With millions under heat warnings from Ontario eastward, new research is shedding light on the health risks from exposure to unusually warm temperatures — particularly in cities with more renters.

The study, released Wednesday by Statistics Canada, found a higher than average risk of death, particularly among those 65 and over, during stretches of extreme heat in many of Canada’s largest cities over the past two decades.

“The risk of dying tends to increase during days defined as extreme heat events compared to days that are not extreme heat events,” said Matthew Quick, the author of the study and a research analyst at Statistics Canada.

Between 2000 and 2020, roughly 670 more deaths than usual were recorded in the country’s 12 largest cities during periods of extreme heat that lasted two days or longer.

The criteria for what is defined as a period of extreme heat varies by region and is defined by Environment and Climate Change Canada.


Cities where extreme heat events are less common saw a greater spike in deaths when the temperature shot up.

“That might point to acclimatization, so your body’s ability to deal with heat might also point to the degree to which adaptation measures are in place,” Quick said.

The research also found higher mortality rates during heat waves in cities where there are more renters. He said this may be because renters are less likely to have air conditioning than homeowners, as he showed in a previous study.

Wednesday’s study adds to a growing body of research showing the effects of extreme heat on vulnerable populations.

Longer heat waves carry greater health risk

A report on the week-long heat dome in British Columbia in 2021 found that most people who died were elderly, had a disability, lived in poorer neighbourhoods or lived alone. During that week, which fell outside the time period for the Statistics Canada study, more than 600 deaths were identified as being heat-related.

Dr. Matthew Bennett, a cardiologist in Vancouver, said he felt the strain on the health system during that exceptionally hot period. Research suggests that longer heat waves, such as the one in B.C., carry greater health risk.

“The ambulance attendants saw it first, then the emergency rooms saw it,” he said. 

“It’s always difficult to ascertain a specific cause of death, but there is really good evidence of why people die or how they could die because of cardiovascular causes.”

Bennett suggested everyone should have a plan for how to stay cool during periods of prolonged heat, either at home or at another location with air conditioning. 

“Everybody who is not at risk should also think of the family members who could be at risk, friends who can be at risk, and check in on them,” he said. 

WATCH | Millions living under heat warnings from Ontario eastward:

Millions living under heat warnings from Ontario eastward

Millions of Canadians from Northern Ontario to Newfoundland are dealing with heat and humidity that puts vulnerable people at risk. The ‘heat dome’ high-pressure system that traps hot air near the ground isn’t expected to subside for days.

The results come amid heat warnings affecting millions in parts of Ontario, Quebec and Atlantic Canada, and large swaths of the United States.

Research shows heat waves are becoming increasingly extreme as the climate warms. A report last month by Climate Central, along with World Weather Attribution and the Red Cross Red Crescent Climate Centre, examined 76 extreme heat waves across 90 countries over a 12-month period starting in May 2023.

The report says that in that period, 6.3 billion people — roughly 78 per cent of the population — experienced at least 31 days of extreme heat that were “made at least two times more likely due to human-caused climate change.”

Environment and Climate Change Canada is forecasting higher-than-normal temperatures throughout most of Canada this summer, after the hottest year on record.

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