In a country where Vancouverites get heat warnings at 29 C, but some in Ontario don't get the same alert until it feels like 42 C with humidity, it's easy to think West Coasters are weather wimps.

But there's science behind those differences, says Environment Canada.

And the system it uses to alert the public to dangerous heat has been quietly changing, prompted by health officials studying the impact of deadly heat waves.

"It gave us the sense that we really needed to move forward with having more evidence-based heat warnings," said Melissa MacDonald, acting senior policy officer and meteorologist with Environment and Climate Change Canada.

What was once a single threshold for nearly everywhere across the country now differs from region to region, about 20 in all, with criteria that considers duration, nighttime lows and local health impacts.

The new system has been rolled out slowly, but its purpose is urgent: to warn people as early as possible that heat can kill and is on its way.

Under the new system, some regions will be seeing heat warnings that rarely, if ever, saw them before. Here, on a map captured July 26, all red sections were heat warnings except those in Ontario and Quebec, which were rain and thunderstorms. (Environment Canada)

Deadly B.C. heat wave

Even before the change from Environment Canada, B.C.'s Lower Mainland had its own threshold for heat warnings, designed after a deadly heat wave in 2009.

Over seven days that summer, the normally balmy South Coast had temperatures that reached 40 C in areas and stayed higher for longer than authorities thought possible there.

That week, 110 more people died than would be seen in an average summer week in the region, according to a 2012 study by the B.C. Centre for Disease Control.

But for the rest of the country, Environment Canada was using a single measure to decide when to warn about heat, whether people were in Whitehorse, St. John's, or Parry Sound, said MacDonald.

"For the remainder of the country, we did use the single criteria … developed in Southern Ontario to begin with and then expanded nationally," she said.

The threshold was a forecast of a humidex value of 40 for an hour, said MacDonald, meaning temperatures so humid they feel like 40 C.

It's a limit that drier and cooler parts of Canada were unlikely to see much of, and one that health officials were learning didn't capture how our bodies respond.

Health authorities warn the dangers of extreme heat will be felt at different thresholds in different regions, depending on what people's bodies are used to. (Mike Laanela/CBC)

Heat risk varies by region

Now, with the exceptions of Quebec and Nunavut, heat warnings are issued when the high temperatures are expected to last two days — rather than an hour — and the threshold varies not just by province but zones within a province.

In Ontario, which piloted the new system in 2015, three different thresholds apply; Alberta, Manitoba and Saskatchewan have two zones; and there's just one for Newfoundland and Labrador.

B.C., with its mountains and sea, has four — ranging from 28 C on the North Coast to 35 C in the Fraser Valley and Southern Interior.

That's partly based on the highest five per cent of temperatures a region is likely to see, said MacDonald.

But it's also based on health data — because if you're not used to heat, it truly is more dangerous, said Kathleen McLean, an environmental health scientist with the B.C. Centre for Disease Control, which worked with Environment Canada on the new criteria in B.C.

"People definitely become physiologically adapted to the temperatures in the place that they live in," said McLean.

Heat warnings are meant to encourage the public to take precautions at extreme temperatures, especially vulnerable people including young children, older adults and pregnant women. (Frederic J. Brown/AFP/Getty Images)

Nighttime relief

Finally, the new criteria factor in what anyone struggling with a sweaty summer sleep knows well: our bodies need to cool down at night.

If the nighttime low between hot days doesn't drop below 13 C — or 16 C, or 18 C, depending where you live — then that's part of the heat warning.

"From a physiological perspective, when temperatures don't cool down at night, people just don't get any relief ... and that makes it harder to deal with the heat," said McLean.

Overall, the goal is to give the public — and authorities with heat response plans — as much notice as possible, she said.

"This helps the most vulnerable people ... cope with the extreme hot weather."