Validation of the environmental stress index (ESI) for physiological variables

A new environmental stress index (ESI), based on ambient temperature (T_a), relative humidity (RH) and solar radiation (SR), was recently suggested as a potential substitute for the wet-bulb globe temperature (WBGT) index. The purpose of this study was to evaluate and validate ESI for three different physiological variables including rectal temperature (T_re), heart rate (HR), and sweat rate (m_sw). A database was taken from a previous study where 12 young men (21±1 y) served as subjects exposed to 120 min of 12 different combinations consisting of three metabolic rates (rest and treadmill walking at 5 km·h^-1 at 0% and 5% grades), two clothing ensembles (BDU and protective MOPP gear) and two outdoor solar radiation levels (shade and open sky). ESI was calculated as follows: ESI=0.63T_a-0.03RH+0.002SR+0.0054(T_aRH)-0.073(0.1+SR)^-1. Significant differences of about 2 units (p<0.05) were found between the ESI calculated in the sun and in the shade for all exposures. Concomitantly, significant differences (p<0.05) were found between m_sw, HR and T_re when measured in the sun and in the shade during all the exercise exposures. Thus, very high correlations (R²>0.838) were found between ESI and T_re, HR, or m_sw. These results indicate that ESI is strongly correlated to the physiological strain, whereby higher stress is reflected in higher strain. Therefore, evaluating heat stress by ESI, which uses the more common, fast response and accurate climatic measures, becomes more predominant.