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Life sciences · Journal article
The Lancet. Planetary Health
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Laboratory-based heatwave simulations are a valuable tool for evaluating physiological and functional effects of heat exposure in vulnerable population groups (eg, older adults) and for testing the efficacy of personal cooling strategies. However, previous studies have been short in duration (2-9 h) and therefore have not captured the cumulative, multiday exposures characteristic of real-world heatwaves. We aimed to investigate physiological and functional outcomes in older adults exposed to a 3-day simulated heatwave. We conducted a single-site, randomised, crossover trial at the Human and Environmental Physiology Research Unit at the University of Ottawa, ON, Canada. Participants were eligible if they lived in the Ottawa-Gatineau region, were aged 65-85 years, did not smoke, spoke English or French, and could provide informed consent. Exclusion criteria included severe physical restrictions, use of medication that made participation inadvisable, or cardiac abnormalities. Enrolled participants were exposed for 3 days (and 2 nights) to a simulated heatwave (34°C during the day and 26°C during the night with 45% relative humidity) and a thermoneutral control condition (26°C during both the day and night with 45% relative humidity) in a specialised climate chamber replicating a studio apartment. The two exposure sessions (ie, heatwave and control conditions) were separated by more than 10 days, and the order in which participants were assigned to each session was randomly allocated using a key generated in Microsoft Excel. Peak 1-h core (rectal) temperature was the primary outcome and was compared across days and conditions using linear mixed-effects models. Participants and investigators could not be masked to the exposure condition, but body temperature and heart rate data were processed by a masked analyst. All randomly assigned participants were included in the analyses; missing data were not imputed. The trial is registered with Clinicaltrials.gov (NCT06633302) and is complete. Between Oct 15, 2024, and Feb 19, 2025, 22 individuals were screened and 18 eligible participants were enrolled; the median age of all participants was 74 years (IQR 71-78), nine (50%) of the 18 participants were female, and nine (50%) were male. All participants completed both intervention periods. Daily peak core temperature was 0·4°C (95% CI 0·3-0·4) higher during the simulated heatwave exposure than during the control exposure (cross-day average: 37·8°C [SD 0·2] vs 37·4°C [0·2]; p<0·0001). No adverse events occurred. Older adults (aged 65-85 years) exposed to a simulated 3-day heatwave showed persistent daytime elevations in core temperature, highlighting the importance of evidence-based cooling strategies and other interventions to minimise physiological strain during even moderate heat stress, and not just during the most extreme conditions. Health Canada and Canadian Institutes of Health Research.