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In a controlled study of 18 adults ages 67 to 84, researchers found that some signs of physiological strain remained the morning after repeated daytime heat exposure, even when the test apartment cooled overnight to 26°C. The study supports efforts to keep indoor temperatures at or below that level during heat events, but the researchers caution that it is an upper limit, not a guarantee of protection.
A controlled University of Ottawa study found that some signs of physiological strain persisted into the morning in older adults after repeated hot days indoors, despite overnight cooling to 26°C (79°F). The small trial, involving 18 adults ages 67 to 84, suggests that nighttime cooling alone may not fully reverse the effects of daytime heat exposure during a multiday heat wave.
Researchers monitored participants in an apartment-style climate chamber over three consecutive days and two nights. During the simulated heat wave, the chamber reached 34°C (93°F) by day and cooled to 26°C overnight. In a comparison session, it remained at 26°C both day and night. The researchers said the test temperatures reflect indoor conditions recorded during real heat events in Canada and the United States.
On hot days, participants’ average peak core temperature was 37.8°C (100.0°F), compared with 37.4°C (99.3°F) in the cooler condition. Their heart rates averaged five beats per minute higher. Blood tests showed higher markers associated with kidney and gastrointestinal stress; researchers also observed lower blood pressure and cardiac autonomic function, along with small declines in balance and cognitive test performance.
Most measured responses moved back toward baseline overnight, but recovery was not complete. Lower morning systolic blood pressure and elevated kidney-stress markers remained evident on the following mornings. The study reported no adverse events. Its results were published in The Lancet Planetary Health as a single-site randomized crossover trial.
Why Overnight Cooling May Fall Short
The findings matter because older adults may spend a heat wave inside homes that stay hot, particularly where air conditioning is unavailable or inadequate. The study shows that being indoors does not necessarily prevent heat strain if indoor temperatures remain high. It also suggests that a stable core temperature should not be treated as proof that the body has fully recovered.
The researchers say keeping indoor temperatures at or below 26°C throughout the day and night can substantially reduce physiological strain. That figure is an upper temperature limit, not a guarantee of safety for every older adult. The trial measured short-term physiological and functional outcomes under controlled conditions; it does not establish how often the observed changes lead to illness in everyday settings.
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A Trial Extending Earlier Heat Research
The University of Ottawa team describes this as the first controlled trial to track older adults through both daytime exposure and overnight recovery across a simulated multiday heat wave. Earlier work by the group examined a single day of heat exposure. According to the researchers, that previous research helped inform Health Canada’s June 2026 guidance recommending an upper indoor temperature limit of 26°C for older adults during extreme heat.
The new trial extends that line of research by testing repeated daytime overheating followed by cooler nights. Glen Kenny, a study author and director of the university’s Human and Environmental Physiology Research Unit, said the study adds evidence for maintaining the recommended limit across a sustained heat event. The trial included equal numbers of women and men, all recruited from the Ottawa–Gatineau region.
“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 study author
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Limits of the Small Controlled Trial
The study involved 18 participants at one site in a laboratory simulation, so it cannot establish how broadly the results apply to older adults with different health conditions, living arrangements or access to cooling. It also does not show whether the measured changes lead to clinical illness, or how long any effects last after the heat exposure ends.
The researchers observed small changes in balance and cognitive test performance, but the source report does not provide detailed effect sizes for those outcomes. It is also unclear from the report how different cooling approaches, such as daytime air conditioning or other interventions, compare in reducing risk outside the chamber. The study’s findings do not mean every older adult will respond in the same way.
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Applying the Findings During Heat Events
The study adds evidence behind advice to keep indoor temperatures no higher than 26°C during extreme heat, including overnight, but it does not evaluate a specific household cooling program. Public-health guidance and future research may help clarify how people can maintain safer indoor conditions when homes lack air conditioning and how cooling strategies affect health outcomes.
The research team’s next steps were not specified in the report. For now, the paper’s key message is that repeated daytime heat exposure can leave measurable effects into the next morning, and core temperature alone may not capture the full degree of strain.
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Key Questions
What did the study find about overnight recovery?
Most measured responses moved toward baseline overnight, but morning blood pressure remained lower and kidney-stress markers remained elevated after repeated daytime heat exposure, even when the chamber cooled to 26°C.
How many people took part?
The trial included 18 adults ages 67 to 84 from the Ottawa–Gatineau region, with equal numbers of women and men.
What temperatures did researchers test?
In the simulated heat-wave condition, the apartment-style chamber reached 34°C (93°F) during the day and cooled to 26°C (79°F) overnight. A comparison session held it at 26°C throughout the day and night.
Does 26°C guarantee protection for older adults?
No. Study author Glen Kenny described 26°C as an upper indoor temperature limit intended to reduce heat-related risk, not a temperature that guarantees protection for every older adult.
What are the study’s main limitations?
It was a small, single-site laboratory trial. The results do not establish how often the measured changes cause illness in daily life or whether they apply equally to older adults with different health and housing circumstances.
Source: rss
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