Evaluation of the environmental stress index for physiological variables

Recently, a new environmental stress index (ESI), based on ambient temperature (T_a), relative humidity and solar radiation, was suggested. ESI was found to be highly correlated to the WBGT index. However, ESI was not correlated with any physiological variable that reflects strain. The purpose of this study was to evaluate ESI for three different physiological variables (rectal temperature (T_re), heart rate (f_c) and sweat rate (m_sw)), and with the physiological strain index (PSI). Twelve young men were exposed outdoors for 12 different experimental combinations consisting of three metabolic rates (rest, moderate and hard exercise intensity), two clothing ensembles (cotton clothing and protective overgarments), and two solar radiation levels (shade and sun). Each exposure lasted 120 min with T_re and f_c being continuously monitored. High correlations (R ≥ 0.838) were found when statistical analysis was done between ESI and T_re, f_c, m_sw, or PSI, which have the potential to be widely accepted and used universally. However, further studies between physiological variables and ESI obtained from other climatic conditions, different exercise intensities, and additional clothing ensembles need to be evaluated.