Heat stress is one of many physical agents to which thousands of workers are under constant exposure. Oftentimes it is necessary to work above the WBGT-based heat stress exposure limits. It is therefore important to consider alternative measures that include an exposure time limit to manage the heat stress. Predicted Heat Strain (PHS) (ISO7933) is one of those alternatives. PHS uses both personal factors like height and weight and job factors of environment, metabolic rate and clothing. The purpose of this project is to determine whether the PHS is an adequate method to predict short term exposure limits. The project’s data were taken from a prior experimental study where twelve participants were exposed to five different heat stress levels while over three different clothing ensembles. A total of 15 combinations of clothing and environment were tried. The PHS process was adapted to an Excel function using Visual Basic for Applications (VBA) (called fPHSTre). fPHSTre predicted a rectal temperature (Tre) at the exposure limit using both personal and job factors and then using standard values for personal factors. Based on analysis of variance, the fPHSTre adequately accounted for clothing, specifically evaporative resistance, using either fixed or individual data for predicted Tre on the experimental trials. In general, the PHS model could be used to reliably assess time limiting safe exposures in occupational settings for workers in hot environments.

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.