Toronto has experienced multiple periods of ‘double exposure’ weather this summer. Here’s what that means

People take in a city view of Toronto on Monday July 14, 2025. The Air Quality Health Index rating for Toronto was over 10, or "very high risk,"from wildfire smoke on Monday morning. THE CANADIAN PRESS/Chris Young (Chris Young/The Canadian Press)

More than once this summer, residents in the Greater Toronto Area have been subject to extreme heat warnings and special air quality statements from wildfire smoke.

This weekend, temperatures are expected to climb again, with Environment Canada calling for a high of 32 C on Saturday in Toronto with humidex values sitting at 40 C. Sunday will be similar with temperatures hitting 33 C.

The national weather agency also says Toronto’s air quality is at a five—for moderate risk—as wildfire smoke from across Canada continues to make for hazy conditions in the city.

When extreme heat and wildfire smoke conditions happen at the same time, it’s known as “double exposure.”

“When we’re talking about these two different exposures, they have two different sets of effects on your body,” Sarah Henderson, Scientific Director of Environmental Health Services at the British Columbia Center for Disease Control, told CTV News Toronto in an interview.

“When we’re talking about wildfire smoke, we’re talking about small particles getting deep into your lungs and causing inflammation and irritation that can affect your whole body. When we’re talking about very hot weather, your heart’s working really hard to pump your blood to the skin so you can sweat and prevent your body from overheating. When we have the two exposures together, both those things are happening at the same time.”

• Wildfires in Canada – News, Tracking and Air Quality – CTV News
• Hourly and 7-day forecast on our CP24.com weather hub

A study released by scientific journal “npj Clean Air” in June that was co-authored by Henderson determined that the combination of heat and smoke increases individual health risks, “leading to worse respiratory, cardiovascular and birth outcomes than either exposure on their own.”

Henderson said those most at risk for experiencing the combined affects are anyone with preexisting lung or cardiovascular conditions, people who are pregnant, infants and young children, seniors, and people with severe mental illnesses like schizophrenia.

“Anybody whose health is not great on a day-to-day basis is probably more likely to struggle with the heat and smoke than people who are in good health,” she said.

Pets and livestock are also susceptible.

“We certainly know that the exposures affect them,” said Henderson.

“There’s enough evidence to suggest that other mammals—like our companion animals and livestock—experience much of the same impacts that we experience when it’s hot or smoky. But, these environmental conditions also have impacts on wildlife as well."

Most of the studies done on the issue of double exposure have been conducted in British Colombia, where the heat is typically on the drier side. Henderson said Ontario’s wetter, humid weather could result in even worse affects.

“When we’re talking about heat and humidity combined, we often talk about apparent temperature, or how hot it feels to your body,” Henderson explained.

“The temperature could be 36 C but feels like 41 and that’s because of the humidity. So, we would expect that humidity is kind of a triple whammy.”

Henderson warned that climate change will only cause incidences of double exposure to become more frequent, and pointed to how the natural landscape is becoming harder for humans to control.

“We are in a position in Canada where decades of forest management have excluded fire from the landscape,” she explained. “We suppressed fires when they started. But prior to widespread human occupation of these lands, those fires maybe would have burned every 30 years, in many cases.”

“Now, we have this build up of fuels on the land and in the changing climate, we’re getting hotter, drier, windier weather, and that’s what we call extreme wildfire weather. So, when we have that combination, plus a lot of fuel on the landscape, it can lead to what is often referred to as mega fires, just these very large wildfires that are simply uncontrollable by humans.”

As for protecting against the elements, Henderson advised to stay indoors where the elements can be monitored and looked after.

“For humans and our companion animals, we do need to be looking to the indoor environment as a way to protect ourselves from these exposures. We can’t control what’s happening outdoors. We generally have more control over what’s happening indoors, so ensuring that the indoor environment is clean and cool can really help to protect people, especially those who are at very high risk.”