Parts of the Greater Toronto Area remained under a special air quality statement and a heat warning on Monday night after the city ranked among the worst in the world for air quality earlier the the day.
Smoke from forest fires over northern Ontario and the Prairies is expected to cause or is causing poor air quality and reduced visibility, Environment Canada says.
Poor air quality may persist into Tuesday for some areas, according to the special air quality statement.
"As smoke levels increase, health risks increase. Limit time outdoors. Consider reducing or rescheduling outdoor sports, activities and events," Environment Canada said.
Those at higher risk of being impacted by wildfire smoke include people aged 65 and older, pregnant people, infants and young children, and those with an existing illness or chronic health condition, the federal weather agency said.
People are advised to limit time outdoors and protect indoor air from wildfire smoke, such as by keeping windows and doors closed as much as possible.
Toronto was the eighth-most polluted major city in the world as of 8 p.m. on Monday, according to a global ranking by IQAir, a Swiss air quality technology company. It was second-most polluted major city in the world as of 8 a.m., according to the ranking.
The area burned by wildfires so far this season is the second-largest on record in Canada, government data shows. Wildfires have consumed 3.7 million hectares this year, which is six times the area of Prince Edward Island.
Environment Canada warned last year that Canadians should be prepared for more extreme weather events, such as wildfires and hurricanes, triggered by human-caused climate change.
WATCH | Cardiologist explains impact of extreme heat and poor air quality on health:
Monday's air quality warning came as extreme heat and humidity continued in the region.
Daytime highs were expected to reach 31 to 35 C, with the humidex making it feel closer to 37 to 42 C, the federal weather agency said in a warning on Monday. Overnight lows are expected to provide little relief, ranging from 19 to 23 degrees.
The heat warning is expected to end on Thursday night, Environment Canada says.
People are advised to stay hydrated and check in on those who are at greater risk of heat illness, including the elderly and those living alone.
Watch for symptoms of heat stroke
Environment Canada is advising people to limit exposure to sun and heat by planning activities during the coolest parts of the day, as well as wearing a wide-brimmed hat and lightweight, light-coloured, loose fitting clothing.
People are advised to watch for symptoms of heat stroke, which is a medical emergency, in themselves and others. Symptoms can include red and hot skin, dizziness, nausea, confusion and change in consciousness.
"While you wait for medical attention, try to cool the person by moving them to a cool place, removing extra clothing, applying cold water or ice packs around the body," Environment Canada said in Monday's warning.
The weather agency also advised people to drink water often, even before feeling thirsty, and close blinds and open windows if the outside is cooler than inside.
People are advised to check in regularly on those at greater risk of heat illness, including the elderly and those living alone.
Signs of heat exhaustion may include headache, nausea, dizziness, thirst, dark urine and intense fatigue. If you experience these symptoms, Environment Canada advises stopping your activity and drinking water.
People with pre-existing conditions at risk: doctor
Poor air quality and extreme heat can put strain on the lungs and the heart, respectively, said cardiologist Dr. Christopher Labos.
"If you are young and healthy, you can probably ride this out without getting seriously sick," he said in an interview with CBC Radio's Metro Morning on Monday.
"But you can easily see that the minute you start having a few additional health problems, that's going to start to cause a cumulative effect as you start piling on all these potential stressors."
He said people who must go outdoors can consider wearing a mask.
Pet owners should also be cautious about the effect of the weather on their dogs, Lobos said.
"Dogs are going to be subject to the same problems we have," he said. "They get hot, they can overheat, they can get heatstroke and unlike us, they can't ask for water."
Labos advised keeping pets indoors as well and giving them plenty of water.