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A controlled University of Ottawa trial published in The Lancet Planetary Health found that older adults exposed to simulated multiday heat waves showed persistent physiological strain — including elevated kidney stress markers and lower morning blood pressure — that did not fully resolve even when indoor temperatures cooled to 26°C overnight. The findings support Health Canada’s June 2026 recommendation of a 26°C indoor upper limit during extreme heat.
Cooling a home overnight may not be enough to protect older adults from the physiological effects of repeated daytime overheating during a multiday heat wave, according to a new controlled trial from the University of Ottawa published in The Lancet Planetary Health. The study of 18 adults aged 67 to 84 found that elevated markers of kidney stress and lower morning blood pressure persisted into the following morning, even when the indoor environment cooled to 26°C (79°F) overnight.
The trial, described by researchers as the first controlled study to examine how older adults respond physiologically across both daytime heat exposure and overnight recovery during a simulated multiday heat wave, involved 18 adults ages 67 to 84 from the Ottawa–Gatineau region, with equal numbers of women and men. Participants stayed in a studio apartment–style climate chamber at the University of Ottawa and were monitored over three consecutive days and two nights.
During the simulated heat wave, the chamber was kept at 34°C (93°F) during the day and cooled to 26°C (79°F) overnight — conditions the researchers said 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 around the clock. On hot days, participants’ peak core body temperature averaged 37.8°C (100.0°F), compared with 37.4°C (99.3°F) under cooler conditions. Their heart rates averaged five beats per minute higher, and blood tests showed higher circulating markers of kidney and gastrointestinal stress. Researchers also recorded lower blood pressure and cardiac autonomic function, along with small declines in balance and cognitive test performance.
Overnight cooling to 26°C allowed most physiological responses to recover to near-baseline levels, but the recovery was not complete: lower morning systolic blood pressure and elevated kidney stress markers remained evident the following mornings. No adverse events were reported during the study. The study is authored by Robert D. Meade and colleagues and titled “Effects of a 3-day simulated heatwave on physiological and functional outcomes in older adults in Canada: a single-site randomised crossover trial.”
Why Overnight Cooling Alone Falls Short
The findings matter because many Canadian homes lack air conditioning, leaving some older adults exposed to high indoor temperatures for days during extreme heat events. The study shows that staying indoors does not necessarily protect older adults from heat-related physiological strain when indoor temperatures remain elevated during the day.
“For the first time, we’ve been able to follow older adults through a full three-day heat wave, including what happens during overnight recovery,” said 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 heat wave, and some effects did not fully resolve overnight, even when the apartment cooled to 26°C (79°F).”
The results also reinforce the practical message that daytime cooling, not just nighttime cooling, is needed to reduce strain. Health guidance based on this research recommends keeping indoor temperatures at or below 26°C throughout the day and night during extreme heat, a standard that homes without air conditioning may not meet on their own.
Link to Health Canada’s 26°C Guidance
The new study builds on earlier work by the same uOttawa research team, which examined the effects of a single day of heat exposure and helped inform Health Canada’s guidance released in June 2026 recommending an upper indoor temperature limit of 26°C (79°F) for older adults during extreme heat. The current trial extends that evidence by testing physiological responses across repeated daytime exposure and overnight recovery in an overheated home environment typical of homes without air conditioning during a heat wave.
“Our earlier work helped inform Health Canada’s 26°C (79°F) recommendation, and this study provides further evidence supporting the importance of maintaining indoor temperatures at or below 26°C (79°F) throughout a sustained heat event,” Kenny said. He cautioned that 26°C is an upper limit intended to reduce risk, not a temperature that guarantees protection for every older adult.
“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.”
— Glen Kenny, professor, uOttawa Faculty of Health Sciences
Limits of the 18-Person Trial
The study was a single-site trial with 18 participants from one region, which limits how broadly the findings can be generalized to older adults with different health conditions, medications, or climates. The simulated heat wave used fixed temperatures of 34°C by day and 26°C overnight; real homes experience more variable conditions, and the study did not test intermediate cooling strategies such as partial daytime air conditioning or fan use.
The long-term health consequences of the observed morning-after effects — such as whether the elevated kidney stress markers translate into clinical harm over repeated heat waves — were not assessed and remain unclear. The researchers also noted that core body temperature alone proved an unreliable indicator of strain, meaning the full picture of heat effects in real-world settings is still developing.
How the Findings May Shape Heat Advice
The results are expected to add evidence to public health guidance on indoor temperature limits, including Health Canada’s 26°C recommendation for older adults during extreme heat. Researchers suggested the findings can inform evaluations of cooling strategies — such as air conditioning, community cooling centres, and home retrofits — aimed at protecting vulnerable groups during multiday heat events.
Further research is likely to examine whether protecting older adults requires maintaining cooler daytime temperatures continuously, and to track whether the kidney and cardiovascular effects observed in the chamber carry over into real-world heat waves and clinical outcomes. Readers concerned about heat exposure should consult local health authorities or a qualified health professional for guidance suited to their circumstances.
Key Questions
Does cooling my home at night protect an older adult from heat effects?
Not fully, according to the study. While overnight cooling to 26°C (79°F) allowed most physiological responses to recover to near-baseline levels, lower morning blood pressure and elevated kidney stress markers persisted. The researchers concluded nighttime cooling alone may be insufficient without controlling daytime temperatures.
What indoor temperature does Health Canada recommend for older adults during heat waves?
Health Canada guidance released in June 2026 recommends an upper indoor temperature limit of 26°C (79°F) for older adults during extreme heat, maintained throughout the day and night. Researchers caution it is a risk-reduction limit, not a guarantee of protection for every individual.
How was the study conducted?
Eighteen adults aged 67 to 84 stayed in a climate chamber apartment at the University of Ottawa for three days and two nights at 34°C (93°F) daytime temperatures, with overnight cooling to 26°C, and were compared against sessions at a constant 26°C. The study was a single-site randomised crossover trial published in The Lancet Planetary Health.
Did participants’ body temperatures keep rising each day?
No. Core body temperature did not worsen from one day to the next, which surprised the researchers. However, other signs of strain — lower blood pressure and elevated kidney stress markers — persisted into the following mornings, indicating that a stable body temperature does not mean full recovery.
Were participants harmed during the study?
No. The researchers reported that no adverse events occurred during the study, which monitored participants under controlled conditions.
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