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In a controlled study of 18 adults ages 67 to 84, some signs of physiological strain remained the morning after three days of simulated indoor heat, despite overnight cooling to 26°C. The findings suggest that stable core temperature and nighttime cooling alone may not indicate full recovery; the study does not establish how these effects translate to health outcomes during real-world heat waves.
Some signs of physiological strain persisted into the morning after older adults experienced repeated daytime heat in a controlled three-day simulation, even though researchers cooled the room to 26°C (79°F) overnight. The University of Ottawa study involved 18 adults ages 67 to 84 and suggests that nighttime cooling may not fully offset repeated indoor overheating during a heat wave.
Researchers monitored 18 adults, ages 67 to 84, in a studio apartment-style climate chamber over three consecutive days and two nights. During the simulated heat wave, the room reached 34°C (93°F) during the day and cooled to 26°C overnight. In a separate comparison session, it stayed at 26°C day and night. The study was published in The Lancet Planetary Health.
On hot days, participants’ peak core body temperature averaged 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 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.
Overnight cooling brought most measured responses close to baseline, but morning systolic blood pressure remained lower and kidney stress markers stayed elevated the following mornings. Researchers reported no adverse events. The trial measured short-term physiological and functional responses; it did not establish whether the observed changes cause illness or other health outcomes in daily life.
Why Daytime Cooling Matters
The results suggest that being indoors may not prevent heat strain when a home stays hot through the day. This is relevant for older adults living in homes without air conditioning or with cooling that cannot keep indoor temperatures down during a prolonged heat event. The study points to a possible gap in relying on overnight cooling alone: some measured effects had not fully resolved by morning.
The researchers say their findings support maintaining indoor temperatures at or below 26°C (79°F) throughout a sustained heat event. That figure is an upper limit intended to reduce risk, not a guarantee of protection for every older adult. Individual responses and real-world conditions may differ from those measured in the chamber.
The study also found that core body temperature alone may not capture all strain. Participants’ temperatures did not keep rising from one simulated hot day to the next, while blood pressure and kidney stress markers showed signs that had persisted into the morning. The authors say a stable temperature should not be taken to mean that full recovery has occurred.
From One Hot Day to Three
The University of Ottawa team says earlier work on a single day of heat exposure 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 following participants through repeated daytime heat exposure and overnight recovery.
The researchers set the chamber temperatures to reflect indoor readings recorded during real heat events in Canada and the United States. A controlled chamber lets researchers track responses under set conditions, but this study involved one site and a small group. It therefore provides evidence about short-term responses in those conditions, rather than a measurement of risk across all older adults or homes.
““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, study author and director of the University of Ottawa’s Human and Environmental Physiology Research Unit
What the Trial Cannot Show
The study included 18 participants at one site and reproduced heat-wave conditions in a climate chamber. It does not establish how often similar changes occur across the broader older population, how long they last outside the laboratory, or whether they lead to clinical complications. The supplied study report gives no longer-term follow-up results.
It is also unclear how the findings apply to people with different health conditions, medications, housing, or access to cooling. The study found small changes in some functional tests, but the report does not show whether those changes affect everyday activity or safety. No adverse events were reported during the trial.
Applying the Findings at Home
The study adds evidence for keeping indoor temperatures at or below 26°C during the day and overnight in a sustained heat event, consistent with the Health Canada guidance described by the researchers. Households and local heat-response programs can use that figure as an upper limit intended to reduce risk, while recognizing that the trial did not test every cooling approach or individual circumstance.
The next research questions include whether the observed morning changes persist in real homes, how responses vary among older adults, and which practical cooling strategies best reduce strain across multiday heat. The published report does not specify a date or plan for a follow-up study.
Key Questions
What temperatures did the researchers test?
The simulated heat-wave condition was 34°C (93°F) during the day and 26°C (79°F) overnight. The comparison condition stayed at 26°C day and night.
Did participants fully recover overnight?
Most measured responses moved close to baseline after overnight cooling. However, morning systolic blood pressure remained lower and kidney stress markers remained elevated the following mornings.
Does a stable core temperature mean an older adult is protected from heat?
Not necessarily, according to the study authors. Core temperature did not rise from one simulated hot day to the next, while other measures, including blood pressure and kidney stress markers, still showed signs of strain.
Does the study prove that indoor heat causes illness?
No. The trial measured short-term physiological and functional responses in 18 adults in a controlled setting. It did not establish whether those changes lead to illness or other health outcomes in real-world heat waves.
What indoor temperature limit did the researchers discuss?
The researchers cited Health Canada’s recommendation of 26°C (79°F) as an upper indoor limit for older adults during extreme heat. They said it is intended to reduce heat-related risk, not to guarantee protection for every person.
Source: rss
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