EFFECTS OF MODERATE HEAT-STRESS ON MENTAL PERFORMANCE

Moderate heat stress is believed to affect mental performance by lowering levels of arousal. Conscious effort can counteract this defect. In most experiments, raised temperatures are perceived at the start by subjects and can act as a stimulus to exert conscious effort. In practice, temperatures usually rise slowly and may therefore have a more marked effect. Thirty-six male and 36 female 17-yr-old subjects in standard cotton uniforms (0.7 clo) were exposed in groups of 4 in a climate chamber to rising air-temperature conditions typical of occupied classrooms, in the range 20-29°C. The maximum rate of rise was 4°C/hr. Each group performed mental work during 3 successive periods of 50 min with 10-min breaks between. During each break the air temperature was reduced by 3°C. Sentence comprehension was significantly reduced by intermediate levels of heat stress in the third hr. A multiplication task was performed significantly more slowly in the heat by male subjects, showing a minimum at 28°C. Recognition memory showed a maximum at 26°C, decreasing significantly at temperatures below and above, and an independent measure of degree of certainty in recall showed a maximum at 27°C. These findings are in accordance with the hypothesis of reduced arousal in moderate heat stress in the absence of conscious effort. It is concluded that moderate heat stress, only a few degrees centigrade above the optimum, has a marked effect on mental performance when temperatures rise slowly. Tasks demanding concentration and clear thinking are adversely affected, but memory and cue utilization can be improved by temperatures up to 26°C, declining rapidly thereafter in the case of male subjects. In hot weather, work involving concentration should therefore be carried out early in the day. Memory and tasks requiring an increased breadth of attention can, with advantage, be postponed until the temperature has risen, provided that it does not rise above 26°C. The positive and negative effects of these moderate levels of heat stress are likely to be greater for men than for women.