Pool halls are often heavily glazed, because daylight makes a large wet space feel less like a basement. That produces a reasonable question from swimmers and from people running sessions: if I can see the sky, am I in the sun.
The answer is mostly no, and the reason is worth understanding because it also tells you where the answer changes.
What glass does to ultraviolet
Ultraviolet is not one thing. The shorter wavelength part is the part principally responsible for sunburn, and it is strongly absorbed by ordinary glass. This is why you do not get sunburnt sitting behind a closed car window or a domestic window, and it is a well established piece of physics rather than a matter of opinion. General sun protection guidance from the NHS and Cancer Research UK, sun, UV and cancer is framed around outdoor exposure precisely because that is where the exposure occurs.
The longer wavelength part of ultraviolet is transmitted more readily by ordinary glass. Public health guidance on ultraviolet exposure, including the ultraviolet index framework described by the World Health Organization, is concerned with total solar ultraviolet exposure outdoors and is not written as guidance for behind glass.
We are not going to publish a transmission figure for any glazing type, tell you what proportion of anything reaches a swimmer, or make a claim about long term skin effects behind glass. Those are questions for dermatology and for the institutional sources above. What we can usefully do is tell you what to check about a building, which is different from telling you what the physics does to your skin.
The question that actually matters is whether it opens
In practice, the variable that changes the answer at a UK pool is not glazing specification. It is whether part of the building opens.
Retractable roofs. Many leisure pools, hotel pools and some competition venues have roofs or roof sections that open in warm weather. When the roof is open, the pool is outdoors. Everything in the outdoors section applies: exposure, reflection off the water, timing, shade. The complication is that nothing else about the facility changes. Same building, same timetable, same habits, same lack of shade, and often no announcement.
Opening walls and doors to a terrace. Same effect over a smaller area, and typically affecting the people sitting and waiting rather than the people swimming.
Attached outdoor areas. An outdoor terrace, a splash area, a sun deck or a queueing area outside the entrance. These are outdoor environments attached to an indoor facility, and on a long day at a leisure centre a child may spend more time in them than in the water.
Does a glazed roof change my answer?
A closed glazed roof leaves you indoors for practical purposes. Ordinary glass absorbs the wavelengths that cause sunburn, and public health guidance is written about outdoor exposure.
The things worth attending to in a glazed hall are the same as any indoor hall: air quality, the pre swim shower, and getting warm and dry afterwards.
An open roof puts you outdoors. So does a terrace, a splash area or a queue outside the door. Those segments need to be planned for and protected as outdoor exposure.
If you are running a session, this is a question to ask before the day rather than to discover at eleven in the morning when the roof starts moving.
Questions worth asking an operator
These are answerable questions that a duty manager can respond to without needing a technical department:
- Does any part of the roof or the walls open, and on what basis is that decided?
- If it does, is there any notice given to session leaders and hirers?
- Is there an outdoor area attached to the facility that groups will use, and is there any shade on it?
- Is there shade at the entrance where people queue?
Notice that all four are about the building rather than about glass. That is deliberate. Glazing specification is not something a duty manager will know, is not something you can verify, and is not where the practical risk sits.
Because you will not get a useful answer and you would not be able to act on it if you did. A building's glazing specification lives in construction documents, may vary between the roof, the walls and later replacements, and does not translate into a swimmer instruction. Whether the roof opens translates directly into an instruction, which is why it is the better question.
If you run sessions in a glazed venue
The practical failure mode is a summer session where the roof opens partway through, the group is in and out of the water for ninety minutes, nobody has any sun protection because it was an indoor session, and there is no shade on the poolside because indoor poolsides do not have any.
Preventing that is a planning task, not a poolside task. It belongs in the session risk assessment described in what a session risk assessment should cover, and it comes down to knowing in advance whether the roof will be open and treating that session as an outdoor one if it will.
Whether this session is actually indoors
Can look up. Can ask at reception. For an individual adult this is a two second decision and rarely matters much.
Owns it properly, and owns finding out in advance rather than at the poolside. A session leader who books a hall with a retractable roof for July needs to know the operator's practice on opening it before the group arrives. Owns treating an open roof session as an outdoor session, including shade, timing and sun protection.
Owns the building, owns the decision to open the roof, and owns telling hirers and session leaders that it may happen. Owns whether there is any shade on an outdoor terrace or at an entrance queue, which in most facilities there is not.
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, with the ultraviolet index framework described by the WHO. The duty to assess the risks of an organised session sits under the Management of Health and Safety at Work Regulations 1999.
Spectator galleries and viewing areas
One group at a glazed pool spends longer in the building than anybody in the water, and it is not the swimmers. It is the parents. A viewing gallery at a children's lesson session holds adults for the full hour, often several times a week, frequently in a heavily glazed space chosen for the view.
For ultraviolet exposure this is not a concern, for the same reason the hall is not: the glazing is between them and the sky. What a glazed gallery does produce is heat, glare and, in summer, a genuinely uncomfortable environment, all of which are comfort and design questions rather than exposure ones. Where a gallery is uncomfortable enough that people stop using it, that is worth telling the operator, because it is an operational fact they can act on.
The exception is the same as everywhere else in this article. Where a viewing area opens onto a terrace, or where spectators wait outside because the gallery is full, those minutes are outdoor minutes and belong to the outdoor plan.
The general principle worth carrying away
Do not try to assess a building. Ask what opens. Buildings block the relevant wavelengths and openings do not, so the useful question is always about openings: roofs, walls, doors, terraces, queues and the walk in from the car park. Every one of those is something a duty manager can answer in a sentence, and every one of them translates directly into whether a segment of the day needs planning as outdoor time.
Summary
A glazed hall is an indoor hall. A hall with the roof open is an outdoor pool with an indoor timetable, which is the more dangerous combination because nothing about the day was planned for it. Ask about opening rather than about glass, and plan the outdoor segments of an indoor day as outdoor segments.