Plate Nº 84 · recorded September 30, 2026
Health & Medicine ResearchReported finding
Overnight Cooling May Not Fully Protect Older Adults From Heat Waves
A University of Ottawa trial shows older adults' kidney and blood pressure strain persisted overnight even when the indoor temperature cooled to 26°C during a simulated heat wave.
By Elena Vasquez4 min read780 words
In brief
- 18 adults aged 67–84 spent three days and two nights in a climate chamber held at 34°C by day and 26°C overnight.
- Peak core temperature averaged 37.8°C on hot days versus 37.4°C in cooler conditions; heart rates ran about 5 bpm higher.
- Lower morning systolic blood pressure and elevated kidney stress markers persisted even after overnight cooling to 26°C.
- The findings support Health Canada's June 2026 guidance recommending a 26°C upper indoor temperature limit for older adults during extreme heat.
Older adults can carry the physiological effects of daytime indoor overheating into the night, even when their homes cool down after sunset. That is the central finding of a new controlled trial from the University of Ottawa, published in The Lancet Planetary Health.
The study is the first controlled trial to track how older adults respond physiologically across both the daytime heat exposure and the overnight recovery periods of a simulated multiday heat wave. Eighteen adults aged 67 to 84 from the Ottawa–Gatineau region, half women and half men, spent three consecutive days and two nights in a studio-apartment-style climate chamber at the university's Human and Environmental Physiology Research Unit.
During the simulated heat wave, researchers kept the chamber at 34°C (93°F) during the day and cooled it to 26°C (79°F) overnight. Those conditions mirror indoor temperatures recorded in real heat events in Canada and the United States. In a separate comparison session, the apartment stayed at 26°C around the clock.
The results were clear. On hot days, participants' peak core body temperature averaged 37.8°C (100.0°F), compared with 37.4°C (99.3°F) under the cooler conditions. Their heart rates ran about five beats per minute higher. Blood tests revealed higher circulating markers of kidney and gastrointestinal stress — biological signals that the organs were working harder under heat.
Researchers also documented lower blood pressure, reduced cardiac autonomic function (the nervous system's regulation of heart activity), and small declines in balance and cognitive test performance.
Recovery was incomplete
Cooling the apartment to 26°C overnight allowed most physiological responses to return close to baseline, but not all the way. Lower morning systolic blood pressure and elevated kidney stress markers persisted into the following mornings.
"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," explains Glen Kenny, full professor at uOttawa's Faculty of Health Sciences and director of the 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."
The message, Kenny says, is that nighttime cooling alone may not fully protect older adults from the cumulative effects of repeated daytime overheating during an extreme heat event.
Being indoors is not always protection
Many Canadian homes lack air conditioning, which leaves some older adults exposed to high indoor temperatures for days at a time. The trial's findings suggest that staying inside offers no guarantee of safety when indoor temperatures stay elevated.
"Most people assume that being indoors means being protected during a heat wave. Our findings show that is not necessarily the case," Kenny says. "Homes and apartments without adequate cooling can remain hot for days during an extreme heat event, exposing older adults to prolonged heat stress."
The results reinforce Health Canada's guidance, released in June 2026, recommending an upper indoor temperature limit of 26°C for older adults during extreme heat. That recommendation drew in part on the uOttawa team's earlier work on single-day heat exposure; the new study extends the evidence to multiday events with overnight recovery periods.
Kenny adds an important caveat: 26°C is an upper limit intended to reduce heat-related risk, not a temperature that guarantees protection for every older adult.
Temperature alone doesn't tell the story
One result surprised the researchers. Participants' core body temperatures did not climb from one day to the next, even as other signs of strain accumulated and, in some cases, persisted into the following morning.
"The most striking thing was what we didn't see," Kenny states. "Body temperature didn't worsen day over day, but that did not mean the body had fully recovered. Blood pressure remained lower, and markers of kidney stress remained elevated the following morning."
The lesson, he says, is that a stable body temperature cannot be read as a sign that an older adult has fully recovered from prolonged heat exposure. Cardiovascular and kidney strain can persist even when the thermometer suggests everything is fine.
No adverse events were reported during the study. As with any single-site trial involving 18 participants, the findings come from a controlled laboratory setting and a specific age group, so broader applications will need continued research.
Publication details: Robert D. Meade et al., "Effects of a 3-day simulated heatwave on physiological and functional outcomes in older adults in Canada: a single-site randomised crossover trial," The Lancet Planetary Health (2026). DOI: 10.1016/j.lanplh.2026.101517
via Medical Xpress (Source)