Comparison of the validity of the main index of heat stress

The validity of nine heat stress indices : Effective Temperature (ET), Effective Temperature star (ET*), Heat Stress Index (HSI), Index of Thermal Stress (ITS), Predicted Mean Vote (PMV), Predicted 4 Hour Sweat Rate (P4SR), Required sweat rate (Sreq), Température équivalente (Teq) and Wet Bulb and Globe Temperature (WBGT) has been evaluated by comparing their results to the physiological responses of 60 workers in 12 different workplaces (5 workers by workplace).
The results show that, in the heat strains analysed during this study, Sreq allows the best prediction of actual heat strain. However, in spite of its accuracy, Sreq does not allow to determine a safe limit of exposure time. In addition, Sreq needs the recording of a large amount of parameters. Thus, a simplified method of heat strain evaluation can be proposed by using WBGT. As long as WBGT is lower than 25°C, heat strain is acceptable. When WBGT is heigher than 25°C, then Sreq must be used.
The physiological recordings (oral temperature and heart rate) done in the present study have demonstrated high reliability and simplicity. These recordings have to be achieved as frequently as possible.