Introduction: Extreme heat events are becoming increasingly frequent, intense, and prolonged, now occurring earlier in spring and persisting later into fall. While physiological responses to prolonged heat exposure in older adults are increasingly studied, most research is conducted during cooler seasons, limiting understanding of seasonal influences. Older adults face elevated heat-related mortality during late-spring and early-summer heatwaves, yet the impact of seasonal transitions on physiological strain, measured via core temperature and heart rate, remains poorly understood. To address this gap, this case study monitored core temperature, heart rate, and thermal sensation in an older adult during a simulated multiday heatwave across five successive seasons over one year. Hypothesis: We hypothesized that physiological strain and thermal sensation would be highest during the winter-to-spring transition, when heat acclimatization is minimal, and lowest during the summer-to-fall period, when residual heat acclimatization is likely to persist. Methods: A physically active older male (61 y; VO 2 max = 40.7 mL/kg/min) completed five separate three-day, three-night exposures in a studio-apartment-style environmental chamber simulating indoor temperatures recorded in dwellings in British Columbia, Canada during the 2021 heatwave (PMID: 35169667). Daytime temperatures were 40°C (09:00–19:00) and nighttime temperatures 36°C (19:00–09:00), with 45% relative humidity. No structured activity was allowed, but free movement was permitted. Core (rectal) temperature and heart rate were continuously monitored, and thermal sensation (7-point scale; 0=neutral, 7=extremely hot) was recorded twice daily (09:00 and 19:00). Results: Baseline core temperature averaged 36.3°C across seasons (36.2–36.5°C). During each three-day exposure, core temperature increased daily by an average of 1.5°C (summer), 1.2°C (fall), 1.0°C (winter), 2.2°C (spring), and 1.2°C (subsequent summer), with the largest single-day increase observed on Day 2 of the spring exposure (+3.2°C, reaching 39.7°C). Baseline heart rate averaged 44 beats·min - ¹, rising daily by 33, 31, 31, 37, and 35 beats·min - ¹ across the five exposures. Thermal sensation rose similarly across seasons (+4 units), except during spring (+5 units). Nighttime was associated with average nightly decreases in core temperature of 0.4, 0.4, 0.1, 0.7, and 0.3°C and heart rate of 14, 7, 4, 4, and 24 beats·min - ¹ across the seasonal exposures. Conclusion: Physiological strain and thermal sensation during multiday indoor heat exposure were generally consistent across seasons except in late spring, when core temperature reached potentially dangerous levels. While heat resilience is largely maintained year-round, vulnerability increases during the winter to spring transition. These findings highlight the need for larger studies on seasonal effects in older adults and the value of controlled heat wave simulations for guiding public health strategies. 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.
Kenny et al. (Fri,) studied this question.