Should You Wipe Off Your Sweat?

Опубликовано 20 декабря 2023, 17:00

Want to donate to the highest-impact charities AND get your donation matched up to $100? Visit Givewell.org/MinutePhysics

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?

Support MinutePhysics on Patreon! patreon.com/minutephysics
Link to Patreon Supporters: minutephysics.com/supporters

**********************************
REFERENCES

Hyperphysics:
hyperphysics.phy-astr.gsu.edu/...

Engineering Toolbox Mollier Diagram:
engineeringtoolbox.com/psychro...

Sweat Info
anaesthesiamcq.com/FluidBook/f...

Other articles:
slate.com/articles/health_and_...
onlinelibrary.wiley.com/doi/10...
lifehacker.com/5921036/dont-wi...
realclearscience.com/2012/06/2...

**********************************
CALCULATIONS

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

Evaporation rate at 25°C and 50% humidity, slight air movement (v~.5m/s) = .35kg/m^2/hr
engineeringtoolbox.com/evapora...

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 (engineeringtoolbox.com/water-t... 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 (en.wikipedia.org/wiki/Body_wei... 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 (weightloss.com.au/weight-loss/... Let’s say 50kJ/kg/h, which for average human amounts to 3000-4000 kJ/hr

**********************************

MinutePhysics is on twitter - @minutephysics
And facebook - facebook.com/minutephysics

Minute Physics provides an energetic and entertaining view of old and new problems in physics -- all in a minute!

Created by Henry Reich