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Suppose that you are in some extremely hot and humid place where your sweat just doesn't evaporate well enough to cool you down. You can then consider putting a liquid other than water on your skin, e.g. methanol would be an excellent liquid with a decent latent heat and high rate of evaporation at room temperature, but this is poisonous. Ethanol is better, but it will also cause problems as you need to use large quantities.

The question is then what liquid that can be used safely, will provide the best cooling.

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  • $\begingroup$ I can't really think of any volatile liquid less harmful than ethanol. Freons, maybe, but these are unsuitable for other reasons. $\endgroup$
    – Ivan Neretin
    Mar 11, 2016 at 21:09
  • $\begingroup$ Ethyl acetate - Higher vapor pressure at room temp than ethanol (73 mmHg vs 44 mmHg), higher LD50 (11.3 g/kg vs 7.1 g/kg, both rats). $\endgroup$
    – SendersReagent
    Mar 11, 2016 at 21:35
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    $\begingroup$ Good! I didn't remember this one. But not that good, come to think of it. Its enthalpy of vaporization is lower, and with higher vapor pressure it will get into your body faster, thus compensating for the marginally lower toxicity. Well, at least it smells nice. $\endgroup$
    – Ivan Neretin
    Mar 11, 2016 at 22:04
  • $\begingroup$ I just got another idea. Isotopes of H2O like e.g. with O-16 replaced by O-18 would work. Even at 100% humidity, the partial pressure of such isotopes in the atmosphere is well below saturation level. Also you can safely drink this, your sweat will evaporate if your body is saturated with this heavier variant of H2O. Unlike D2O this is safe to drink. But I think this is way too expensive to be of practical use. $\endgroup$
    – Count Iblis
    Mar 12, 2016 at 0:34
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    $\begingroup$ Isotopes just don't work that way. You won't see any difference from regular water. $\endgroup$
    – Ivan Neretin
    Mar 12, 2016 at 15:05

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There aren't many liquids other than alcohols which you can easily use. Liquid butane, for example, would be a coolant, but it would probably cause frostbite (and is flammable, not to mention from cylinders it stinks).

If you can't use ethanol for any reason, you can use isopropanol (rubbing alcohol): its cooling properties are similar. Note that you should only use isopropanol outdoors, because not only does it smell nasty in high concentrations, it is slightly toxic. Make sure to use the water solutions (~75%), as gels will leave sticky and nasty residue upon evaporation.

Even ethanol is not completely "safe", because it has the potential to cause drunkenness, even in vapor form.

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