This is one of the few questions on this site with a short answer, and the short answer is no. It is worth spending an article on anyway, because the reasoning matters more than the conclusion, and because the exceptions are the parts people get wrong.
Both sides of the ledger, indoors
Every decision about applying a product before swimming has two sides. What it does for you, and what it does to the water. Outdoors both sides are real and the balance favours protection. Indoors one side is empty.
| Indoors | Outdoors | |
|---|---|---|
| What it protects you from | No meaningful ultraviolet exposure to protect against | Real and compounding exposure: dwell time, minimal clothing, reflection |
| What it adds to the water | Organic material, disinfectant demand, filtration load | The same, and worth paying |
| Net | Cost with no benefit | Benefit clearly outweighing the cost |
| Correct action | Do not apply. Shower off any applied earlier. | Apply properly, well before entry, and reapply after getting out |
People sometimes resist this because sun protection has been framed, correctly, as an unambiguously good habit. Habits are useful precisely because they do not require thought. But a habit applied where it has no effect is not caution, it is just a habit, and in this case it has a small cost attached to it for everyone else in the pool.
Arriving with it already on
This is the common real world case. A family spends the morning outside, correctly applies sun protection, and then goes to an indoor pool in the afternoon. Nobody did anything wrong. But the product is now on the skin of several people who are about to get into treated water.
The answer is a proper pre swim shower, with soap, of the kind described in why pools ask you to shower first. This is exactly the scenario the pre swim shower exists for, and it is one of the few times when the difference between a token rinse and a real shower is obvious and large.
Showering off sunscreen before an indoor swim does not "waste" it and does not leave you unprotected, because you are going indoors. If you are going back outside afterwards, reapply then, following the product instructions and general guidance from the NHS. Protection is for the outdoor segment, not for the day as an undifferentiated block.
The edge cases, in order of how often they occur
1. Retractable roofs. A pool with a roof that opens is outdoors when it is open. Some leisure centres and hotel pools open a roof on warm days without announcing it as a change of environment. If you are running a session, ask. If you are swimming and the roof is open above you at midday in summer, you are outdoors.
2. Outdoor segments attached to an indoor session. Waiting outside for doors to open. A gala with an outdoor warm up area. A school swimming trip where the coach parks ten minutes away. A long competition day with breaks on an outdoor terrace. These add up, and for children on a full day they can be the dominant exposure of the day while everybody is thinking about the pool.
3. Extensively glazed halls. Discussed properly in glazed roofs and rooflights. The practical answer for almost every hall is that ordinary glazing blocks what matters, and the question worth asking is whether the roof opens rather than whether the glass transmits.
4. Outdoor training on the same day. A squad that does an open water session in the morning and a pool session in the evening has one outdoor session and one indoor one, and they need different preparation.
I am at a pool. Do I apply anything?
No. Nothing to protect against. Shower properly if you have product on from earlier, and get on with the swim. The things worth attending to indoors are the air, the water, goggles and getting dry afterwards.
Yes. Apply generously and well before you get in rather than at the water's edge, and reapply after you get out and towel off, following the instructions and general NHS and Cancer Research UK, sun, UV and cancer guidance. Then use shade and timing so the product is not carrying the whole load. See timing an outdoor session.
If you are responsible for other people's children
A swim teacher, a school staff member or a club volunteer is answering this question on behalf of children who cannot answer it. The useful thing is to answer it once, before the session, as part of the plan, rather than in the changing room with fifteen children waiting.
The questions that resolve it: is any part of this session outdoors, will the roof be open, is there any outdoor waiting time, and is there any outdoor break. If all four answers are no, sun protection is not part of this session and the pre swim shower is. If any answer is yes, that segment needs handling and it needs handling before anyone changes into swimwear.
If a child or an adult uses a prescribed topical treatment, or has a skin condition, the question of what goes on before swimming is a clinical one and belongs to whoever prescribed it, or to a pharmacist. We do not give advice about applying anything to affected skin, and a session leader should not be improvising it either. The right route is a written note from the family about what the child uses and when.
Whether sunscreen is applied before this indoor session
Straightforward for an adult swimming alone: do not apply, shower off anything already on. Owns knowing whether any part of the day is actually outdoors.
Owns it for the group, and owns finding out about roofs, outdoor waiting and outdoor breaks in advance. Owns not improvising with anybody's prescribed treatment, and owns making the pre swim shower part of the routine so that product arriving on children is dealt with.
Owns telling people when the roof will be open, providing shade at outdoor waiting areas and entrances where that is possible, and providing showers that are warm and usable enough that people actually use them.
Where a duty attachesGeneral sun protection guidance from the NHS, Cancer Research UK, sun, UV and cancer and NICE NG34, sunlight exposure: risks and benefits. Pre swim hygiene provision sits in Health and Safety Executive, HSG179 Health and safety in swimming pools and the code of practice from Pool Water Treatment Advisory Group.
Why this question keeps being asked
It is worth understanding why an apparently simple question generates so much uncertainty, because the reason is not ignorance. It is that sun protection has been promoted, correctly and effectively, as an unconditional good habit. Habits are useful precisely because they do not require a judgement each time, and the whole point of a public health message is that it should survive being applied without thought.
So people arrive at an indoor pool carrying a rule that was designed to be applied without thinking, and applying it without thinking is exactly what they do. That is not a failure of the individual. It is what a successful habit looks like when it meets an environment the habit was not written for.
The resolution is not to weaken the habit. It is to attach it to the right trigger. The trigger for sun protection is going outdoors, not going swimming. Once the trigger is being outdoors, the indoor pool question answers itself, the outdoor pool question answers itself, and the awkward cases such as a roof that opens or an outdoor gala answer themselves too, because in each of them the question is simply whether you are outdoors.
That is also why this publication puts an indoors or outdoors block on almost every page. It is not a design flourish. It is the recognition that the environment, rather than the activity, is what determines the answer, and that a reader who leaves with that distinction will get the awkward cases right without needing to have read the article about them.
Summary
Indoors, no. Shower off anything already applied. Outdoors, yes, properly and with timing and shade doing part of the work. The only real skill here is noticing which one you are actually in, which sounds easy and is the thing most often got wrong on a long day with children and a roof that opens.