Hot occupational exposures: a corrected maximum exposure period to allow for sweating onset

The major defence available to the body in high-temperature occupational exposures is the evaporation of sweat and the calculation of the evaporation rate forms a central part of the assessment procedure for determining a safe working pattern. This calculation is conventionally carried out on the basis of a steady-state sweat rate, which neglects the gradual onset of sweating at the beginning of an exposure. The maximum exposure period determined on this basis will overestimate the safe working time. This paper presents the background to a correction, which can be applied to such a conventionally-determined maximum exposure period to take account of sweating onset. In an occupational hot exposure situation, the assessment of a safe working period is critically dependent on an accurate evaluation of the body energy loss by sweat evaporation. Numerical assessment methods conventionally calculate this component on the assumption of a step change to the steady-state value immediately on commencement of an exposure. As sweating onset is a gradual process, this can lead to a serious overestimation of the accumulated sweat loss and safe exposure period. The outcome of the accompanying paper is a numerical relationship which allows the maximum exposure period calculated on a steady-state basis to be corrected to take account of sweating onset. The form of this correction shows that its application is particularly important in short exposures with a high evaporative loss.