must function. However, much can be done to prevent harmful effects on personnel
from exposure to these extremes and subsequent deterioration of performance.
b. Heat injuries are the result of excessive elevation of body temperature and/or
water electrolyte imbalance due to inadequate replacement of fluid lost through
perspiration. The danger of heat injuries is much greater for freshly exposed
unacclimatized personnel than it is for seasoned troops. In normal individuals, a
physiologic adjustment occurs over a prevention and treatment of heat injuries period of
2 to 3 weeks. Careful attention to heat indices is one of the most effective ways of
preventing heat injuries. FM 21-10 and TB MED 507 provide valuable guidance on the
prevention and treatment of heat injuries.
c. The wet-bulb globe temperature (WBGT) index is a single numeral by which
the effects of air temperature, air movement, relative humidity, and radiant heat can be
expressed for certain types of activities. This index of environmental factors is obtained
by: (1) measuring the air temperature, the air movement and relative humidity, and the
radiant heat; (2) multiplying the air temperature reading by 0.1, the air movement and
relative humidity reading by 0.7, and the radiant heat reading by 0.2; and then (3)
adding the three resulting numerals. For example, if the air temperature is 90F, the air
movement and relative humidity reading is 80F, and the radiant heat reading is 100F,
the WBGT index would be 85 for Fahrenheit readings and 29.7 for Celcius readings:
(F-32 = C).
(32C) x 0.1
32oC x 0.1 =
(27C) x 0.7
27oC x 0.7 =
(38C) x 0.2
38oC x 0.2
The several environmental factors are measured with the illustrated devices set up in
the area where exposure is to take place (see Figure 1-3 to view equipment needed to
perform the WBGT index).
(1) Air temperature (dry-bulb temperature) is measured with a dry-bulb
thermometer shielded from the direct rays of the sun.
(2) Air movement and relative humidity (wet-bulb temperature) are
measured with a wet-bulb thermometer. (See the WBGT kit at Figure 1-4.) A wet-bulb
thermometer is a regular thermometer which has a mercury bulb covered by a wet wick.
The mouth of a water bottle, in which the wick is placed, should be about three-fourths
of an inch below the tip of the thermometer bulb. The higher the relative humidity and
the slower the air movement, the closer the wet-bulb reading will be to the dry-bulb
(3) Radiant heat (globe temperature) is measured with a thermometer
inserted into the center of a hollow, flat black, 6-inch copper globe exposed to the full
effects of the sun and wind.