loss by conduction to the air occurs where the air temperature is below body
temperature. Conversely, heat gain by conduction occurs when air temperatures
exceeds that of the body. Similarly, the body loses heat by radiation when the
surrounding objects have surface temperatures lower than the temperature of the body
surface, and gains heat by radiation when the temperature of surrounding surfaces is
above that of the body surface. Accordingly, temperature combinations of the air and
the surrounding objects may exist which result in heat gain by the body due to radiation,
even though the air temperature is below that of the body. The enlargement of blood
vessels in the skin and an increase in the rate of the heartbeat are normal body
responses to heat stress. While these adjustments increase the temperature of the skin
and thus increase heat loss by conduction and radiation, they place a strain on the
circulatory system. These factors play important roles in the cause of heat stress
reactions.
b. When the body cannot lose sufficient heat by conduction and radiation, the
activity of the sweat glands increases, and heat loss by evaporation of sweat becomes
the important means of maintaining normal body temperature. However, as the relative
humidity of the air increases, evaporation of sweat decreases, thus reducing the cooling
function of the sweat. Excess sweating may lead to a marked loss of water and salt
from the body. The resulting dehydration and loss of body salt add to the stress on the
circulatory system.
c. Other major factors influencing physiologic responses incident to high
temperature exposures relate to the physical work performed prior to and during the
exposure, and the physical condition of the individual. The body is continually
producing heat, but during physical work, the production of heat is increased in
proportion to the type, intensity, and duration of the work. Thus, physical work
accentuates the effects of high temperature. Individual susceptibility to heat stress may
be increased by a large number and variety of conditions, including acute and chronic
infections, febrile conditions, reactions to immunizations, conditions affecting sweat
secretion, heat rash or acute sunburn, previous occurrence of heatstroke, use of
alcohol, dehydration, lack of sleep, fatigue, and obesity. The risk of heat injury is very
much higher in overweight persons than in those of normal weight, and special care
should be exercised when such persons are exposed to high temperatures.
d. The temperature of the human body is regulated within extremely narrow
limits, although there may be marked variations in the environmental heat. Three fairly
distinct types of heat injury may occur, depending upon the manner of breakdown of the
individual's heat adjustment. These injuries are heat cramps, heat exhaustion, and
heatstroke. The three conditions produce distinctive signs and symptoms that should
be recognized at once--not only by AMEDD personnel, but also by unit personnel--if the
victim is to receive proper care and attention. All military personnel in the field should
be familiar with first-aid treatment of these conditions.
MD0008
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