Graduation Year

2011

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Public Health

Major Professor

Thomas E. Bernard

Keywords

clothing adjustment factors, Heat storage, Heat Stress, Predicted Heat Strain

Abstract

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.

ix

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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