Much of southern Quebec, including the Montreal region, is draped in a hazy cloud of smog as massive forest fires continue to burn in northern sectors of the province.
Experts say this can have an effect on your health, especially if you're already vulnerable.
Dr. Shawn Aaron, a professor in the department of medicine at the University of Ottawa, recommends wearing an N95 mask if you must work outdoors.
"Ideally on days like today, you should stay inside with the windows closed and the air conditioning on," he said in a statement.
"If you have HEPA filters in your house, turn them on. Avoid exercising or heavy work outdoors on poor air quality days."
According to Environment and Climate Change Canada (ECCC), which has issued a smog warning for Montreal and other regions Tuesday, the poor air quality especially affects asthmatic children and people with respiratory ailments or heart disease.
Overall, ECCC recommends staying inside if you feel unwell or are experiencing shortness of breath, wheezing, or an asthma attack, and contacting your care provider.
Aaron said babies, young children and the elderly are particularly vulnerable.
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Laval schools keep kids inside
In Laval, Que., Dr. Jean-Pierre Trépanier, who heads public health for the city, said those who are pregnant should also use caution.
His agency, CISSS de Laval, encouraged school boards and service centres to keep children indoors Monday afternoon and all day Tuesday. The school boards agreed.
"We wanted to get kids away from the small particles," he said. "It's quite an uncommon event. It's the first time I've recommended that in 15 years of practice."
There is poor air quality from the Quebec City region all the way to Gatineau.
The Eastern Townships have acceptable air quality, and so do areas farther north like Lac-Saint-Jean and Saguenay.
But the poor air quality is extending westward and even south of the border. Ottawa's air quality is considered dangerous to human health and is expected to stay that way through most of the week.
ECCC has issued a special weather statement for Toronto as well.
In the United States, the Environmental Protection Agency is alerting most of New England about air quality. States farther west and south have also been affected.
According to IQAir, which tracks air quality globally, New York City and Toronto ranked among the worst in the world on Tuesday.
Long-term exposure could lead to health risks
Jill Baumgartner, an associate member of the McGill School of Environment, studies exposure to environmental pollutants and its effects on human health.
She said the biggest health threat from wildfires is the microscopic airborne particles.
"When you breathe them in, they get deep into your lungs and they can cause a bunch of different health problems," said Baumgartner on CBC's Radio Noon Quebec.
Long-term exposure to wildfire smoke and air pollution can lead to more serious chronic heart and lung diseases, she said.
But what makes wildfire smoke dangerous? The chemical makeup of the smoke is complex, said Sarah Henderson on CBC's White Coat Black Art. She's an epidemiologist who researches wildfire smoke.
"There's a lot of material when we're talking about communities or houses that are burning, and so it's a very thick type of smoke," she said.
Any particles smaller than 2.5 microns — known as PM2.5 — can infiltrate the lungs and cross the blood-brain barrier.
When PM2.5 gets into your lungs, your body's immune system kicks into gear, similar to the way it would to attack a virus or bacteria, said Henderson.
But those tiny smoke particles can't be killed the way viruses or bacteria can be, so your immune system just keeps working.
That can bring on systemic inflammation in the body, which can lead over time to chronic issues such as heart disease, said Dr. Courtney Howard, an ER physician in Yellowknife who researches wildfires and health.
Particles get more toxic as they travel
Scott Weichenthal, an assistant professor at McGill University who specializes in identifying and evaluating environmental risk factors for chronic diseases, said along with PM.2.5, wildfires can also lead to an increase in ozone in the atmosphere.
And, he added, there is evidence that suggests wildfire particles become more toxic as they travel.
"Just because you're not living close to the forest fire, doesn't mean you're not necessarily impacted by the air pollutants," he told CBC News.
Rain can reduce emissions and remove particles from the air. There is the potential for rain this week in Montreal, but there is growing concern that this summer could be particularly dry.
"I think the concept of normal is a bit difficult to frame," said Weichenthal.
"This might be the new normal. As we have more and more forest fires of greater intensity, and happening more often, I think we can expect more days like this during the summer."