Volunteers with Project Food Chain drop off water at the St. Felix 24-hr respite site, while a heat wave blankets parts of Ontario, in Toronto on June 19.Cole Burston/The Canadian Press
There were nearly 700 excess deaths linked to extreme heat events in Canada’s largest cities in the first two decades of this century, according to a new Statistics Canada report released as much of Central and Eastern Canada sweats through the first major heat wave of the year.
Statscan analyzed deaths from non-accidental, cardiovascular and respiratory causes in a dozen major cities over 21 years and concluded that the risk of dying rose during scorching periods in most of them, particularly for senior citizens.
Montreal and Toronto experienced the most substantial mortality impact related to extreme heat, possibly because the two metropolises have higher proportions of renters than most other cities, and renters are less likely to have air conditioning, the report noted.
Vancouver and Surrey, B.C., had the fewest extreme heat events during the study period, but among the highest relative risk of excess deaths when temperatures soared.
“The thinking here goes that if you live in a place that has lots of extreme heat events that are pretty common every summer, then maybe you’re acclimatized physiologically to the heat. Maybe you have adaptation resources in place, like air conditioning,” said Matthew Quick, a Statscan research analyst and author of the new study. “Whereas if extreme heat events are less common, then you don’t necessarily have those characteristics.”
The report includes data from 2000 to 2020, before the deadly heat dome that killed 619 people in British Columbia in June of 2021.
Stepping back to examine national averages over a two-decade time horizon produced some counterintuitive results. Statscan found that the average longer-term impact of extreme heat events on mortality was “considerably smaller than those previously estimated from analyses of single events.”
One possible explanation, the report noted, is that earlier studies focused on catastrophic heat waves that were known to be quite deadly, whereas the new analysis looked at all extreme heat events in a 21-year period, some of which caused little or no excess mortality.
Sarah Henderson, the scientific director of Environmental Health Services at the BC Centre for Disease Control, said she is concerned the Statscan report may be underestimating heat-related mortality.
She pointed out that limiting the analysis to non-accidental deaths excludes cases where heatstroke was explicitly listed as the cause of death. Those deaths are considered accidental in the Canadian Vital Statistics death database, which Statscan maintains and used in the new analysis.
“This study confirms that there is an increase in non-accidental mortality on hot days and that is not surprising,” Dr. Henderson said. “I think the conclusion that the increase might not be as large as we expect it to be needs to be questioned.”
Figuring out when to attribute deaths to extreme heat is a challenge because those who die during heat waves often have multiple underlying illnesses that contribute to their deaths.
The code that denotes a death caused solely by excessive natural heat is rarely used, according to previous research by Dr. Henderson and her colleagues. One of their papers notes that, according to Canadian national vital statistics data, an average of only 16 deaths a year are coded that way.
Different approaches to classifying and counting heat-related deaths can yield different estimates. For example, Quebec researchers published a separate study on Wednesday estimating that heat is linked to 470 deaths and 225 hospital admissions in that province alone every summer.
The Statscan report examined death data from the 12 Canadian cities with populations greater than 500,000 in the 2021 census: Toronto; Montreal; Calgary; Ottawa; Edmonton; Winnipeg; Mississauga; Vancouver; Brampton, Ont.; Hamilton; Surrey, B.C.; and Quebec City.
It concluded that, across all the cities, there were approximately 668 excess non-accidental deaths, including 115 excess cardiovascular deaths and 114 excess respiratory deaths during periods that met the Environment Canada definition for an extreme heat event – which differs from place to place.
Regardless of how deaths are attributed, doctors agree that high temperatures pose serious health risks, particularly to the elderly, the very young and people with certain chronic diseases.
“We need to take temperature seriously,” said Anna Gunz, a pediatrician, associate professor at Western University and medical director of the Children’s Environmental Health Clinic of Ontario. “We need to make sure that we are finding ways to keep cool at a population level and finding ways to make sure that there’s equity in terms of cooling.”