Should You Wipe Off Your Sweat?

Video Description

Want to donate to the highest-impact charities AND get your donation matched up to $100? Visit https://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! http://www.patreon.com/minutephysics Link to Patreon Supporters: http://www.minutephysics.com/supporters/ ********************************** REFERENCES Hyperphysics: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/sweat.html Engineering Toolbox Mollier Diagram: http://www.engineeringtoolbox.com/psychrometric-chart-mollier-d_27.html Sweat Info http://www.anaesthesiamcq.com/FluidBook/fl3_3.php Other articles: http://www.slate.com/articles/health_and_science/explainer/2012/06/should_you_wipe_away_your_sweat_or_does_that_keep_you_from_cooling_down_.html http://onlinelibrary.wiley.com/doi/10.1111/j.1748-1716.2012.02452.x/abstract http://lifehacker.com/5921036/dont-wipe-your-sweat-off-your-brow-itll-cool-you-down-faster http://www.realclearscience.com/2012/06/28/does_wiping_sweat_prevent_you_from_cooling_off_247729.html ********************************** 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 ********************************** MinutePhysics is on twitter - @minutephysics And facebook - http://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