Prediction of strain for intermittent heat exposures

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Abstract
Under intermittent heat exposures, predictions of body sweat loss ()Δm) could be derived either from the ambient conditions or from their time-weighted average. To compare the accuracy of these two procedures, five subjects exercised, semi-nude, for 120min at constant work load (50 W) under six environmental conditions: four conditions with 20 min duration ‘square pulse(rsquo; variations in either air temperature (T a or ambient vapour pressure (P a, and two conditions with constant T a and P a, levels. Selected ambient conditions involved skin wettedness (w) levels ranging from 0·35 up to 1. The recorded Δm were compared to the values predicted (PΔm) using the ‘Analytical determination of thermal stress’ model (ISO-DP 7933). Results showed that PΔm were lower than observed Δm by 2 to 10% depending on the condition. This trend could mainly be ascribed to the proposed relation between the evaporative efficiency of sweating and wettedness levels. For exposures consisting of T a variations, the prediction error was not markedly different whether a time-weighted average of the ambient variables was used or not For exposures consisting of P a, variations, the expected dripping of sweat during the humid exposures was limited by the short duration of these periods and the averaging procedure gave the best prediction of the actual sweat loss. It is concluded that under intermittent heat stress conditions, a time-weighted average of the ambient conditions may be used for thermal strain predictions, provided that the heat exposure periods are of short duration.

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