Open water swimming is the clearest case in the whole category of a gap between what a claim certifies and what a person is doing. The claim on the pack was substantiated in a tank. You are in a loch, a lake, an estuary or the sea, moving, for a period that bears no relationship to the test, exiting and re-entering, drying yourself on a towel, and doing it in the middle of the day because that is when the water is warmest and the safety cover is out.
None of that makes the claim untrue. It makes it narrow. This file sets out what the conditions actually impose, what gives way first, and what a protection strategy looks like when you accept that the film on your skin is a consumable rather than a coating.
The conditions, set out plainly
Open water swimming, UK summer
Conditions block- UV load
- Moderate to high on a clear summer day in the UK, with reflected UV added from the surface. The Met Office publishes a daily UV index forecast, and it is the number to check before a long swim.
- Water exposure
- Continuous immersion, broken by exits. Salt or fresh, cold, moving.
- Sweat
- Low while immersed, then significant during any walk in or out and on the bank afterwards.
- Friction
- High. Wetsuit neck seal, arm strokes across the shoulder, towel drying, swim cap edge.
- Typical duration
- Thirty minutes to several hours, often in the middle of the day.
- What fails first
- The film on the shoulders, the back of the neck and the backs of the hands, in that order.
- Protection strategy
- Apply the full quantity well before you get changed, cover what you can with textile, accept that reapplication after every exit is the only thing that works, and treat the swim cap line and the neck seal as the two places to check every single time.
Two features of that block do most of the work. The first is friction. The second is duration. Both are absent from the way sun protection is usually discussed, and both dominate what actually happens on a swim.
What fails first, and why
People assume water washes sunscreen off. Water is part of it, but mechanical removal is the larger effect and it happens in three places on a swimmer.
The shoulders. Every stroke drags the upper arm across the deltoid and the trapezius. In a wetsuit, the seal and the shoulder panel move over the same band of skin thousands of times in an hour. That is abrasion, and abrasion removes a film far more efficiently than immersion does. The shoulders are also the part of a swimmer most consistently presented to the sky.
The back of the neck. A swim cap ends somewhere, and wherever it ends there is a line. Above it the cap protects completely. Below it the skin is exposed at close to the worst possible angle, and it is rubbed by the cap edge and by the wetsuit collar at the same time.
The backs of the hands. Hands break the surface on every stroke, so they receive both direct and surface reflected ultraviolet. They are also the part of the body most often missed at the application stage and most thoroughly wiped during towelling. Cancer Research UK's public guidance on sun, UV and cancer is worth reading alongside this, because the cumulative exposure of the hands and the neck over a swimming career is not trivial.
Towelling deserves its own sentence. Drying yourself briskly with a towel removes more of the film than the swim did. It is the single most underrated event in the whole sequence, and it happens at exactly the moment people feel the session is over and stop thinking about protection, while standing on an exposed bank in the middle of the day.
The reapplication clock
Reapplication clock: A long open water session
What has usually happened to the film on your shoulders by each point. This is a description of mechanism, not a measured decay curve, and no laboratory figure is implied.
The protection strategy that survives contact with a swim
The strategy has four parts, and the order matters.
1. Apply the full quantity, early, dry
The quantity used in the laboratory to establish a sun protection factor is specified in the test method, and it is considerably more than most people apply. The practical consequence is that a product applied thinly does not deliver the labelled factor, and the shortfall is not proportional in a friendly direction. Apply to dry skin, before you put on a wetsuit or a costume, and give it time to settle rather than getting straight into the water. Our note on the tested quantity covers why this single variable outranks almost everything else on the pack.
2. Move as much as possible onto textile
A long sleeved swim top, a swim cap that covers the whole scalp, and a rash vest for the walk down and the recovery afterwards are all doing something a film cannot do, which is stay put. Textile ultraviolet protection is a rated property with its own standards, and it does not degrade over an afternoon. Our file on textile UPF as a real alternative explains what the ratings mean and what a wet garment does differently from a dry one.
3. Treat every exit as a reapplication event
Not every two hours. Every exit. If you get out, dry off and stand about, you have just removed a large and unknown fraction of the film. Reapplying then is cheap, quick and the only way to return to a state you understand.
4. Protect the swim cap line and the neck seal by habit
These two bands are the ones that burn, and they burn on people who consider themselves careful. Make them the first thing you do rather than the last, and check them on every reapplication.
Reflected ultraviolet on the water
A swimmer is exposed from above and from the surface at once. The World Health Organization's material on the ultraviolet index describes reflection from water and, in stronger terms, from sea foam, which is why a choppy, breaking surface is a more demanding environment than flat water. The effect is not that reflection doubles anything. It is that shaded parts of you, the underside of the chin, the inside of the upper arm, the nostrils, receive ultraviolet they would not receive on land. Our file on reflected ultraviolet from water, snow and sand covers this properly.
What the label will and will not tell you
Water resistant
What it certifies
That the product was assessed using a published immersion procedure, in which protection is measured, the subject is immersed for defined periods in still water, and protection is measured again. ISO 16217 defines the immersion procedure and ISO 18861 defines how the percentage of water resistance is expressed. A claim of this kind is a statement that a defined procedure was followed and a defined retention threshold was met.
What it does not
It does not certify performance in moving water, in salt water, in cold water, under a wetsuit, or for the duration of your swim. It does not survive towelling, which is mechanical removal rather than dissolution. It does not mean the film is intact after the tested periods have elapsed, and it is not a permission to skip reapplication.
The word missing from all of this is "waterproof", and its absence is deliberate. In the UK and the EU it is not a permitted claim, for reasons set out in our standards note on why waterproof is not a permitted claim. If you see it on a pack, you are almost certainly holding a product bought outside the UK and EU, or a product whose marketing has drifted ahead of what the law allows.
What is not verifiable from where you are standing
We publish no measurements, and there are things about a swim that nobody can measure from the bank. You cannot know how much of the film survived a particular hour. You cannot know how evenly it was applied across your own back. You cannot infer, from a water resistance claim, how a specific formulation behaves in cold salt water under a neoprene seal, because that is not what the procedure tests. Anyone who gives you a percentage for your swim is guessing.
What you can do is control the two variables that are genuinely in your hands: quantity applied, and frequency of reapplication. Everything else is weather.
After the swim
Rinse, dry, get covered, and pay attention to how the skin looks over the following days. The NHS sets out what to do about sunscreen and sun safety, and it is the right first stop for anything that looks wrong. We do not give diagnostic guidance here and we never will. If a mole or a patch of skin is changing, that is a conversation with a GP, not with a website. Our note on checking your skin after a season outdoors explains how to make that a habit rather than an event.