# Should You Wipe Off Your Sweat?

From “Minute Physics”. ( YouTube / Nebula )

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If you’re in scorching heat, or when your body is working hard and you’ve got hot, hot sweat all over, sticky and stifling – does wiping off the sweat help you cool off? Or is it better to leave it on?

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REFERENCES

Engineering Toolbox Mollier Diagram:
http://www.engineeringtoolbox.com/psychrometric-chart-mollier-d_27.html

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CALCULATIONS

Typical adult human body surface area ~ 1.5-2 m^2
https://en.wikipedia.org/wiki/Body_surface_area

Evaporation rate at 25°C and 50% humidity, slight air movement (v~.5m/s) = .35kg/m^2/hr
http://www.engineeringtoolbox.com/evaporation-water-surface-d_690.html

So in these conditions, a sweat-covered human can expect to evaporate ~.5-.75 L of water in an hour (For higher humidity (60-70%) it goes to ~.37-.5 L of water/hr). That amounts to ~0.25-0.35mm of sweat (covering the whole body) evaporated in an hour, or 6 micrometers every minute.

Water has latent heat of 2,270 kJ/kg (http://www.engineeringtoolbox.com/water-thermal-properties-d_162.html), so in an hour a human can lose ~1100-1700 kJ of energy. (2270/4.1868 ~ 542 Cal)

BUT that assumes all of the energy came from the person. If some proportion of it came from the air (~1/3-1/2?) then the person is only cooled down partially.

Mass of a human ~ 60-80kg (https://en.wikipedia.org/wiki/Body_weight), assuming ~specific heat of water, ie 4 kJ/kg/K, could decrease temp by ~4.5-5°C.

Energy used in moderate-hard exercise is ~20-30 kJ/kg/30 min, or ~40-60kJ/kg/h (http://www.weightloss.com.au/weight-loss/weight-loss-tools/exercise-energy-charts.html). Let’s say 50kJ/kg/h, which for average human amounts to 3000-4000 kJ/hr

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