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There is a popular belief that wet skin burns or tans faster. However, I've never heard a believable explanation of why this happens.

The best explanation I've heard is that the water droplets on the skin act as a lens, focusing the sunlight onto your skin. I don't see how this would affect an overall burn, because the amount of sunlight reaching the skin is the same (ignoring reflection).

Is this 'fact' true, and if so, what causes it?

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I only have a non-causal explanation, i.e. a correlation. Large surfaces of water (or snow) reflecting light will make your skin burn much faster. Large bodies of water tend to make your skin wet as well. –  Glen The Udderboat Jul 16 '13 at 14:02
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However, thinking very loosely about it, there could be some biology involved. Something like "the wet skin makes the body think it is cold and stops cooling it/starts warming it". –  Glen The Udderboat Jul 16 '13 at 14:08
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Finally, I'd propose that wet skin makes you produce less sweat. Perhaps sweat is better/worse at preventing sunburn than water? –  Glen The Udderboat Jul 16 '13 at 14:20
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While the total sunlight power will be the same, sunburning need not be a linear phenomenon. If the water were to concentrate the total power on (say) half of the skin surface, and then on the other half, then some 'threshold' intensity could be overcome and sunburning would take place. To make it even you would need the droplets to move around, which they do anyway. –  Emilio Pisanty Jul 16 '13 at 14:30
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@EmilioPisanty so what you're saying is water droplets are strategically plotting to work together to give us all sunburns at lower than normally required sunlight intensities? What a dastardly devious plan! –  Jim Jul 16 '13 at 14:41

4 Answers 4

up vote 29 down vote accepted

I don't know of any research to find out if skin sunburns faster when wet, though someone did a comparable experiment to find out if plants can be burnt by sunlight focussed through drops of water after the plants have been watered.

You need to be clear what is being measured here. The total amount of sunlight hitting you, and a plant, is unaffected by whether you're wet or not. The question is whether water droplets can focus the sunlight onto intense patches causing small local burns.

The answer is that under most circumstances water droplets do not cause burning because unless the contact angle is very high they do not focus the sunlight onto the skin. Burning (of the plants) could happen if the droplets were held above the leaf surface by hairs, or when the water droplets were replaced by glass spheres (with an effective contact angle of 180º.

My observation of water droplets on my own skin is that the contact angles are less than 90º, so from the plant experiments these droplets would not cause local burning. The answer to your question is (probably) that wet skin does not burn faster. I would agree with Will that the cooling effect of water on the skin may make you unaware that you're being burnt, and this may lead to the common belief that wet skin accelerates burning.

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+1 I like that you experimented with droplets on your own skin. –  Will Jul 16 '13 at 14:42
    
What if the water contains some % of oil, such as ocean water at a tarry beach? –  Travis J May 7 at 19:18
    
@TravisJ: can you be a bit more precise? What effects do you think the oil might cause? It's not obvious to me that the presence of oil would have much effect on the shape of droplets on the skin - well, not unless you picked up the oil and rubbed it in :-). –  John Rennie May 8 at 5:45
    
@JohnRennie - I thought that the oil might cause a film to form on the skin. I am not sure what the result of that would be, perhaps it would refract the light? I know that some people will use mineral oil in order to tan faster, but it will also burn you faster. –  Travis J May 8 at 16:32

I'm no expert in biology, and biology may describe this phenomena? I am applying what I know about physics.


Sunburn is caused by excessive exposure to skin-damaging UV light.

I tend to believe that wet skin by itself does not cause you to burn faster, one merely feels cool in water, so one is prepared to stay out in the sun longer. I can't see any reason for why wet skin would amplify the effect of burning (barring localized droplets, possibly causing what I will call "spotted sunburn"). In fact, some frequencies of UV are absorbed by water, which would seem to reduce the effect (by how much, I'm not sure). In fact, having a layer of water on your skin would seem to help reduce sunburn (however slightly) because some of the UV will be reflected by the water.


That being said, consider a large body of water. Here are a couple of situations you might find yourself in:

If you have your body submerged in water it is possible that more UV could strike your skin due to the water surface not being flat (a lensing effect).

If you are above the water (for example, on a boat, or just your head and shoulders poking out of the water, etc.) there will definitely be more UV striking your skin, due to there being not only direct light from the sun, but reflected light from the surface of the water.


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Another reason is that if your skin is wet then you probably just washed off all your sunscreen... –  Nathaniel Jul 16 '13 at 14:05
    
@Nathaniel I was going to mention that point :P. However, if you have followed the instructions of application correctly (giving it enough time to be absorbed by the skin, etc.), and it is designed for use in water, there should be no issue. –  Will Jul 16 '13 at 14:07
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Your two "large body of water" points are rather contradictory though, at least if you happen to believe energy is conserved... –  leftaroundabout Jul 16 '13 at 15:47
    
@leftaroundabout How so? I'm giving possible scenarios. Do you want to argue that because light is being reflected I cannot have a lensing effect? All I am saying is that if the lensing effect is strong enough, this could occur, irrespective of the reflection (assuming all is not reflected), and vice versa. I agree that the latter point is more likely as the water's surface is forever changing and will probably average out a potential lensing effect. –  Will Jul 16 '13 at 15:51
    
What I want to argue is, if some of the UV light is reflected by the surface (which is certainly the case), then this part of doesn't get into the water body and you can't really receive a higher intensity while immersed in the water, than without any water around. Yes, with a very strong lensing effect this might come about, but it seems quite implausible. –  leftaroundabout Jul 16 '13 at 15:54

Consider these facts: [cdc.gov]

  • Grass reflects from 2.5 to 3 percent of UV rays hitting its surface.
  • Sand reflects 20 to 30 percent of UV rays.
  • Snow and ice can reflect 80 to 90 percent of UV rays.
  • Depending on the angle of reflection, water can reflect up to a full 100 percent of UV rays striking the surface.

As such, one explanation might be:

Most UV rays from the sun (which cause tans and sunburns) can reflect off the surface of water. This is actually why so many people get sunburnt while swimming or wading in water -- UV radiation will reflect off the surface of the water onto whatever part of a person is above the water, giving a swimmer or wader a double dose of UV radiation, versus someone far enough inland to not be affected by reflection off the water. The UV reflection is strongest when sunlight's angle to the water is at its lowest, within a few hours of sunrise or a few hours before sunset. By contrast, more UV rays penetrate water and can cause sunburns on completely submerged divers during mid-day, when the sun is high overhead.

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Do you have any reputable reference for water reflecting 100% of the UV rays, especially when the sun is close to overhead? –  Peter Shor Jul 18 '13 at 18:45

The reflectance off water is only about 2%, so about 98% of the solar rays pass into the water layer on your skin, and the bottom surface of that water is optically contacting your skin, so the water acts as an anti reflection coating, that more efficiently couples the rays into your skin layers.

Also a lot of "wet skin" burning, is actually wet tee shirt burning, and the water in the pores of the wet shirt very efficiently couples the rays through to your skin.

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Actually, the UV reflectance off water seems to be higher ... I've seen figures on the web between 5% and 50%. But the most trustworthy ones seem to be 10% or less. –  Peter Shor Jul 17 '13 at 2:23

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