In several 100 climatic chamber experiments young healthy acclimatized men were exposed successively to combinations of ambient temperature and air humidity that were varied systematically from day to day within the range of 15 to 57° and S to 97% relative humidity, with air speed kept constant at 0.3m/s.The subjects who were nearly nude were exposed at rest and at different treadmill work levels (metabolic rates up to about 1350kJ/h). Each exposure lasted between two and six h. It was found that body temperatures and heart rate, but not loss of sweat, were suitable criteria for indicating equal stress conditions. Equations were derived describing those climatic combinations which were equi-valent at the various activity levels of each subject. The slope of the respective equivalence curves plotted in psychrometric charts varied systematically with work level. The curves obtained for various sub-jects at any given activity level showed similarities, so that the calculation of an equivalence system common to all subjects seemed to be justified. A detailed comparison of these results with previous indices is presented. The combinations of ambient temperatures and humidities that were found to be equivalent under the given conditions of physical activity corresponded particularly well with the Index of Physiological Effect (ROBINSON et al.). There was also good agreement with the P4SR Index (MCARDLE et al.).The combinations corresponded with other indices (Heat Stress Index of BELDING and HATCH, Index of Thermal Stress of GIVONI) and with Basic Effective Temperature and Wet-bulb Globe Temperature only within limited ranges of climate depending upon work level. A procedure is presented which allows one to evaluate in terms of physiological responses how far various indices describe equivalent stress conditions correctly.

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