Physiological responses to 9 hours of heat exposure in young and older adults. Part III: Association with self-reported symptoms and mood-state.

Older adults are at greater risk of heat-related morbidity and mortality during heat waves, which are commonly linked to impaired thermoregulation. However, little is known about the influence of increasing age on the relation between thermal strain and perceptual responses during daylong heat exposure. We evaluated thermal and perceptual responses in 20 young (19-31 years) and 39 older adults (20 with hypertension and/or type 2 diabetes; 61-78 years) resting in the heat for 9 hours (heat index: 37°C). Body core and mean skin temperature areas under the curve (AUC, hours 0-9) were assessed as indicators of cumulative thermal strain. Self-reported symptoms (68-item environmental symptoms questionnaire), and mood-disturbance (40-item profile of mood states questionnaire) were assessed at end-heating (adjusted for pre-scores). Body core temperature AUC was 2.4°C·h [1.0, 3.7] higher in older relative to young adults (P<0.001), while mean skin temperature AUC was not different (-0.5°C·h [-4.1, 3.2] P=0.799). At end-heating, self-reported symptoms were not different between age groups (0.99-fold [0.80, 1.23], P=0.923), with or without adjustment for body core or mean skin temperature AUC (both P≥0.824). Mood-disturbance was 0.93-fold [0.88, 0.99] lower in older, relative to young adults (P=0.031). Older adults with and without chronic health conditions experienced similar thermal strain, yet those with these conditions reported lower symptom scores and mood disturbance compared to young adults and their age-matched counterparts (all P≤0.026). Although older adults experienced heightened thermal strain during the 9 h heat exposure, they did not experience greater self-reported symptoms or mood disturbance relative to young adults.