Sweating

 

Strenuous exercise performed in the hot or humid conditions put severe stresses on the body. Such heat related stresses must be dealt with by the body in order to perform at optimal levels and more importantly to prevent heat stroke. Strenuous exercise causes dehydration, even if one consumes fluids during exercise.

Movement requires the contraction of muscles which is an exothermic (heat releasing) process. The energy that the muscle releases during exercise is predominantly heat. Only 20% of the energy output of the muscle is converted to motion the remainder is released as heat. This large amount of heat must be removed from the body in order to maintain a safe core temperature and fortunately the human body has a mechanism of removing heat called sweating.

Sweat is predominantly made of water with electrolytes, such as sodium(Na+) and chloride(Cl-) ions, dissolved in it. As the water evaporates from the surface of the skin it cools it down. Evaporation is an endothermic (heat absorbing) process, so evaporation of sweat absorbs heat from the skin and hence cools the athlete.

On hot and humid days sweating is not as effective as on hot and dry days and therefore heat is not removed from the body as efficiently. During high humidity the air is full of water molecules and the rate of evaporation from the skin is severely reduced.

100% humidity means that the air, at a given temperature, is saturated with water. Saturated means that no more water can dissolve in a given volume of air at that temperature. The table on the right shows the water content of saturated air at different temperatures.

The relative humidity is the percent of saturation humidity. For example at 10 oC 1 m3 of air saturated with water vapour contains 9.4 grams of water, according to the table on the right. So 50% relative humidity at 10 oC means that 4.7 grams of water is present in 1 m3 of air.

 
At very high humidity levels, water simply accumulates on the surface of the skin, beads up and rolls off the body, much like water on a glass surface.
Little evaporation, of water from the skin, occurs when the air is very humid, as shown on the right. With so little heat being lost through evaporation, blood vessels to the skin dilate in order to remove heat by convection of heat into the surrounding air and radiation of heat away from the body. With so much blood going to the skin, little blood goes to the active muscles and vital organs such as the brain. Physical strength is severely compromised and the athlete quickly fatigues with a decline in mental capacity. This condition is called heat stroke.

Does 100% relative humidity mean that you are literally swimming in water?

A 2 m3 sample of air at 100% relative humidity at 20 oC contains how much water?

What is the water content in grams of 1 m3 of saturated air at 10 oC?

How much water is present in 1 m3 of air at 30 oC if there is 50% relative humidity?

At 80% relative humidity at 20 oC what is the mass of water of 2 m3 air ?

Explain why the rate of evaporation is very low at high humidity?

Why are athletes in danger of heat exhaustion in high humidity?

What is heat stroke?

How does exercise in high humidity and hot temperatures often cause heat stroke?
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