The body constantly produces
heat, especially during exercise. To maintain a constant normal
temperature, it must pass this heat on to the environment. Life-threatening
circumstances can develop if the body becomes too hot or too cold.
Body temperature must be maintained within fairly narrow limits,
usually between 74 and 110 degrees Fahrenheit. However, hypothermia
and heat injuries can occur within much narrower limits. Therefore,
extreme temperatures can have a devastating effect on the body's
ability to control its temperature.
Overheating is a serious threat
to health and physical performance. During exercise, the body
can produce heat at a rate 10 to 20 times greater than during
rest. To survive, it must get rid of the excess heat.
The four ways in which the body can gain or lose heat are the following:
- Conduction - the transfer of heat from a warm object to a cool one that is touching it. (Warming boots by putting them on is an example.)
- Convection - the transfer of heat by circulation or movement of air. (Using a fan on a hot day is an example.)
- Radiation - the transfer of heat by electromagnetic waves. (Sitting under a heat lamp is an example.)
- Evaporation - the transfer of heat by changing a liquid into a gas. (Evaporating sweat cooling the skin is an example.)
Heat moves from warm to cool
areas. During exercise, when the body is extremely warm, heat
can be lost by a combination of the four methods. Sweating, however,
is the body's most important means for heat loss, especially during
exercise. Any condition that slows or blocks the transfer of heat
from the body by evaporation causes heat storage which results
in an increase in body temperature.
The degree to which evaporative
cooling occurs is also directly related to the air's relative
humidity (a measure of the amount of water vapor in the air).
When the relative humidity is 100 percent, the air is completely
saturated at its temperature. No more water can evaporate into
the surrounding air. As a result, sweat does not evaporate, no
cooling effect takes place, and the body temperature increases.
This causes even more sweating. During exercise in the heat, sweat
rates of up to two quarts per hour are not uncommon. If the lost fluids are not
replaced, dehydration can occur. This condition, in turn, can result in severe heat injuries.
Thus, in hot, humid conditions
when a survivor's sweat cannot evaporate, there is no cooling effect
through the process of evaporation. High relative humidities combined
with high temperatures can cause serious problems. Weather of
this type occurs in the tropics and equatorial regions such as
Central America and southern Asia.