Physiological and perceptual responses of young adults to multiday indoor heat exposure during a simulated heatwave

Background: As climate change intensifies, extreme heat events (EHEs) are becoming more frequent and severe, posing increasing risks to human health. Although older adults are often considered most vulnerable, young adults may also be affected by prolonged heat exposure, which can disrupt sleep and impose cumulative thermal and cardiovascular strain. However, the physiological effects of multiday indoor heat exposure in young adults remain poorly understood. To address this gap, we investigated thermal, cardiovascular, and perceptual responses during a three-day simulated EHE, with daytime and nighttime temperatures of 36°C and 31°C, respectively. Hypothesis: We hypothesized that physiological strain, measured by core (rectal) temperature and heart rate, and thermal sensation would progressively increase over the three-day exposure, with the highest elevations occurring on the third day compared to the first. Methods: Sixteen habitually active young adults (8 females; median [IQR], 24 [21–28] y; VO2max, 45.1 [40.9-47.8] ml/kg/min) were housed in a studio-apartment-style environmental chamber for three consecutive days and two nights. Daytime temperatures were maintained at 36°C for 10 hours (09:00–19:00) and reduced to 31°C overnight (19:00–09:00), with 45% relative humidity. Participants were allowed free movement but no structured activity. Core temperature and heart rate were recorded continuously, and thermal sensation was assessed twice daily (9:00 and 19:00) using a 7-point scale (0 = neutral, 7 = extremely hot). Outcomes were analyzed using baseline-adjusted linear mixed-effects models (α = 0.05). Results: Core temperature increased significantly above pre-exposure baseline levels (mean (SD): 36.9°C (0.2)) on all three days (all, P < 0.001). End-of-day increases were similar across days, rising by 0.7°C (0.2), 0.7°C (0.2), and 0.8°C (0.1) on Days 1, 2, and 3, respectively (P > 0.93). Nighttime core temperature decreased by 0.5°C (0.2) on both nights (P > 0.84). Heart rate increased above baseline resting values (68 beats·min-1 (12)) by 15 (6), 16 (6), and 18 (9) beats·min-1 on Days 1, 2, and 3, with no significant day-to-day differences (P > 0.68). Nighttime heart rate decreased by 6 (12) and 7 (7) beats·min-1 on Nights 1 and 2. Thermal sensation increased similarly across days by 3 (1) units (P > 0.95). Conclusion: Although young adults experienced elevated thermal and cardiovascular strain during daylong heat exposure, these responses, and their subjective perception, did not worsen over three consecutive days. Nighttime cooling likely provided partial relief, preventing the accumulation of physiological burden. These findings suggest that young adults can tolerate short multiday indoor heat exposure. However, longer or more intense exposures, typical of EHEs, may pose greater risks, particularly in the absence of nighttime temperature reductions. Understanding these indoor heat dynamics is increasingly important as EHEs become more frequent and severe with climate change. Funding: Health Canada / Santé Canada (The views expressed herein do not necessarily represent the views of Health Canada) (held by Kenny GP). This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.