Validity of eighteen empirical heat stress indices in predicting the physiological parameters of workers under various occupational and climatic conditions

Thermal strain indices have been developed to assess the risk level of heat strain on workers in occupational settings. To date, no comprehensive study has validated these indicators. The present study aims to investigate the validity of eighteen empirical heat stress indices in predicting the physiological parameters of workers under various occupational and environmental conditions. This cross-sectional study was conducted with 201 male employees working in warm environments. First, Individual and background information was collected. After participants had rested for 30 min, their heart rate and tympanic temperature were measured. People then returned to their workplace and started their routine work. At the end of 90 min, workers' heart rate and tympanic temperature were measured again. Additionally, researchers estimated their metabolic rate and clothing thermal insulation. Environmental parameters were also collected including dry temperature, wet temperature, globe temperature, relative humidity and air flow speed at 30, 60 and 90 min time points. Additional information was recorded to calculate environmental indicators. Then, the values of each of the environmental indicators were calculated. Finally, the validity of each index was evaluated under these different conditions. Results showed that highest regression coefficients with tympanic temperature were CET (0.6822), TSI (0.6697), WBGT (0.6692), and ESI (0.6415) indices, respectively. The greatest regression coefficients with heart rate belonged to TSI (0.4687), OT (0.4545), CET (0.4449), and Teq (0.4449) indices, respectively. Based on these results, the highest diagnostic accuracies of ROC curves for tympanic temperature were related to indices of WBGT, CET, and HI with AUC of 0.913, 0.912, and 0.910, respectively. In total, the results showed that the two indices of WBGT and CET have the strongest validity among the empirical indices. However, each of these empirical indices has the best accuracy under specific conditions. Therefore, the development of new indices is required for the precise assessment of heat strain under various conditions.