New research published in The Lancet Planetary Health reveals that older adults experienced persistent physiological strain during a simulated three-day heatwave, even when indoor environments overnight cooled to a recommended 26°C. The study challenges assumptions that indoor shelter provides adequate protection against prolonged extreme heat.
Simulating a Three-Day Heatwave at the University of Ottawa
Researchers at the University of Ottawa examined how older adults respond physiologically across both daytime heat exposure and overnight recovery during a simulated multi-day heatwave. The trial monitored 18 adults aged 67 to 84 from the Ottawa-Gatineau region, evenly split between women and men, inside a specialized studio-apartment-style climate chamber. These conditions reflect indoor temperatures recorded during real heat events in Canada and the United States.
Participants remained inside the chamber across three consecutive days and two nights. During the simulated heatwave conditions, the daytime temperature reached 34°C while the nighttime temperature was cooled to 26°C. In a separate comparison control session, the apartment stayed at 26°C day and night. The exposure sessions were randomized and separated by more than 10 days.
Persistent Physiological Strain Despite 26°C Overnight Cooling
While participants’ core body temperature did not continue climbing day after day, other markers of physical stress remained evident. On hot days, peak core body temperature averaged 37.8°C compared with 37.4°C in cooler conditions, and heart rates averaged five beats per minute higher. Blood tests also revealed lower blood pressure, reduced cardiac autonomic function, small declines in balance, and diminished cognitive test performance. Data gathered by the research team showed that absolute standing systolic blood pressure dropped by 27 mm Hg during heat wave exposure relative to the control condition (P < .0001).
Although lowering the indoor temperature to 26°C overnight allowed most responses to recover toward baseline, the recovery was incomplete. Elevated markers of kidney stress and lower morning systolic blood pressure persisted into the following mornings.
Multisystem Impacts and Health Canada Guidance
The study observed clear evidence of strain across multiple organ systems. The change in plasma volume suggested kidney stress, while intestinal fatty acid-binding protein concentrations indicated gastrointestinal distress with elevated levels during the heatwave simulation. Cognitive function was similarly impacted, with Stroop composite scores dropping on days two and three.
Most people assume that being indoors means being protected during a heatwave.
Glen Kenny, director of the Human and Environmental Physiology Research Unit at the University of Ottawa, via News-Medical
The most striking thing was what we didn’t see,
Glen Kenny via News-Medical
What we saw are consequences for their general physiology and their cognitive ability,
Glen Kenny via Medscape
The findings align with Health Canada’s guidance released in June 2026, which recommends maintaining an upper indoor temperature limit of 26°C for older adults during extreme heat. Researchers emphasize that 26°C serves as an upper limit to reduce risk rather than a guarantee of protection for every individual.
Broader Housing Implications for Vulnerable Populations
Experts note that many homes lack adequate air conditioning, leaving older residents exposed for days or weeks. Samir K. Sinha, a geriatrician and clinician scientist at Sinai Health System and University Health Network, pointed out that real-world patients often manage comorbidities and take medications that increase their vulnerability.
Sinha co-chaired an emergency preparedness project with the Canadian Red Cross and National Institute on Aging that called for clear thresholds for temperature regulation in housing for older adults during warmer months. Financial backing for the research was provided by the Canadian Institutes of Health Research alongside Health Canada’s Climate Change and Health Capacity Building Program.
