Greater perceptual, but not thermophysiological, responses to heat exposure in heat-sensitive compared with heat-resilient individuals

Inter-individual vulnerability to environmental heat stress varies considerably, yet in healthy humans, the mechanisms underlying individual differences in heat tolerance and symptom severity remain poorly understood. Therefore, thermophysiological, perceptual, orthostatic, and motor-cognitive responses were evaluated in healthy individuals rating themselves to be either heat-sensitive (n = 22) or heat-resilient (n = 23). Participants completed a standardized 4-h heat exposure protocol where outcomes were assessed during 1.5-h passive heat exposure (supine rest in 40°C; 12% humidity; wet-bulb globe temperature 25°C), after simulated sunlight (total UV exposure equal to 3 standard erythema doses and total radiant heat approximately 430 W·m−2; raising wet-bulb globe temperature to 36°C), and during 30 min of mild exercise (1.5 W·kg−1). Heat-sensitive participants reported greater thermal discomfort already 5 min into the exposure protocol and substantially higher discomfort following simulated sunlight and exercise (group average 84% vs 50% in the heat-resilient group, p < .001). The heat-sensitive group also experienced a slightly larger mismatch between water intake and losses and reported lower ability to concentrate and reduced alertness at the end of the exposure. In contrast, body temperatures and objective motor-cognitive performance were similar between groups. Severe dizziness (rating ≥ 75%) during supine-to-stand orthostatic testing occurred in one-third of all participants, with no differences between groups in prevalence or severity. In conclusion, self-rated ability to tolerate hot environments was predictive of perceptual responses such as thermal discomfort and alertness, despite similar thermophysiological strain and without difference in the occurrence of severe dizziness or impairments in motor-cognitive performance.