These are the 5 things you should know about heat warnings in Canada

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MONTREAL — Environment Canada has issued heat warnings this summer for parts of the country used to baking in the heat, but also in places unaccustomed to extended periods of hot weather. Earlier this month, the Northwest Territories and northern Quebec were under heat warnings as temperatures climbed into the 30s. On Wednesday, warnings were in effect not just in southern Ontario but in much of Nova Scotia, northeastern Newfoundland and even Old Crow, Yukon, north of the Arctic Circle.

But the temperature that leads to a heat warning in one province could be lower than the temperature that triggers an alert in another part of the country.

Here are five things to know about heat warnings in Canada:

What exactly is a heat warning?

Environment Canada issues heat warnings when temperatures begin posing a health risk. Generally, the warnings act as forecasts that include daytime highs and nighttime lows above a specific threshold for two consecutive days.

Heat can be dangerous because it forces the body to work harder than normal to maintain its internal temperature, says Ray Bustinza, a scientific adviser on environmental health at Quebec's public health institute. Older people, those with chronic medical conditions and young children are particularly at risk because their bodies have to work even harder, Bustinza says.

How hot is too hot?

Provinces have different thresholds, and the levels warranting an alert can even vary within a province. In general, there are only a few degrees difference in the threshold between provinces.

For example, heat warnings are issued in Nova Scotia when daytime high temperatures are expected to reach 29 C or warmer for two or more consecutive days, and when nighttime low temperatures aren't expected to fall below 16 C. In southwestern British Columbia, it takes two days at 33 C or warmer with nighttime lows of 17 C or warmer to trigger an alert.

As well, British Columbia has a second extreme-heat warning system that is used when temperatures get so high everyone is at risk.

Sarah Henderson, scientific director of environmental health services at the British Columbia Centre for Disease Control, said the Environment Canada system is intended to alert people when there is risk associated with higher temperatures, especially for vulnerable people. The extreme-heat warning is issued when conditions become really dangerous, she said.

Quebec, like B.C., also has an extreme-heat warning system in addition to the Environment Canada-issued warnings. The levels that trigger those alerts vary from region to region and are based on temperatures that have been associated with significant increases in mortality.

Why are heat warnings triggered at different temperatures in different provinces?

Environment Canada has 15 heat warning regions across Canada, with different criteria for each one --- though those criteria often differ by just a few degrees.

Jean-Philippe Begin, a meteorologist with Environment Canada, says the criteria were developed in conjunction with provincial and territorial health departments, which are responsible for setting the thresholds.

Henderson, who is also a professor in the medical faculty at University of British Columbia, said people's susceptibility to higher temperatures is based on what they're used to. "That's partially because they're physiologically adapted to the climate that they live in, partially because they're behaviourally adapted to the climate they live in and partially because the built environment reflects the climate," she said.

In B.C., there are five different heat regions to account for its varied topography and climate. In each of those regions, the threshold for a warning is based on the minimal temperature that leads to an increase in mortality of around five per cent.

How does the humidex factor in?

In every province, except B.C. and Alberta, heat alerts can also be triggered if the humidex reaches a specific level.

The humidex is an attempt to link the temperature with the relative humidity, a measure of how much moisture is in the air, said Djordje Romanic, a professor of atmospheric and oceanic sciences at McGill University.

As temperatures rise, people start sweating, which cools the body when the sweat evaporates. But that process doesn't work as well if there's a lot of moisture in the air. "We are sweating, but that sweat is not evaporating, and our body is not capable of reducing its temperature," Romanic said.

Humidex readings quantify the combined effect of humidity and temperature on a healthy person at rest in the shade, Romanic said, adding that high humidex levels can cause heatstroke.

The Canadian Centre for Occupational Health and Safety says humidex levels of 46 and higher are dangerous and that people should avoid exertion when the humidex rating is over 40.

In western Canada, hot weather tends to be drier than in eastern and central Canada, Henderson said. B.C., she added, considered adding the humidex to its thresholds when they were updated in 2018, "but we didn't see any meaningful differences between the temperature and humidex models."

With climate change, will heat warnings become more common and could people stop caring?

B.C. is currently evaluating whether to update its thresholds, Henderson said, as warnings become more common. "We worry constantly about over-alerting," Henderson said, adding that last summer some parts of B.C. had six heat warnings, a frequency she worries will lead people to become desensitized to the warnings.

One of the challenges to maintaining people's attention to the health risks of heat warnings is that many people enjoy hot weather, she said.

While poor air quality caused by wildfire smoke, for example, isn't pleasant for anyone -- even those who may not be at risk of serious health effects -- temperatures that can kill vulnerable people can also lead to a pleasant day at the beach for others.

"It really is a very challenging communications problem," she said.

This report by The Canadian Press was first published July 26, 2023.