HEAT AND SOLAR INJURIES - Medicine for Mountaineering and Other Wilderness Activities - page 414

Evaporativecoolingislimitedbecausethemaximalsweatingrateforindividualsnotac-
climatized to heat is about 1500 ml per hour. Evaporation of that much perspiration would
eliminate the heat produced by running six miles at a pace of ten minutes per mile, which
is not a particularly fast pace.
Acclimatization to heat takes about one week, results in an increased tolerance for ex-
ercise in a hot environment, and is produced by mechanisms that increase the maximum
sweating rate but reduce salt loss. Water deprivation does not accelerate or otherwise con-
tribute to the acclimatization process.
Exertional and Nonexertional Heat Illness
Heat illness results from the inability of the body to get rid of heat it has produced. Two
variants are recognized. Exertional heat illness results from an increase in heat production
so large that all the heat cannot be dissipated even though the heat losing mechanisms are
functioning well. Nonexertional heat illness results from impairment of heat-losing pro-
cesses by disease and can occur even though heat production is low or normal. Heat ill-
nesses occurring in healthy individuals participating in vigorous wilderness activities are
essentially all the result of exertion, but the initial treatment for both disorders is the same.
A fever raises body temperature essentially by resetting the body's “thermostat” at a
higherlevelandcanbecontrolledwithdrugssuchasaspirin,ibuprofen,oracetaminophen.
Such drugs have no effect upon the elevated temperature associated with heat illness be-
cause the thermostat has not been changed. Only heat production has changed.
Preventing Exertional Heat Illness
Theonlywayexertionalheatinjuriescanbepreventedisbyrecognizingclimaticcondi-
tions in which heat cannot be dissipated and curtailing physical activity. Those conditions
are easily defined: an environmental temperature of 95°F (35°C) or higher and a high rel-
ative humidity.
The average skin temperature is 95°F (35°C), slightly lower than core temperature. At
environmentaltemperaturesabovethislevel,heatcannotbelostbyconvectionbecausethe
air temperature is higher than skin temperature. Heat cannot be lost by radiation because
the environment is hotter than the skin surface. (Heat would be gained by radiation.) The
only way heat can be lost in such circumstances is by evaporation. (Mechanisms of heat
loss are described in Chapter 26: Cold Injuries .)
At a high relative humidity, evaporation is greatly diminished and heat cannot be lost
by that route. In fact, in humid conditions at temperatures greater than 95°F (35°C), heat
cannot be lost effectively by any route. In the southeastern United States, where hot, humid
conditions are common in the summer, experienced residents know that a person dripping
with perspiration is in danger of heat illness. If perspiration were evaporating and provid-
ing cooling, it would not accumulate on the skin.