Laboratory heatwave simulations can help researchers assess how heat exposure affects vulnerable groups like older adults and evaluate cooling strategies to reduce those effects. This new study is the first controlled trial to examine how older adults physiologically respond across both the daytime heat exposure and overnight recovery periods of a simulated multi-day heatwave.
The trial involved 18 adults between the ages of 67 and 84 from the Ottawa-Gatineau region, with equal numbers of women and men. Participants remained in and were monitored within a specialized studio-apartment-style climate chamber at the University of Ottawa across three consecutive days and two nights.
“For the first time, we've been able to follow older adults through a full three-day heatwave, including what happens during overnight recovery,” explains Glen Kenny, Full Professor at uOttawa’s Faculty of Health Sciences and Director of the Human and Environmental Physiology Research Unit. “What we saw is that physiological strain persisted across each day of the heatwave, and some effects did not fully resolve overnight, even when the apartment cooled to 26°C. This tells us that cooling only at night may not be enough to fully protect older adults from the effects of repeated daytime overheating occurring during an extreme heat event.”
During the simulated heatwave, the apartment was kept at 34°C during the day and cooled to 26°C overnight. These conditions 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 throughout the day and night.
On the hot days, participants’ peak core body temperature averaged 37.8°C, compared with 37.4°C under the cooler conditions. Their heart rates were, on average, five beats per minute higher, while blood tests showed higher circulating blood markers of kidney and gastrointestinal stress.
Researchers also observed lower blood pressure and cardiac autonomic function, along with small declines in balance and cognitive test performance.
Although cooling the apartment to 26°C overnight allowed most physiological responses to recover to near-baseline levels, the recovery was not complete. Lower morning systolic blood pressure and elevated markers of kidney stress remained evident the following mornings.
“For the first time, we've been able to follow older adults through a full three-day heatwave, including what happens during overnight recovery”
Glen Kenny
— Full Professor, Faculty of Health Sciences & Director of the HEPRU
Being indoors does not always mean being safe
Many Canadian homes do not have air conditioning, leaving some older adults vulnerable to high indoor temperatures during extreme heat events. The findings show that remaining indoors does not necessarily protect older adults from heat-related physiological strain when indoor temperatures remain elevated.
"Most people assume that being indoors means being protected during a heatwave. Our findings show that is not necessarily the case," says Professor Kenny. "Homes and apartments without adequate cooling can remain hot for days during an extreme heat event, exposing older adults to prolonged heat stress. Maintaining indoor temperatures at or below the recommended upper limit of 26°C throughout the day and night can substantially reduce physiological strain in older adults during extreme heat.”
The results reinforce Health Canada's recently released guidance (June 2026) recommending an upper indoor temperature limit of 26°C for older adults during extreme heat, informed in part by the uOttawa research team’s earlier work examining the effects of a single day of heat exposure.
The new study builds on that evidence by examining physiological responses across repeated daytime heat exposure and overnight recovery during a simulated multi-day heatwave in an overheated home environment, typical of what can occur in homes without air conditioning during an extreme heat event.
"Our earlier work helped inform Health Canada's 26°C recommendation, and this study provides further evidence supporting the importance of maintaining indoor temperatures at or below 26°C throughout a sustained heat event,” adds Professor Kenny. “Importantly, 26°C is an upper indoor temperature limit intended to reduce heat-related risk; it should not be interpreted as a temperature that guarantees protection for every older adult.”
Core body temperature does not tell the whole story
One result surprised the researchers: participants’ core body temperatures did not continue to rise from one day to the next, even though other signs of physiological strain were evident, with some persisting into the following morning.
“The most striking thing was what we didn't see," states Professor Kenny. "Body temperature didn't worsen day over day, but that did not mean the body had fully recovered. Blood pressure remained lower, and markers of kidney stress remained elevated the following morning. The lesson is that we can't judge the physiological effects of heat by temperature alone or assume that a stable body temperature means an older adult is fully protected from the effects of prolonged heat exposure.”
Older adults may still experience cardiovascular and kidney-related strain even when their core temperature remains relatively stable from one day to the next. No adverse events were reported during the study.
The study was funded by Health Canada's Climate Change and Health Capacity Building Program and the Canadian Institutes of Health Research.
The study, titled “Effects of a 3-day simulated heatwave on physiological and functional outcomes in older adults in Canada: a single-site randomised crossover trial”, was published in The Lancet Planetary Health.
Authorship: Robert D Meade, Kelli E King, James J McCormick, Sarah M Taggart, Gil Bourgois, Nicholas Goulet, Caroline Li-Maloney, Pierre Boulay, Ronald J Sigal, Glen P Kenny. DOI: 10.1016/j.lanplh.2026.101517