Journal of Thermal Biology 28 (2003) 43–49 Evaluation of the environmental stress index for physiological variables D.S. Moran a, *, K.B. Pandolf b , Y. Shapiro a , A. Laor a , Y. Heled a , R.R. Gonzalez b a Heller Institute of Medical Research, Sheba Medical Center, Tel Hashomer 52621, Israel b US Army Research Institute of Environmental Medicine, Natick, MA 01760-5007, USA Received 6 March 2002; accepted 18 June 2002 Abstract 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 120min with T re and f c being continuously monitored. High correlations (RX0:838) 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. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Heat-stress; Indices; Rectal temperature; Heart rate; Sweat rate 1. Introduction Heat stress evaluation is generally determined through meteorological parameters that enable the estimation of the influence of several environmental factors on thermal comfort and physiological ability. The variables included in heat stress indices and their relative weights have changed over the years. The existing indices can be divided into two main categories (Gonzalez, 1986): effective temperature (ET) scales, which are based on meteorological parameters only (e.g., ambient tempera- ture, wet-bulb temperature, black-globe temperature), and rational heat scales, which include a combina- tion of environmental and physiological parameters (e.g., radiative and convective heat transfer, evaporative capacity of the environment, and metabolic heat production). In 1923, Houghten and Yaglou developed the ET index from which at least five additional indices were derived. Among them, in 1957, Yaglou and Minard introduced the wet bulb globe temperature (WBGT) index, which gained popularity mainly due to its simplicity and convenience of use. This index is obtained primarily from three parameters: black globe tempera- ture (T g ) which mainly reflects the solar radiation, wet bulb temperature (T w ), and dry bulb temperature (T a ). This index is calculated as follows: WBGT ¼ 0:7T w þ 0:2T g þ 0:1T a : The WBGT is in use in the field by the US Army and is the index from which training safety orders are based (Burr, 1991). It has been adapted by the World Health Organization (WHO) and the American College of *Corresponding author. Tel.: +972-3-5303564; fax: +972-3- 7377002. E-mail address: dmoran@sheba.health.gov.il (D.S. Moran). 0306-4565/02/$-see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII:S0306-4565(02)00035-9