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In a controlled trial of 18 adults ages 67 to 84, researchers found that repeated daytime exposure to 34°C indoor heat produced physiological strain that did not fully resolve overnight when the room cooled to 26°C. The study supports guidance to keep indoor temperatures at or below 26°C during extreme heat, but that limit does not guarantee protection for every older adult.
Some signs of heat-related physiological strain persisted overnight in older adults after repeated daytime exposure to high indoor temperatures, according to a University of Ottawa study that simulated a three-day heat wave. The findings suggest that cooling a home only at night may not fully offset daytime overheating, a concern for older people living in homes without adequate cooling.
The trial included 18 adults ages 67 to 84, with equal numbers of women and men, from the Ottawa–Gatineau region. Participants spent three consecutive days and two nights in a studio apartment-style climate chamber at the University of Ottawa. On heat-exposure days, the chamber reached 34°C (93°F) during the day and was cooled to 26°C (79°F) overnight. During a separate comparison session, it stayed at 26°C throughout.
At peak, participants’ average core body temperature was 37.8°C (100.0°F) in the hot conditions, compared with 37.4°C (99.3°F) in the cooler session. Their heart rates averaged five beats per minute higher during the hot days. Blood tests showed higher circulating markers of kidney and gastrointestinal stress; researchers also observed lower blood pressure and cardiac autonomic function, plus small declines in balance and cognitive test performance.
Cooling overnight allowed most measured responses to return close to baseline, but recovery was incomplete. Lower morning systolic blood pressure and elevated kidney-stress markers remained evident the following mornings. The researchers reported no adverse events during the study. The findings were published in The Lancet Planetary Health as a single-site randomised crossover trial by Robert D. Meade and colleagues.
Why Daytime Cooling Matters
The results matter because indoor heat can remain a risk during a heat wave, particularly in homes without air conditioning. The researchers said many Canadian homes lack air conditioning, and that indoor temperatures can stay high for days during extreme heat. Staying inside, by itself, may not prevent physiological strain when the home remains overheated.
The study reinforces Health Canada guidance released in June 2026 recommending an upper indoor temperature limit of 26°C (79°F) for older adults during extreme heat. That temperature is intended to reduce heat-related risk; the researchers stressed that it does not guarantee protection for every older person. The trial points to the value of keeping indoor spaces cool during the day as well as at night.
The findings also caution against using one measure, such as body temperature, to judge the effects of sustained heat exposure. Participants’ core temperatures did not rise further from one hot day to the next, yet some other indicators remained altered the following morning. A stable reading therefore did not mean that all measured responses had recovered.
From One Hot Day to Three
The study builds on earlier work by the University of Ottawa research team examining older adults’ responses to a single day of heat exposure. That earlier research helped inform the 26°C indoor upper limit in Health Canada’s June 2026 guidance, according to Glen Kenny, the study’s senior researcher.
The new trial extended the observation period to repeated daytime heat and overnight recovery. Its chamber conditions were selected to reflect indoor temperatures recorded during real heat events in Canada and the United States, the report said. Researchers could monitor participants under controlled conditions while comparing the simulated heat-wave schedule with a consistently cooler environment.
Kenny, a professor in the University of Ottawa’s Faculty of Health Sciences and director of its Human and Environmental Physiology Research Unit, said the work followed older adults through a full three-day simulated heat wave, including overnight recovery. The team said the controlled setting can help researchers study how heat affects vulnerable groups and assess cooling strategies.
““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).””
— Glen Kenny, University of Ottawa professor and study researcher
What the Trial Cannot Establish
The study involved 18 participants at one site, so the results describe the responses measured in this small group under controlled conditions. The source report does not establish how frequently the same effects occur across the wider older adult population, or how individual health conditions and living situations may change responses.
The trial compared a simulated heat-wave schedule with a consistently cooler chamber, but it did not test every possible way of cooling a home or measure whether particular interventions prevent illness during real-world heat waves. The report also does not specify how long the remaining changes persisted beyond the following mornings. Researchers said the 26°C limit is intended to reduce risk, not assure safety for each person.
Further Heat and Cooling Research
The researchers say laboratory simulations can help evaluate how older adults respond to heat and test cooling strategies. The next questions include how physiological effects vary across a broader range of people and whether practical ways of keeping homes cool through the day reduce strain during real heat events.
For now, the study adds evidence behind Health Canada’s 26°C upper indoor limit during extreme heat while underscoring that overnight cooling alone may not fully restore measured responses after repeated hot days. The report does not set out a new guideline or a guarantee of protection; it identifies the need to account for daytime indoor temperatures and continued monitoring of heat effects.
Key Questions
What did the study find?
In a controlled simulation involving 18 adults ages 67 to 84, researchers found that some signs of physiological strain after repeated hot days remained the next morning, even after the room cooled overnight to 26°C.
What temperatures did the researchers test?
During the simulated heat wave, the chamber reached 34°C (93°F) by day and cooled to 26°C (79°F) at night. In a comparison session, it remained at 26°C around the clock.
Did participants’ body temperatures keep rising each day?
No. Their core temperatures did not worsen day over day, but some other measures had not fully recovered. The researchers pointed to lower morning systolic blood pressure and elevated kidney-stress markers.
Does 26°C guarantee protection for older adults?
No. The study researcher described 26°C as an upper indoor limit intended to reduce risk, not a temperature that guarantees protection for every older adult.
Where was the research published?
The trial by Robert D. Meade and colleagues was published in The Lancet Planetary Health in 2026.
Source: rss
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