Most people's mental model of sun exposure is a single source directly overhead, blocked by anything between you and it. That model works reasonably well on a pavement. It works badly near water, and the places it fails are exactly the places where people get caught out.
Light reaches you by more than one route
Solar ultraviolet reaching a person outdoors arrives by three routes rather than one.
- Direct. Straight from the sun. This is the one everybody models, and it is the one that a hat, a parasol and a roof block.
- Scattered. Diffused through the atmosphere and arriving from the whole sky rather than one point. This is why you can burn in the shade of a single umbrella on an open beach, and why an overcast day is not a day off. The World Health Organization ultraviolet index framework accounts for total incident ultraviolet rather than direct beam alone, and Cancer Research UK, sun, UV and cancer makes the point about cloud explicitly.
- Reflected. Bounced off surfaces. Different surfaces reflect very differently, and water is a notably reflective one, particularly when the surface is disturbed and when the sun is lower in the sky.
At a swimming venue you have all three, plus wet skin, plus pale concrete or tiled surrounds which also reflect, plus several hours.
We are not going to publish reflectance percentages for water, sand, concrete or anything else. Figures for reflectance vary enormously with surface state, sun angle, turbidity and wavelength, and a single number carried out of context is misleading rather than helpful. The institutional sources for ultraviolet exposure are the WHO, Cancer Research UK, sun, UV and cancer and NICE NG34.
Where the burning actually appears
This is the practically useful part. Because a proportion of the exposure arrives from below and from the side, sunburn at swimming venues shows up in a distinctive pattern that a purely overhead model does not predict:
| Area | Why | Practical note |
|---|---|---|
| Under the chin and jaw | Upward reflected component from the water surface | The classic open water and lido burn. Frequently missed on application. |
| Under the nose and around the nostrils | Same upward path | Small area, disproportionately noticeable |
| Backs and tops of the ears | Direct and reflected, and usually forgotten entirely | Also relevant to anybody wearing a cap that does not cover them |
| The underside of the arms | Upward reflection while swimming or floating | Rarely considered at all |
| The back of the neck | Direct, while looking down at the water | Swimming with the head down is the whole activity |
| The tops of the feet | Direct, while sitting on the side | Long dwell time, no movement |
| Parting of the hair and scalp | Direct, and no protection unless covered | A cap or hat is the only realistic answer here |
Notice how many of those are places people would never think to apply anything, and how many are covered by simply wearing a shirt and a hat with a brim between swims.
Eyes and glare
The reflected component matters for eyes as much as for skin, and it is the reason glare off water is so pronounced and so tiring. Open water swimmers and outdoor pool users looking along a water surface are looking directly into the reflected path. This is covered properly in eyes, goggles and glare, which also covers what tinted goggles do and do not do.
Shade from a single overhead object does not solve the problem near water, because shade blocks the direct route and leaves the scattered and reflected ones open. Deep shade, meaning shade with something above and around you, does far more than a parasol on an open deck. That is a design point for operators and it is covered in shade at an outdoor pool.
Being in the water is not shelter
Two beliefs are worth correcting. The first is that being in the water protects you. Water does attenuate ultraviolet with depth, but a swimmer at the surface has their back, shoulders and neck at or just under the surface for the entire swim, and the attenuation over that distance is not protection you should be planning around. Front crawl in particular presents the back of the neck and the shoulders to the sky continuously.
The second is that a cloudy day is a safe day. Scattered ultraviolet passes through cloud, and Cancer Research UK, sun, UV and cancer is explicit that you can burn on an overcast day. This matters more at swimming venues than elsewhere because a cool overcast day at a lido or on an open water swim removes the heat signal entirely while leaving a meaningful exposure in place.
Does reflection matter to me?
No. There is no meaningful incident ultraviolet in an enclosed hall, so there is nothing to reflect. Glare indoors is an artificial lighting and surface issue, and it is a comfort and supervision question rather than an exposure one.
Lifeguards indoors do have a genuine visibility problem from surface reflection of artificial light and from window glare, which is a supervision issue rather than a skin one.
Yes, and it changes where you need cover. Reflection is why sunburn at outdoor pools and in open water appears under the chin, under the nose, on the ears and on the undersides of arms.
It is also why glare on the water is so much worse than glare on land, which affects both swimmers and the people supervising them.
Covering the parts that reflection reaches
Owns their own application, including the places that are easy to miss, and owns wearing a hat and shirt between swims rather than relying on product alone. Owns sunglasses out of the water.
Owns it for a group, especially children, who will not apply anything to the underside of their own chin and will not think about their ears. Owns building reapplication into the session as a scheduled stop rather than an instruction. Owns their own exposure too, since supervising a water surface for two hours means looking at the reflected path for two hours.
Owns whether there is deep shade rather than a single parasol, because the difference between the two is exactly the difference between blocking the direct route and blocking the scattered one. Owns shade over lifeguard positions.
Where a duty attachesGeneral guidance from the NHS, Cancer Research UK, sun, UV and cancer and NICE NG34, sunlight exposure: risks and benefits, with the ultraviolet index framework from the WHO.
Water is not the only reflective surface at a pool
Swimming venues are unusually pale. Concrete surrounds, white or light tiling, painted decking, rendered walls and light coloured furniture all reflect, and a lido is often surrounded by all of them at once. That is a design consequence of building somewhere that has to drain, be cleaned constantly and not get too hot underfoot, and it is not going to change.
The practical effect is that even a swimmer sitting away from the water, on a lounger, under a parasol, is sitting in a bowl of reflective surfaces. It is a further reason why deep shade with something above and around outperforms an overhead object, and why the shade at a venue with grass and trees behaves differently from the shade at a venue that is entirely hard landscaped.
It also explains an observation people make and then dismiss: that they burn more at a pool than they do doing something else outdoors for the same length of time. That is not imagination. It is the combination of a reflective water surface, reflective surrounds, minimal clothing and no subjective heat signal because they keep getting wet.
What to do with this, practically
Nothing complicated. Apply to the places the overhead model misses, wear a hat with a brim and a shirt between swims, sit in shade that has sides rather than just a top, and treat an overcast day at a lido as a day that still counts. None of that requires understanding the physics, but the physics is why those four things are the ones that matter.
Summary
Exposure near water arrives from more directions than people plan for. The result is a recognisable pattern of burning in places a hat does not reach and nobody applies anything to. The fix is not more product, it is remembering the underside of the chin, the ears and the arms, using deep shade rather than a parasol, and wearing something between swims. And do not treat an overcast day as a day off, because at a swimming venue it removes the warning without removing the exposure.