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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 physiological strain — including kidney stress markers and lower blood pressure — that did not fully resolve overnight even when their apartment cooled to 26°C (79°F). The findings support keeping indoor temperatures at or below 26°C day and night.
Older adults exposed to repeated daytime indoor heat during a multiday heat wave continued to show signs of physiological strain the next morning — even when their living space was cooled overnight to 26°C (79°F) — according to a new controlled trial from the University of Ottawa published in The Lancet Planetary Health. The study, the first controlled trial to track older adults’ physiological responses through both daytime heat exposure and overnight recovery across a simulated multiday heat wave, found that lower morning systolic blood pressure and elevated markers of kidney stress persisted after hot days despite overnight cooling.
The trial 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 for three consecutive days and two nights. During the simulated heat wave, the chamber was held 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 the 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 were on average five beats per minute higher, and blood tests showed higher circulating markers of kidney and gastrointestinal stress. Researchers also observed lower blood pressure, reduced cardiac autonomic function, and small declines in balance and cognitive test performance. The study authors reported no adverse events during the trial.
Although overnight cooling to 26°C allowed most physiological responses to recover to near-baseline levels, the recovery was incomplete, according to the researchers. Lower morning systolic blood pressure and elevated kidney stress markers remained evident the following mornings. Notably, core body temperature did not rise progressively from one hot day to the next — but other measures of strain persisted.
Why Overnight Cooling Isn’t Enough
The findings challenge a common assumption that being indoors — or cooling a home at night — is sufficient protection during a heat wave. Many Canadian homes lack air conditioning, and the study suggests older adults in such homes can experience prolonged heat stress when indoor temperatures stay elevated for days. Lead researcher Glen Kenny, director of uOttawa’s Human and Environmental Physiology Research Unit, said the results show that “cooling only at night may not be enough to fully protect older adults” from repeated daytime overheating.
The study also carries a practical warning for caregivers and clinicians: a stable core body temperature does not mean an older adult has recovered. “We can’t judge the physiological effects of heat by temperature alone or assume that a stable body temperature means an older adult is fully protected,” Kenny said, noting that cardiovascular and kidney-related strain may persist even when temperature appears stable.
Link to Health Canada’s 26°C Guidance
: “The results reinforce 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 guidance was informed in part by the same uOttawa research team’s earlier work on a single day of heat exposure. The new trial extends that evidence by examining repeated daytime exposure and overnight recovery across a simulated multiday heat wave typical of homes without air conditioning.
Kenny cautioned that 26°C is an upper limit intended to reduce risk, not a guarantee: “It should not be interpreted as a temperature that guarantees protection for every older adult.”
“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, professor, University of Ottawa Faculty of Health Sciences
Limitations of the Ottawa Trial
The study was a single-site trial with 18 participants from one region, which limits how far the findings generalize to older adults with different health conditions, medications, or climates. The simulated heat wave used fixed day and night temperatures, which may not capture the variability of real-world conditions, including hotter nights than the 26°C tested. It is not yet clear from this study how quickly the persistent effects — such as elevated kidney stress markers — resolve after a heat wave ends, or whether they translate into longer-term health outcomes. The trial also could not determine whether strategies such as daytime air conditioning, fans, or hydration changes would eliminate the residual strain, since those interventions were not tested.
Heat Guidance and Future Research
The findings are likely to feed into ongoing public health messaging as extreme heat events become more frequent. Health Canada’s 26°C indoor limit, updated in June 2026, is the current benchmark, and this study adds evidence for maintaining that limit throughout the day and night during sustained heat events rather than only overnight. Researchers say laboratory heat wave simulations can continue to be used to evaluate cooling strategies for vulnerable groups, and further work is expected to test whether additional daytime interventions can prevent the residual cardiovascular and kidney strain observed in this trial. Readers concerned about heat exposure and older family members should consult local public health guidance or a qualified health professional.
Key Questions
Does cooling my home at night protect an older adult during a heat wave?
Not fully, according to this study. While overnight cooling to 26°C (79°F) allowed most physiological measures to recover to near-baseline, lower morning blood pressure and elevated kidney stress markers persisted. The researchers recommend keeping indoor temperatures at or below 26°C throughout the day and night.
What temperatures did participants experience in the study?
During the simulated heat wave, the climate chamber apartment was kept at 34°C (93°F) during the day and cooled to 26°C (79°F) overnight for three days and two nights. A comparison session kept the apartment at 26°C around the clock.
If an older adult’s body temperature is normal, are they safe from heat effects?
The study suggests not necessarily. Core body temperature did not rise from one hot day to the next, yet other signs of strain — including lower blood pressure and elevated kidney stress markers — persisted into the following morning. The authors caution against judging heat effects by temperature alone.
How large was the study?
It involved 18 adults aged 67 to 84 from the Ottawa–Gatineau region, evenly split between women and men, in a single-site randomised crossover trial. No adverse events were reported during the study.
What does Health Canada recommend for indoor temperatures during extreme heat?
Health Canada guidance released in June 2026 recommends an upper indoor temperature limit of 26°C (79°F) for older adults during extreme heat events, informed in part by this research team’s earlier work. The researchers note it is a risk-reduction limit, not a guarantee of protection for every individual.
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