Response to intermittent work in hot environments. Final report on CEC Contract 7247/22/003

Hot workplaces in coalmines are characterised by warm air with high ambient humidity. Although such conditions have not been a significant problem in Britain, it has been predicted that problems of heat and humidity will become more prevalent with the current trend towards deeper and more highly mechanised workings.A number of heat stress criteria have been published with the aim of preventing acute heat illness in industrial workers. Most of these criteria were originally derived from physiological studies involving extended periods of dynamic work in stable thermal environments. Continuous dynamic workloads are rarely found in modern industry however, physical workload usually being of an intermittent nature. For example, physical work in modern mining is characterised by relatively short periods of combined dynamic and static work. While “”steady state”” criteria may be applied to intermittent work patterns by calculating hourly time weighted averages of metabolic load, doubts have been expressed concerning the physiological validity of this procedure. In particular, the inherent assumption that work pattern has no overall effect on metabolic heat load would appear to be based on limited physiological evidence.The nature of work may also be significant. It has been shown that the pattern and magnitude of cardiorespiratory responses differ in dynamic and static muscular work. Little is known, however, about the effects of static work on thermoregulatory responses.This project was established in order to fill some of the gaps in knowledge by examining the effects of the composition and pattern of physical workload on human heat tolerance. By undertaking a systematic study of these factors, the need for modifications to existing heat stress criteria could be assessed. A supplementary aim of the project was to compare the predictive abilities of prominent thermal indices such as ET(A) and WBGT using the intermittent combined workloads which are characteristic of modern industry.The average physiological responses to intermittent dynamic workloads, of a frequency and intensity comparable to those encountered in modern mining, did not significantly differ from those to continuous dynamic work of the same average intensity. However, the average values concealed the inevitable high degree of intersubject variability in rsponse (Williams, 1956) and also 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 the intermittent combined workloads typical of modern mining tasks. 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.Of the indices examined, ET(A), already widely used in the UK mining industry, was found to be the most reliable indication of thermal strain. “”