Heat stress impairment of mental performance: a revision of tolerance limits

A time-related, heat stress tolerance curve for unimpaired mental performance was constructed by Wing from a summary of 15 studies. The tolerance limits, more properly described as the lower limits for heat impaired mental performance, were subsequently adopted by the National Institute for Occupational Safety and Health as the recommended standard of tolerance times for sedentary work in heat stress. Although Ramsey and Morrissey have reported a series of isodecrement curves which indicate that mental performance impairment in heat may not be a simple function, a reappraisal of the upper limit for unimpairment has not yet been advanced. The present review reevaluates results of early studies, apparently supportive of Wing's position, and proposes an alternate interpretation. Further, analysis of more recent data suggests a mental performance impairment/heat stress relationship closely related to human thermophysiological tolerance limits.