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A University of Ottawa controlled trial published in The Lancet Planetary Health found that older adults exposed to simulated heat wave conditions showed physiological strain, including kidney stress markers and lower blood pressure, that persisted overnight even when the room cooled to 26°C (79°F). The findings support Health Canada’s guidance that indoor temperatures should stay at or below 26°C day and night during extreme heat.
A controlled laboratory trial from the University of Ottawa, published in The Lancet Planetary Health, has found that cooling a home overnight may not fully protect older adults from the physiological effects of repeated daytime indoor overheating during a multiday heat wave. The study is described by its authors as the first controlled trial to examine how older adults respond physiologically across both the daytime heat exposure and overnight recovery periods of a simulated heat wave, and its results support Health Canada’s recommendation of an indoor upper limit of 26°C (79°F) during extreme heat.
The trial involved 18 adults aged 67 to 84 from the Ottawa–Gatineau region, with equal numbers of women and men. Participants stayed in a studio-apartment-style climate chamber at the University of Ottawa for three consecutive days and two nights. During the simulated heat wave, the chamber was held at 34°C (93°F) during the day and cooled to 26°C (79°F) overnight — conditions the researchers said reflect indoor temperatures recorded during real heat events in Canada and the United States. In a separate comparison session, the apartment remained at 26°C around the clock.
On the hot days, participants’ peak core body temperature averaged 37.8°C (100.0°F), compared with 37.4°C (99.3°F) under cooler conditions. Heart rates were on average five beats per minute higher, and blood tests showed higher circulating markers of kidney and gastrointestinal stress. The researchers also observed lower blood pressure and reduced cardiac autonomic function, along with small declines in balance and cognitive test performance. The study authors reported no adverse events during the research.
Although overnight cooling to 26°C allowed most physiological responses to recover to near-baseline levels, recovery was not complete, according to the findings. Lower morning systolic blood pressure and elevated kidney stress markers remained evident on the mornings following hot days.
Indoor Safety Assumptions Challenged
The findings matter because many Canadian homes lack air conditioning, leaving some older adults exposed to high indoor temperatures during extreme heat events. The study indicates that simply staying indoors does not guarantee protection when indoor temperatures remain elevated. “Most people assume that being indoors means being protected during a heat wave. Our findings show that is not necessarily the case,” said Glen Kenny, the study’s senior researcher and director of uOttawa’s Human and Environmental Physiology Research Unit.
The results reinforce Health Canada guidance released in June 2026 recommending an upper indoor temperature limit of 26°C for older adults during extreme heat. Kenny said maintaining indoor temperatures at or below 26°C throughout the day and night can substantially reduce physiological strain in older adults during extreme heat, though he cautioned that 26°C is an upper limit intended to reduce risk, not a temperature that guarantees protection for every individual.
Building on Earlier Heat Exposure Research
The study, led by Robert D. Meade and colleagues, builds on earlier work by the same uOttawa research team, which examined the effects of a single day of heat exposure in older adults and helped inform Health Canada’s 26°C recommendation. The new trial extends that evidence by examining repeated daytime exposure combined with overnight recovery, replicating conditions typical of homes without air conditioning during a sustained heat event. Laboratory simulations of this kind allow researchers to assess how heat affects vulnerable groups under controlled conditions and to evaluate cooling strategies. The paper is titled “Effects of a 3-day simulated heatwave on physiological and functional outcomes in older adults in Canada: a single-site randomised crossover trial.”
“What we saw is that physiological strain persisted across each day of the heat wave, and some effects did not fully resolve overnight, even when the apartment cooled to 26°C (79°F). This tells us that cooling only at night may not be enough to fully protect older adults.”
— Glen Kenny, professor, University of Ottawa Faculty of Health Sciences
Limits of the Laboratory Findings
The trial involved a small sample of 18 participants at a single site, which limits how broadly the results can be generalized. The simulated conditions, while based on real indoor temperatures recorded during heat events, may not capture every variation in home environments, individual health status, or behaviour during actual heat waves. The researchers also noted an unexpected result: core body temperatures did not rise progressively from one day to the next, even though other markers of strain persisted — meaning the full relationship between temperature stability and internal physiological strain is not yet completely understood. It remains unclear how these findings translate to older adults with pre-existing medical conditions or to longer heat events beyond three days.
Implications for Heat Health Guidance
The researchers say the study provides further evidence supporting the importance of keeping indoor temperatures at or below 26°C throughout sustained heat events, not only overnight. The findings may inform future public health messaging about daytime cooling strategies for older adults, including access to air-conditioned spaces during the hottest hours. The team suggests continued research is needed on how prolonged and repeated heat exposure affects vulnerable populations and on which cooling interventions best reduce cumulative physiological strain.
Key Questions
Does cooling my home at night protect an older adult from heat effects?
Not fully, according to the study. Even when the apartment cooled to 26°C (79°F) overnight, some effects of daytime overheating — lower systolic blood pressure and elevated kidney stress markers — were still present the next morning.
What indoor temperature does Health Canada recommend for older adults during extreme heat?
Health Canada guidance released in June 2026 recommends keeping indoor temperatures at or below 26°C (79°F) for older adults during extreme heat, throughout both day and night. The study authors note this is an upper limit to reduce risk, not a guarantee of protection.
How was the study conducted?
Eighteen adults aged 67 to 84 stayed in a climate chamber at the University of Ottawa for three days and two nights at 34°C during the day and 26°C overnight, with a comparison session at 26°C around the clock. Physiological measurements and functional tests were taken throughout.
Did participants’ body temperatures keep rising during the heat wave?
No. Core body temperature did not worsen from one day to the next, but other signs of strain — including lower blood pressure and elevated kidney stress markers — persisted, meaning a stable temperature does not indicate full recovery.
Were participants harmed during the trial?
The researchers reported no adverse events during the study.
Source: rss
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