulder, a wetsuit seal at the throat, a towel, a sleeve, a hand wiping a forehead, sand, a seat, a harness.
There is no standardised rub resistance claim in UK and EU sunscreen labelling and no immersion procedure that includes rubbing. Nothing on the front of any pack tells you how a formulation behaves under abrasion.
This is the single largest gap between what is certified and what happens outdoors, because abrasion is the dominant mechanism in a long list of activities:
- Surfing, where the board removes the chest film in minutes.
- Swimming, where the stroke abrades the shoulders and the neck seal abrades the throat.
- Hill walking, where pack straps work two bands across the shoulders all day.
- Cycling and running, where wiping removes the forehead film repeatedly.
- Outdoor work, where gloves, harnesses, collars and tools do it continuously.
- Any use of a towel, ever.
What a combined claim is worth
Sweat and water resistant
What it certifies
The water resistance half points at a published immersion procedure and means something specific. Where the sweat resistance half is claimed, the manufacturer is required under the common criteria for cosmetic claims, retained in UK law, to hold adequate evidence supporting it, so it is not an empty phrase.
What it does not
It does not mean the two halves were tested the same way, or that either was tested in a way you can look up. There is no public single procedure for sweat resistance in the UK and EU equivalent to the immersion standards, so two products carrying the phrase may rest on entirely different evidence.
What to do about the gap
Assume abrasion is uncovered
Not badly covered. Uncovered. If your activity involves rubbing, no claim on the pack speaks to it and you should plan on the basis that the film in contact areas is gone.
Move contact areas onto textile
This is the recurring conclusion of this publication and it is not a rhetorical flourish. Anywhere a strap, a board, a seal or a bag touches you, fabric is the only reliable protection, because it does not abrade away. Our note on UPF clothing covers how fabric protection is rated.
Judge sweat resistance empirically
You can observe the failure mode directly: does it run into your eyes on a hard effort? That is more informative than the phrase on the pack, because it is your sweat rate, your skin and your formulation.
Treat every towel dry as a reset
Because it is one.
Why is there no rub standard?
Fairly, because it is hard. Immersion can be controlled: temperature, movement, duration. Abrasion has no natural parameters. What pressure? What material? What speed? A standard would have to pick values, and any values picked would resemble some activities and not others. The result would risk being less informative than saying nothing.
That is a reasonable position for a standards body to hold. It is not a reasonable position for a person on a surfboard to be unaware of, which is why we set it out.
Testing a product on yourself, properly
Since the abrasion and sweat questions are not answered on the pack, the only remaining source of information is your own experience. It is worth gathering it deliberately rather than accidentally, and the method is not complicated.
Test on a training day, not on the event. The first time you find out that a product runs into your eyes should not be at kilometre thirty of a marathon or halfway through a race day on the water.
Test the failure mode that matters for your activity. For runners and cyclists, that is migration under sweat: does it reach your eyes, and how long does it take. For surfers and swimmers, it is whether anything remains on the chest and shoulders after an hour. For walkers, it is whether the strap bands are still covered at the summit.
Notice the practical properties, not the marketing ones. Does the container open with one wet or cold hand. Does it apply over damp skin. Does it fit in the pocket you will actually carry it in. A product that fails any of those will be used less often, and frequency is the dominant variable.
None of this produces a number and none of it is transferable to anyone else. It is, however, the only evidence available about the two mechanisms the labelling does not describe, and it costs nothing to collect.
What is not verifiable
We cannot tell you which formulations resist abrasion best, because there is no published method producing comparable results and we run no tests. Anyone who tells you otherwise is describing an impression. We are also not going to characterise a specific product's sweat resistance evidence, because we have not seen it and neither has anyone outside the company.
The one line version
Water resistance is tested and comparable. Sweat resistance is substantiated but not standardised. Rub resistance is neither, and it is the mechanism most likely to be removing your protection right now.