View Full Version : Should I mop my brow?
08-02-2001, 11:45 AM
If I'm taking strenuous exercise, and sweating from it, should I leave the sweat there (to let it cool my overheating body) or wipe it off?
Discussed this with a couple of friends last week whilst climbing a hill on a hot day, and we came up with a couple of theories between us:
* Sweat already done its job when it reaches the surface, by taking some heat out with it, and it's actually more efficient (cooling-wise) to wipe it off.
* It's the act of evaporating while it's on the surface that cools you down (hazy memories of chemistry lessons makes me think that evaporation needs energy, and the water molecules in the sweat take this from your body, thereby cooling you).
So what's the straight dope?
Evaporation cools you off so obviously wiping off the sweat is counter-productive (except when it is getting into your eyes).
The University of Arizona has worked on the idea of cooling off open areas around malls, etc., by having ponds of water and other devices that would cool thru evaporation. They have been very successful with it, except I don't know where they plan to get all the water from.
08-02-2001, 12:22 PM
Whaddaya think your eyebrows are for? They're rain gutters for your eyes.
Your memories of chemistry lessons are correct. The evaporation of your sweat is what cools you off. But since your forhead is such a small area, wiping the sweat off won't affect your body's cooling very much. Besides, all you need is a thin film of water on your skin to take advantage of evaporative cooling. Rivers of sweat running into your eyes aren't helping. Wiping your brow will leave that thin film behind anyway.
And make sure to replace that sweat by drinking plenty of water when you exercise.
08-02-2001, 01:57 PM
Evaporation is by far the greatest agent in cooling your core temperature, but not the only one. There are 3 others: conduction, convection, and radiation.
There are two main ways your brain can effect heat loss thru your skin: increase the blood flow through the skin, and increased sweat. When blood reaches the surface of your skin, calories are lost to the surrounding environment, and calories are a measure of heat. You are also cooled by evaporation of invisible sweat. But when your temperature rises more, the brain turns on the sweat glands, increasing the sweating.
The exchange of heat from the skin to the environment depnds on several factors. Radiation is the most important, accounting for 60% of heat loss under resting conditions. Your body is a metabolic furnace that constantly radiates heat. You can also gain heat by the sun's radiation.
You also lose or gain heat by conduction. If you sit on a hot surface, you will gain heat. In contrast, cold water causes heat from the skin to be conducted to the water.
Closely related to conduction is convection, which is moving the warmed water or air away from the skin and replacing it with cooler water or air. That is why a cool breeze helps, but not a hot breeze.
At room temperatures at rest, about 25% of your calories of heat are eliminated by insensible perspiration. As you exercise and increase the need to eliminate heat, you sweat more and the heat loss is increased. That is why humidity causes heat stress. It limits your ability to cool by evaporation. In humid environments, the body must rely mostly on radiation, conduction, and convection to cool.
95% of the calories you consume and eliminate are eliminated through your skin. Only about 25% of the calories expended with exercise are used for the mecanical effort. The other 75% are from heat produced by the muscles. That is why vigorous exercise causes the body temperature to rise.
A healthy body has an amazing ability to adjust to its environment and people can become acclimiated to either heat or cold. Acclimitazation to heat requires 2-6 weeks. Before acclimitazation occurs, the sweat glands will lose a lot of salt when you sweat. Sweat glands adapt to recycle the sweat in a way to permit reabsorption of some salt. But in vigorous exercise, the capacity of the sweat glands to reabsorb salt will be overloaded.
There is a change within the cell structure (mitochondria) that enables the cells to metabolize energy compounds from food more efficiently, using less oxygen. Since delivery of oxygen is dependent upon the capacity of the circulation to increase blood flow, this increased efficiency decreases the overall work the heart must do during heat stress.
( The Health Letter August 1991)
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