We sweat to cool our bodies by evaporation, but when we wipe the sweat away – a natural reaction in humans – are we defeating that function and making ourselves hotter? Do we also hasten dehydration by making our bodies sweat more, to replace what we’ve wiped away? In other words, should we just leave the sweat to do its job?
• The damp squib answer is that neither leaving the sweat nor wiping it away will make any difference to how well or badly you keep cool and hydrated.
Why? First, we are constantly sweating, but, by the time we feel it on our skin, sweat has reached the limit of its evaporative cooling capacity. Leaving it won’t make more of it evaporate or make what is there evaporate faster.
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Second, sweating is governed by how hot we are, not by how sweaty or dry our skin is. Because the sweat on our skin is no longer evaporating and cooling us, our sweat glands continue to pour it out. This is why beads of sweat accumulate on our brows – to the point of dripping down our noses and proverbially getting up them. Leave it, and you are just letting gravity wipe it away for you. You will dehydrate just as fast.
And the muscular heat that our wiping actions generate? This could increase or decrease our rate of overheating, depending on how sedentary or vigorous the activity we are interrupting is. So, on balance, while it may be a hot topic for theoretical debate, it will make no palpable difference.
You would do better to switch on an electric fan, or invest muscular effort in wafting a hand-held one. This will disperse the layer of insulating air that clings to your skin and allow more evaporative cooling. If the ambient air temperature is below body heat, it may directly cool you somewhat. Alternatively, wear cotton garments. The material will soak up the sweat and provide a greater evaporative surface.
Len Winokur, Leeds, UK
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