Predicted sweat rate in fluctuating thermal conditions.

The ISO 7933 Required Sweat Rate Index is inappropriate under conditions of fluctuating exposure. Time weighted average values can introduce large underestimations as, for hot conditions, the relationship between physical stress and physiological strain is not linear. Mean skin temperature and sweat rate vary as a first order system, that is, according to an exponential averaging system, in response to a step function of the primary parameters. A very simple algorithm has been derived to take into account this exponential averaging and to predict the sweat rate at any time, taking into account the past exposure of the subject. It is based on time constants of 3 min for the mean skin temperature and of 25 min (up to a sweat rate of 50 W.m-2) and 10 min for the sweat rate. The correlation coefficient is 0.780 between observed and predicted sweat rates in independent experiments used for the validation of the model. This emphasises the necessity for exponential averaging as differences between individuals are very important and the precision of the observed data in these experiments was lower than for laboratory experimental data.