Wildfires may keep you inside more often this summer. Is it safe to run the AC?

As wildfires rage across the country, casting a dark haze over Canadian skies, many people are staying indoors seeking refuge from the smoky air.

But as temperatures heat up in some parts of the country, Canadians may be left wondering if turning on their air conditioners will bring unwelcome smoke and pollutants into their homes.

“It’s a little bit complicated. For most Canadians, if they have air conditioning in their home, there is no outdoor air that comes in with that air conditioning,” said Jeffrey Siegel, an engineering professor at the University of Toronto, who also studies indoor air quality.

“And so turning on the air conditioning is a good thing to do to keep the temperature down, and it’s an especially good thing to do if you have a good filter in place.”

But it depends on the filter and the air conditioner, he said.

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While some air conditioners can effectively reduce pollutants from wildfire smoke and improve indoor air quality, there are others that might draw smoke into your home.

“Wildfire smoke is made up of a whole soup of pollutants,” explained Dr. Samantha Green, a family physician at Unity Health Toronto. “And the one that we worry most about is referred to as PM 2.5, and that’s because it not only irritates the nose, the throat and the eyes, but it can get deep into the lungs and then into the bloodstream.”

Short-term exposure to smoke can cause eye irritation, runny nose, sore throat, wheezing, shortness of breath and a tight sensation in the chest, she said.

“And the longer you spend out in the poor air, the greater your symptoms will be.”

With worsening air quality alerts spreading across Canada, experts such as Green and Siegel advise staying indoors, if possible, and closing your doors and windows. If you opt to utilize air conditioning, fans or air filters, here are some essential tips to consider.

The key to using a central air conditioner during wildfire season is a “really good filter,” Siegel said.

While many air conditioners typically do not take in air from the outdoors, he warned some new homes, as well as most commercial and non-residential buildings, have outdoor air that comes in as part of the central air system. In this case (and even in units that don’t do this) Siegel recommends investing in a good filter that is “high on the MERV scale.”

The minimum efficiency reporting value (MERV) scale measures the effectiveness of air filters, like those used in air conditioners. It rates filters on a scale from one to 20, with a higher MERV rating indicating a higher level of effective filtration.

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“I certainly would like to see people using MERV 11 filters, even higher would be better,” he said, which can easily be found online or at a home improvement store.

Most air filter manufacturers recommend changing air filters every three months, but, “if you’re running your fan continuously, just make sure that that filter is changed right after the wildfire episode,” he said.

Most window air conditioning units don’t have a connection to outdoor air, Siegel said, meaning they recirculate the air that’s already inside your home, cooling and filtering it in the process.

However, window air units have “terrible filters in them,” he stressed.

“That filter is not doing anything for wildfire smoke, but it won’t make things worse,” Siegel said.

Window-mounted air conditioners could draw in smokey air from outside if the unit is not properly sealed and there are gaps and openings.

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Siegel recommends checking your window unit for any gaps and using “good weather stripping” to make sure it is sealed tightly.

Alberta engineering professor Lexuan Zhong, recommended against a wall-mounted air-conditioned altogether if there is an air quality advisory near you.

“They usually don’t have filters at all, and it will only bring more pollutants from outside to inside,” she said.

While exhaust fans (in your kitchen or bathroom) can be beneficial for improving indoor air quality by expelling polluted air, Siegel explained they can also draw in wildfire smoke if the outdoor air quality is poor.

“You need some ventilation, but we want to avoid using those more than we need to,” he said.

“A kitchen rangehood fan can push out all the pollutants from cooking, but it also draws air in. This is a situation where you want to use them only when you need to.”

A portable high-efficiency particulate air (HEPA) filter forces air through a fine mesh that traps air particles and can remove smoke from your home.

Because wildfire smoke predominantly consists of tiny particles, a good air purifier will be able to remove them from the air.

“Portable filters are great. They’re really important during wildfire season,” Siegel said. “The thing you want to look for is the clean air delivery rate or the CADR. The higher it is, the better.”

The CADR measurement shows the effectiveness of an air purifier in removing pollutants. A higher CADR means the purifier can clean the air quickly and more efficiently.

Whether you’re using an air conditioner, window unit or air purifier, Siegel emphasized that because of the threat of wildfires persisting throughout the summer, it is crucial to also prioritize the potential health risks linked with extreme heat.

“Extreme heat is a really serious issue and is just as serious of a health issue, if not more so, than wildfire smoke. And so keeping people cool is really important, too,” he said.