Influence of intermittency and static components of work on heat stress

This paper summarizes the results of a series of experiments established to examine the effects of the composition and pattern of physical workload on human heat tolerance and to compare the predictive abilities of a selection of prominent heat stress indices. The experiments were carried out in a climatically controlled chamber and involved mixtures of dynamic (treadmill walking) and static (weight holding) activities as appropriate. The average physiological responses to intermittent dynamic workloads did not significantly differ from those to continuous dynamic work of the same average intensity. However, the average values masked periods of peak physiological loading. The inclusion of a static load component had a negligible effect on the average level of physiological thermal strain. As a consequence, the presence or absence of a static work component did not influence the predictive abilities of the five heat stress indices examined. It was concluded that, where peak loading was unlikely to be a problem, time weighted averaging of workload was a reasonable approach in assessing industrial thermal strain and that heat stress criteria, derived originally for continuous dynamic workloads, were likely to be equally appropriate for intermittent combined workloads. However, care should be taken to ensure that peak loading did not present a health risk to industrial populations carrying out intermittent work in hot environments.