Toronto and Montreal rank among the deadliest cities for Canadian heat waves, according to new research by Statistics Canada (StatCan).

Released Wednesday, the 20-year data review compared mortality rates during extreme heat events among the 12 most populous Canadian cities. Researchers found that while extreme heat is a common contributor to excess deaths in national totals, the country's two biggest cities have seen the largest spikes.

StatCan counted roughly 700 excess deaths attributed to heat between 2000 and 2020, with 230 attributed specifically to heart or lung problems.

Of the total, an estimated 250 were counted among Toronto residents and 295 in Montreal, the latter municipality known as particularly vulnerable to environmental and heat-related deaths, research shows.

"One explanation … may be the concentration of rental housing and lower levels of air conditioning access among renters, compared with owners," the study reads.

Quebec City, Ottawa, Surrey, B.C., Vancouver, Brampton, Ont. and Mississauga, Ont. saw lesser effects on excess mortality that Toronto and Montreal, ranging between 22 and 34 excess deaths in the timeframe of the review. In Calgary, Winnipeg, Edmonton and Hamilton, mortality figures during recognized extreme heat events were actually slightly lower than the baseline.

"With a few exceptions, the mortality risks during extreme heat events in the other 10 cities were not significantly different from the daily averages," the study results read.

Cooler cities, renter hotspots more vulnerable

The June study found a correlation between the proportion of rented households in a city and its risk of excess heat deaths.

Census data shows that Montreal and Toronto are among Canada's most renter-heavy cities, with 61 and 48 per cent of households, respectively, rented to their occupants. In addition, the StatCan study cited research showing that tenants had significantly less access to air conditioning, when compared to provincial averages in Quebec, Ontario and British Columbia.

Another possible factor in the mortality spikes were the relative frequency of extreme heat events in the affected areas. Statistical analysis found that the more individual extreme-heat days a city experienced, the less of an impact they would have on the whole — a pattern StatCan suggests may be because those cities are better prepared, both in resources and biology.

"Past research has suggested that people living in areas with lower ambient temperatures and fewer extreme heat events may have lower levels of physical acclimatization," the study reads, pointing as well to the lack of air conditioning in cooler-weather cities.

"Surrey and Vancouver, in particular, had the fewest extreme heat events … but among the highest [extreme heat relative risks] attributable to non-accidental and cardiovascular causes."

Heat deadlier for seniors

Extreme heat has long been associated with excess deaths, whether through exhaustion, stroke or exacerbation of existing medical issues. Research shows that Canadian seniors have borne the brunt of heat mortality.

"Extreme heat events were found to be more strongly associated with mortality for people aged 65 and older," the study reads.

"People aged younger than 65 did not have significantly higher mortality risks during extreme heat events for any causes or in any cities."