Performance Assessment of Predicted Heat Strain in High Heat Stress
Heat stress is a common physical agent associated with many occupations. The most commonly used method of assessing heat stress exposure is an empirical method using the Wet Bulb Globe Temperature Index but his method is limited in its ability to parse out individual contributors to the heat stress....
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| Format: | Others |
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Scholar Commons
2011
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| Online Access: | http://scholarcommons.usf.edu/etd/3212 http://scholarcommons.usf.edu/cgi/viewcontent.cgi?article=4407&context=etd |
| Summary: | Heat stress is a common physical agent associated with many
occupations. The most commonly used method of assessing heat stress
exposure is an empirical method using the Wet Bulb Globe Temperature Index
but his method is limited in its ability to parse out individual contributors to the
heat stress. The International Organization for Standardization (ISO) published a
rational model called Predicted Heat Strain (PHS) in 2004, and rational methods
have the advantage of separating out the individual pathways for heat exchange.
The objective of this research was a performance assessment of the current PHS
model. This experimental design consisted of 15 trials (3 clothing ensembles and
5 heat stress levels) involving 12 men and women. The clothing ensembles were
work clothes, NexGen® (microporous) coveralls, and Tychem® QC (vaporbarrier)
coveralls. The heat stress levels were 1.0 , 2.0 , 3.5 , 5.5 and 9.0 °CWBGT
above the average critical environment for each ensemble determined in
prior studies. The metabolic rate was 190 W/m2. The two outcomes of each trial
were an exposure time when core temperature reached 38 °C (ET38) and a Safe
Exposure Time (SET) defined as the amount of time required to reach either a
core temperature (Tre) = 38.5 ºC, a heart rate of 85% age-estimated maximum, or
fatigue.
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Trial data for environment, metabolic rate and clothing were inputs to the
(PHS) model to determine a predicted amount of time for the participants to
reach a Tre = 38 ºC, which was the limiting condition in PHS for acute exposures.
The first consideration was predictive validity for which PHS-Time was compared
to ET38. The expectation would be that PHS-Time would predict the mean ET
response. Results for predictive validity indicated a moderate agreement
between ET38 and PHS-Time (r2 of 0.34 and Intraclass Correlation Coefficient at
0.33). When the method for accounting for clothing was changed to that
recommended by ISO, the PHS predicted times moved systematically toward a
shorter exposure time and modest agreement (r2 of 0.39 and Intraclass
Correlation Coefficient at 0.31). Protective validity was the ability of the PHSTime
to predict an exposure time that would be safe for most people. In this
case, PHS-Time was compared to SET. The PHS was protective for 73% of the
cases. When it was modified to account for clothing following the ISO method,
the protective outcomes were 98%.
In addition, the PHS model examined with respect to starting core
temperature and fixed height and weight. Using the actual core temperature
improved the outcomes somewhat, but changing from 36.8 to 37.0 would be
sufficient. There is a strong tendency to over-predict PHS-Time for individuals
with a low body surface area, usually short and lower than average weight. |
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