I have a friend who claims her dermatologist told her than sunscreen was what was burning her skin. (I think it's much more likely that her rash/burn was either a sunburn, a skin reaction to some other product, or a "sun poisoning" type rash from using a sunscreen that doesn't block UVA -- my wife gets that.)

Said doctor told her it's because sunscreen converts the light energy to heat, and that heat was burning her skin. I know this is technically the way those organic sunscreen ingredients work, but can anyone quantify how much heat would actually be produced by this process?

Some web sites claim that some users feel hot when they're using absorbent-type sunscreens -- and again I find that hard to believe. Make me want to slather a few hot dogs with different sunscreens (and a control with none) and leave them in the sun and stick them with my meat thermometer to see if there's any discernable difference.

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    $\begingroup$ Bwahahaha :D I suggest not to go to this doc unless to laugh him out. $\endgroup$
    – Mithoron
    Jun 2 '18 at 17:14

Well, the sunscreen is obviously not creating any additional heat. All the light that falls on skin is either reflected/scattered immediately or absorbed and turned to heat. Unless the sunscreen reduces the albedo of the skin -- that is, unless it reduces the amount of light that gets reflected or scattered -- then it can't possibly increase the amount of heat energy that is generated by the absorption of light.

What it does do is cause the absorption of UV light to occur right there in the uppermost layers of the skin, where the sunscreen is absorbed, instead of further down in the living dermis, where it can do damage. So I suppose the amount of heating in the upper (dead) layers of the epidermis goes up, with a compensating decrease in the amount of heating in the lower (live) layers.

To roughly quantify the heat, we could take the solar irradiance at sea level, which obviously varies by latitude, of about 1 kW/m^2, and then multiply by the reflectance of human skin, which varies considerably by individual and race, but let's say about 50%. That means about 500 W/m^2 = 50 mW/cm^2 of light energy is being turned into heat for your average white person on a clear day. If 1/4 of one's total skin area (about 2 m^2) is exposed, that's about 250W of energy you need to get rid of. Ordinary resting metabolism is about 80 W, so you're asking the body to get rid of 3x more heat than it normally does. Hence you start to sweat, become attracted to the idea of going into the ocean, et cetera. If we want to quantify the sweating necessary, we divide by the latent heat of vaporization for water (2260 kJ/kg) and find about 100 mg/s of sweat need to be produced and evaporate to get rid of 250W.


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