A great deal of writing about swimming and sun protection quietly assumes an outdoor pool. Most swimming in the United Kingdom does not happen at one. It happens in an enclosed, heated, artificially lit hall, and in that environment the sun question does not have a smaller answer. It has a different answer, and the difference is worth stating plainly because it changes what a swimmer should actually do.
Why a building removes the problem
Sunburn and the longer term skin effects that public health guidance is concerned with are driven by ultraviolet radiation, and specifically by the shorter wavelength part of it. That part is strongly absorbed by ordinary window glass and by building materials generally. Sitting behind a normal window is not a meaningful sunburn exposure. Being inside a building with a roof is not one either.
This is why guidance on sun protection from the NHS, Cancer Research UK, sun, UV and cancer and NICE is framed around being outdoors, around the time of day, around shade, and around the ultraviolet index. It is guidance about outdoor exposure. It is not written to be carried indoors, and carrying it indoors produces behaviour with no benefit attached to it.
Indoors, sun protection is not less important. It is doing nothing, which is a different statement.
What replaces the sun question indoors
Indoors, the environmental problem is not radiation. It is air and water. Specifically:
- Air quality in the hall. The volatile byproducts of disinfection do not disperse in an enclosed space. They sit above the water and are breathed by everyone. This is the dominant indoor environmental issue and it is covered in indoor pool air and ventilation.
- Water contact. Longer sessions, warmer water, repeated immersion, and everything that comes with that for skin and hair. Covered in skin after chlorine.
- Shared facilities. Changing rooms, floors, benches. Covered in changing room hygiene.
- Heat and humidity. Which affect staff on poolside far more than swimmers in the water.
None of these are helped in any way by sun protection. That is the whole point of separating the environments.
Does sun protection do anything useful for me here?
No. There is no meaningful ultraviolet exposure in an enclosed pool hall, so a sunscreen applied before an indoor swim protects you against an exposure that is not occurring.
Meanwhile it adds organic material to the water, which consumes disinfectant and contributes to the compounds that make halls smell. So the cost is small but real and the benefit is nil. If you arrived with sunscreen on from earlier, shower it off.
Yes, and it matters. Outdoors you have long dwell time, minimal clothing, reflection off the water surface, and frequently the middle of the day.
The load it adds to the water is a cost worth paying. See outdoor pools and lidos and timing an outdoor session.
Where indoors is not quite indoors
There are real exceptions and it would be sloppy to pretend otherwise. Three matter.
Extensively glazed halls and rooflights. A hall with a large glazed roof or full height glazing is still a building, and ordinary glass still blocks the relevant wavelengths. But glazing specification varies, some architectural glazing is designed to transmit more daylight than a domestic window, and the practical question becomes what the glazing actually is. This is covered separately in glazed roofs and rooflights.
Retractable roofs and openable walls. A pool with a roof that opens is an outdoor pool when the roof is open. That sounds trivially obvious and it is routinely missed, because the facility, the timetable and the habits are all indoor ones. A session leader running a squad under an open roof at midday in July is running an outdoor session.
Getting to and from the building. The walk across a car park, the wait outside for a lift, the twenty minutes in the sun before a lesson, the time on a terrace between sessions. These are outdoor exposures attached to an indoor swim, and for children on a long swimming day they can add up to more than the swim.
The exceptions do not mean you should apply sunscreen for an indoor swim just in case. They mean you should be clear about which parts of the day are indoor and which are outdoor, and treat them differently. Sun protection belongs to the outdoor parts. An open air gala warm up, an outdoor lunch break at a competition, or a wait outside a leisure centre are outdoor parts.
The year round swimmer and the vitamin D question
People who swim indoors regularly and seriously, particularly competitive swimmers training early mornings and evenings, can spend a very large amount of their week inside. That is a general question about sunlight exposure rather than a swimming question, and it is one that NICE NG34, sunlight exposure: risks and benefits addresses directly: there is a balance between the risks of ultraviolet exposure and the benefits of sunlight for vitamin D, and the guidance discusses both sides.
Vitamin D status, supplementation and what any individual should do about it are clinical matters. We will not give a recommendation, a dose or a threshold. The relevant institutional sources are NICE NG34, sunlight exposure: risks and benefits and the NHS, and the person to ask about your own situation is a GP or a pharmacist. It is a legitimate thing to raise if you are indoors most of the time, and it is not something a swimming publication should be answering.
What this means if you are responsible for children
Indoor lessons are the most common way children in the United Kingdom swim. For an indoor lesson, there is no sun protection task. What there is instead is a set of quite different tasks: a proper pre swim shower, a toilet trip, goggles that fit, a plan for getting warm and dry afterwards, and awareness that a hall which stings everybody's eyes is worth mentioning.
Where the sun protection task reappears is on the way in and out, at outdoor galas, at open air sessions, and on any day where the timetable puts a child outside in the middle of the day. Being clear about that distinction means the protection happens when it matters instead of being applied uniformly and then forgotten on the one afternoon it counted.
Whether sun protection is needed for this session
Owns the decision for their own skin and can generally answer it by looking at the roof. Indoors, no. Outdoors or under an open roof, yes. Owns noticing the outdoor parts of an indoor day.
Owns it for the group, and this is the harder version. A leader has to answer it for children who cannot, has to know whether the session is indoors or outdoors including whether a roof will be opened, and has to have covered it before the group arrives rather than at the poolside. Owns the outdoor segments of the day: warm ups, waiting, breaks, travel.
Owns whether the environment is what it appears to be: whether the roof opens, what the glazing does, whether there is shade outside the entrance, and whether an outdoor area attached to an indoor pool is usable in the middle of the day. Owns telling session leaders when a roof is going to be open.
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. For the session itself, the risk assessment duty under the Management of Health and Safety at Work Regulations 1999.
The short version
Inside a normal enclosed pool hall, sun protection is not doing anything, and there is no trade off to weigh because there is nothing on the benefit side. Do the indoor things instead: shower properly, wear goggles, dry off, and pay attention to the air. Then, when the day includes an outdoor segment, treat that segment as outdoors and protect it properly. The mistake to avoid is not applying sunscreen indoors. It is treating an indoor day and an outdoor day as though they were the same day.