Geoscience Reference
In-Depth Information
15.8.5.2 Additional Thermal Stress Indices
1. The
effective temperature
(ET) index combines the temperature, the humidity of the air,
and air velocity. This index has been used extensively in the field of comfort ventilation and
air-conditioning. ET remains a useful measurement technique in mines and other places
where humidity is high and radiant heat is low.
2. The
heat stress index
(HSI) was developed by Belding and Hatch in 1965. Although the
HSI considers all environmental factors and work rate, it is not completely satisfactory for
determining an individual worker's heat stress and it is also difficult to use.
15.8.6 h
eat
s
tress
s
ample
m
easurement
and
C
alCulation
The wet bulb globe temperature index (usually abbreviated WBGT) is the most widely used alge-
braic approximation of an “effective temperature” currently in use today. It is an index that can
be determined quickly, requiring a minimum of effort and operator skill. As an approximation
to an “effective temperature,” the WBGT takes into account virtually all the commonly accepted
mechanisms of heat transfer (e.g., radiant, evaporative). It does not account for the cooling effect
of wind speed. Because of its simplicity, WBGT has been adopted by the American Conference of
Government Hygienists as its principal index for use in specifying a heat stress related threshold
limit value (TLV).
■
EXAMPLE 15.10
Problem:
What would be the wet globe temperature index (in °C) for a quarry worker in Connecticut,
who must work on a sunny morning when the outdoor dry bulb temperature is 88°F, the wet bulb
temperature is 72°F, and the globe temperature is 102°F?
Solution:
WBGT
= 0.7(
NWB
) + 0.2(
GT
) + 0.1(
DB
) = 0.7(72) + 0.2(102) + 8.8 = 50.4 + 20.4 + 8.8 = 78.8°F (26°C)
■
EXAMPLE 15.14
Problem:
Later is the same afternoon, at the same quarry identified above, rain clouds have gath-
ered, and rain has commenced to fall. The quarry manager has covered the work area in the quarry
pit with a large tarpaulin to protect his employees. If the wet bulb temperature under the tarp has
increased to 78°F, while the globe temperature has remained unchanged, what will be the new
WBGT for this slightly different situation? (
Hint:
This is an indoor environment.)
Solution:
WBGT
= 0.7(
NWB
) + 0.3(
GT
) = 0.7(78°) + 0.3(102) = 54.6 + 30.6 = 85.2°F
15.8.7 C
old
h
azards
Temperature hazards are generally thought of as relating to extremes of heat. This is natural because
most workplace temperature hazards do relate to heat. However, temperature extremes at the other
end of the spectrum—cold—can also be hazardous. Employees who work outdoors in colder cli-
mates and employees who work indoors in such jobs as meatpacking are subjected to cold hazards.
There are four factors that contribute to cold stress: cold temperature, high or cold wind, damp-
ness and cold water. These factors, alone or in combination, draw heat away from the body. Other
cold stress factors include age, disease, and overall physical condition. OSHA expresses cold stress
though its cold stress equation:
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