This is a long post. Hope some of you find it useful.
With summer approaching, many hikers are preparing for warm weather hikes in different parts of the country. In addition to a general aerobic conditioning program, many undertake to also condition themselves to anticipated heat.
Two important elements of heat acclimation are: 1. maintaining cardiovascular efficiency and; 2. slowing the rate at which body core temperatures reach a critical temperature or increasing thermo-regulation efficiency. Cardiovascular efficiency is important because this is the mechanism for moving exercise-generated heat to the external and internal body surfaces. Thermo-regulation is important for efficiently transporting body heat to the surrounding environment. The literature I have reviewed on the subject indicates that cardiovascular efficiency can be improved through a general aerobic exercise program while improving thermo-regulation efficiency requires exercise in a hot environment.
I became interested in the effects of humidity on thermo-regulation because of some personal military experiences in humid and arid regions and from reading of differences in water consumption by people exercising in hot-humid and in hot-dry climates. It appeared to me that those in humid environments required measurably, or substantially, more water during exercise than those exercising in low relative humidity. A paper by L.E. Armstrong (1998) in the Proceedings, Internet Society for Sport Science (
http://sportsci.org) confirmed my empirical observation without postulating a mechanism. Armstrong stated that “
Heat acclimatization performed in a hot-humid condition stimulates a greater sweat rate than heat acclimatization in a hot-dry environment. Also, the absolute rate of sweating influences thermo-regulation.” I believe that the difference is related to the differences in the role of evaporative cooling in humid versus arid environments.
Water evaporation can remove 540 calories per cubic centimeter (cc or ml) in the evaporation process with no change in temperature; this is called the heat of evaporation and is the reason that evaporative coolers are effective in the arid SW but not in the humid SE.
A backpacker or mountaineer can easily burn 3000 to >5000 calories per day. A portion of the calories are used to do work; the remainder, most of the calories, become heat that must be removed from the body. Heat calories are removed by sweating or by heat transfer to breathed air via the lungs.
In a very humid environment, evaporative cooling is almost non-existent and heat removal is simply the difference between the temperature of water when drunk and body temperature (37 deg. C). Assuming water was 15 deg. C when drunk then a ml of water will remove 22 calories of heat when it leaves the body as sweat. The same ml of water can remove 562 calories if it is able to evaporate.
Loss of heat by breathing is also reduced in humid environments; water evaporation, and cooling in the alveoli is much reduced by humidity. In contrast, water loss by breathing is much increased in low relative humidity.
I suspect that an individual acclimated to humid heat thermo-regulates more by moving more water, and thus more heat, through the skin. A person adapted to an arid environment has adapted to a thermo-regulation regime more dependent on evaporative cooling and water conservation.
According to Armstrong, acclimation to a different heat regime requires at least two weeks of exercising in the new environment. People adapted to exercise in humid environments probably should consider their greater potential for water loss during acclimation if planning a hike in hot-dry environments. Conversely, those adapted to hot-dry environments should consider their lower capacity for heat loss by liquid sweat if planning a trip in the humid areas of our country.
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