Every list of sun protection measures published by a health body puts timing near the top. Avoid the middle of the day. Seek shade when ultraviolet is strongest. It is the cheapest and most effective intervention available and it requires no product, no equipment and no compliance from anybody.
At an organised swimming session it is also the measure that individual swimmers cannot take, because the session is at the time the session is at. That makes it a timetabling decision, which is why this article is aimed at whoever sets timetables.
The ultraviolet index, and what it is for
The ultraviolet index is an internationally standardised scale describing the strength of solar ultraviolet at ground level. It was developed to give the public a single, comparable number to act on, and the framework is described by the World Health Organization. It is forecast daily and is available in ordinary weather forecasts.
The practical properties worth understanding:
- It varies through the day, peaking around the middle of the day, and the peak is sharper than most people expect.
- It varies with season and latitude, so a British summer figure and a Mediterranean one are not comparable.
- It is not a temperature measure and does not track how hot it feels. A cool, breezy, bright day can carry a meaningful index, which is exactly the day people at outdoor pools and open water get caught out.
- Cloud reduces it but does not remove it, as Cancer Research UK, sun, UV and cancer makes clear.
We are not going to tell you what index value to treat as the point at which to act, because that is public health guidance and it belongs to the bodies that publish it. Use the NHS, Cancer Research UK, sun, UV and cancer and NICE NG34, and read the index alongside the guidance those bodies attach to it rather than a threshold from a swimming website.
What moving a session actually changes
Everything, in a way that no other measure does. Moving an outdoor session out of the middle of the day reduces exposure for every person present, including staff, without depending on anybody applying anything correctly, without cost, and without a compliance problem. It is the only measure on the list with no failure mode.
Timing is the only sun protection measure that works even when nobody cooperates.
Against that, timing is constrained by everything else: pool availability, school timetables, staffing, daylight, water temperature, and the fact that people want to swim outdoors when it is warm. Those constraints are real. The point is not that a session can always be moved. It is that the decision to move it or not is somebody's, it should be conscious, and it should not default to the middle of the day simply because that is when the slot was free.
Why swimming makes timing harder
Several features of swimming push sessions towards exactly the wrong time.
- Water temperature. Outdoor water is warmest in the afternoon, and with children in particular that is a genuine welfare consideration pulling against the exposure one.
- Facility hours. Outdoor pools open when they open. Early slots are often reserved for lane swimming and squads.
- School timetables. A curriculum swimming session happens when the coach is booked and the pool is available, which is very often the middle of the day.
- Open water safety cover. Organised open water sessions run when safety cover is available, which for volunteer run groups is often weekend middays.
- Weather chasing. Sessions get moved to the hot day, which is the high index day.
None of those are irrational. All of them mean the timing decision has to be made deliberately against the other constraints rather than assumed away.
A workable approach for a session leader
- Check the forecast index the day before, not on the morning, so that decisions can actually be communicated.
- Decide in advance what you will change at what point: the start time, the length, the number and placement of breaks, whether the group sits in shade, whether there is a shirt and hat expectation out of the water.
- Communicate it before people arrive. Telling twenty families at the poolside that everybody needs a hat is too late.
- Schedule the reapplication stops rather than instructing people to reapply. With children an instruction is not a plan.
- Include the waiting and the travel. The exposure starts when they arrive, not when they get in.
- Include yourself and your staff. The people supervising are outdoors for the whole session and often for longer, and they are at work.
A club committee deciding a summer training timetable is making a sun exposure decision whether or not it thinks it is. Naming it as such in the minutes changes the conversation, because it turns an unexamined default into a decision somebody has taken. That is also what makes it appear in a risk assessment, which is where it belongs under the Management of Health and Safety at Work Regulations 1999.
Does timing matter for this session?
No, other than for the outdoor parts of the day: waiting outside, travel, an outdoor break, or a roof that opens. Indoor session timing is driven by pool availability and by the group, not by the sun.
Do check whether a roof will be open, because that converts the session into an outdoor one without changing the booking.
Yes, and it is the most effective lever available. Start time, session length, break placement and whether the group is in the open through the middle of the day.
Applies to open water sessions as much as to outdoor pools, and open water adds cold, glare and safety cover availability to the same decision.
What time this outdoor session happens
Owns almost nothing. Can choose when to swim recreationally, which is a real and underused freedom, but cannot move an organised session. Can raise it with the organiser, and a club that hears it from several members will consider it.
Owns the session plan within the slot: the length, the breaks, where the group sits, the shirt and hat expectation, the scheduled reapplication stops, and the decision to shorten or cancel in extreme conditions. Owns checking the forecast in time to communicate a change.
Owns the slot itself, the opening hours, and whether early and late sessions are available at all. A lido that offers only midday public sessions in July has made a timing decision on behalf of everyone who swims there.
Where a duty attachesThe ultraviolet index framework from the WHO, with guidance from the NHS, Cancer Research UK, sun, UV and cancer and NICE NG34, sunlight exposure: risks and benefits. The duty to assess and control risks for an organised activity sits under the Management of Health and Safety at Work Regulations 1999.
Summary
Timing works when nothing else does, costs nothing, and protects the staff as well as the swimmers. For an individual it is the easiest change available. For an organised session it is a decision made by whoever sets the timetable, and the useful thing is to make it consciously, check the forecast in time to act on it, and write it down so it is a decision rather than a habit.