2007, Piran, Slovenia

Environmental Ergonomics XII
Igor B. Mekjavic, Stelios N. Kounalakis & Nigel A.S. Taylor (Eds.), © BIOMED, Ljubljana 2007
Figure 1. Safe exposure time related to WBGT adjusted for clothing and metabolic rate with
a suggested protective limit.
The reference metabolic rate for the data in Figure was 190 W m -2 or about 380 W for the
participant population. At 35, 40 and 45 °C-WBGT, the time limits are 62, 21 and 14
minutes. Using the US Navy PHEL 280 W curve for the same WBGTs, the times are 78, 30,
and 18. The general trend is the same, but the comparison is somewhat difficult because of
the 100 W difference in the reference metabolic rate. The US EPRI 430 W (Heavy Work)
curve for the same WBGTs have times of 35, 15 and 0; while the 290 W (Moderate Work)
curve is 180, 52, and 17 minutes. The EPRI curves bracket the proposed protective limit.
The adjustment of WBGT for the metabolic demands was applied to a rather narrow range of
metabolic rates, but the adjustment factor described in the results was developed for a range
from low to high metabolic rates. To compare to the previous recommendations, let the
following define AdjWBGT based on a nominal body surface area of 2.0, which was typical
of the subjects in the study.
AdjWBGT [°C] = WBGTmeasured + CAF - 0.02 (380 - M[W]) (2)
Using 285 W as the metabolic rate and CAF = 0 (Work Clothes), the equation recommends
time limits of 140, 29, and16 minutes. These fall closely into alignment with EPRI and are
close to the PHEL times.
Previous efforts at using Clothing Adjustment Factors to account for different clothing
ensembles in WBGT exposure assessment schemes for prolonged exposures have
demonstrated that the CAFs are robust to metabolic rate and different levels of relative
492
150
135
120
105
90
75
60
45
30
15
0
30 35 40 45 50
Adjusted WBGT [ C]
Work Clothes
NexGen
Tychem QC
Protective Limit