Geoscience Reference
In-Depth Information
C
= Convection heat gain.
E
= Evaporation (cooling).
The ideal balance when applying the equation is no new heat gain. As long as heat gained from radi-
ation, convection, and metabolic processes do not exceed that lost through the evaporation induced
by sweating, the body experiences no stress or hazard. However, when heat gain from any source is
more than the body can compensate for by sweating, the result is
heat stress
.
15.8.4 W
orK
-l
oad
a
ssessment
Under conditions of high temperature and heavy workload, the industrial hygienist should deter-
mine the work-load category of each job (refer to applicable ACGIH tables). Work-load category is
determined by averaging metabolic rates for the tasks and then ranking them:
• Light work—up to 200 kcal/hour
• Medium work—200 to 350 kcal/hour
• Heavy work—350 to 500 kcal/hour
15.8.4.1 Cool Rest Area
Where heat conditions in the rest area are different from those in the work area, the metabolic rate
(
M
) should be calculated using a time-weighted average, as shown in Equation 15.17:
Averagemetabolic rate =
(
)()
+
(
)()
++
(
)()
Mt
Mt
22
…
MMt
11
nn
(15.17)
t
12
…
+++
t
t
n
where
M
= Metabolic rate.
t
= Time in minutes.
15.8.5 s
ampling
m
ethods
Currently used thermal exposure sampling methods are discussed below.
•
Body temperature measurements
—Although instruments are available to estimate deep
body temperature by measuring the temperature in the ear canal or on the skin, these
instruments are not sufficiently reliable to use in compliance evaluations.
•
Environmental measurements
—Environmental heat measurements should be made at, or
as close as possible to, the specific work area where the worker is exposed. When a worker
is not continuously exposed in a single hot area but moves between two or more areas hav-
ing different levels of environmental heat, or when the environmental heat varies substan-
tially at a single hot area, environmental heat exposures should be measured for each area
and for each level of environmental heat to which employees are exposed.
•
Wet bulb globe temperature index
—Wet bulb globe temperature (WBGT) should be cal-
culated using the appropriate formula (see Equation 15.17). The WBGT for continuous
all-day or several hour exposures should be averaged over a 60-minute period. Intermittent
exposures should be averaged over a 120-minute period. These averages should be calcu-
lated using Equation 15.18.
=
(
)()
+
(
)()
++
(
)()
WBGT
t
BGTt
22
…
WBGT
t
11
nnn
Average
WBGT
(15.18)
t
12
…
+++
t
t
n
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