Comparison and validation of heat stress indices in experimental studies

The present study aimed at (1) deriving the best methodology for using heat stress indices in fluctuating ambient conditions, (2) assessing various ways for improving the prediction ability of the Required Sweat Rate index (SW(req)). The data base included the results from five experimental series involving 32 volunteers; these series involved environmental variations at constant metabolic rate, work-rest cycles in constant ambient conditions and alternate periods of work in heat and rest in neutral conditions. Observed and predicted variations in sweat rate were compared. During exposures to rapid changes in climatic conditions, the best prediction of the body sweat loss was provided by a model involving the simulation of the response of skin temperature and sweat rate to a step change in ambient conditions. The model used exponential weighting algorithms with time constants of 3 and 10 minutes respectively for these two variables. The study showed also that the prediction accuracy of SW(req) index can be significantly improved by adopting a new expression for the calculation of the evaporative efficiency of sweating, by predicting the mean T(sk) value as a function of the parameters of the work situation and by applying the exponential weighting to the predicted values of sweat rate. Among the various heat stress indices tested in the study, the Required Sweat Rate Index gave the best approximation of the body sweat loss