There's no escaping wildfire smoke indoors, experts say — but it can help

Hamilton Spectator

31-07-2025

It's been a
smoky summer for Toronto
, with forest fires in the Prairies once again pushing the city's Air Quality Health Index into the 'high risk' category Thursday morning.
Environment Canada has issued a special statement
urging Torontonians to limit their time outdoors where possible. But there's no escaping the smoke even when inside, experts tell the Star.
Most buildings are only able to keep about half — 40 to 60 per cent — of outdoor air pollutants from coming inside, according to the research of Jeffrey Brook, an associate professor at the University of Toronto's Dalla Lana School of Public Health.
As climate change advances, I cannot grasp the hugeness of the calamity. What once was notable is becoming a habit, a toxic, sinister pattern.
As climate change advances, I cannot grasp the hugeness of the calamity. What once was notable is becoming a habit, a toxic, sinister pattern.
That means, on especially smoky days, indoor air quality can still reach dangerous levels for susceptible individuals, Brook and other experts previously told the Star. These can include young children, pregnant people, elderly individuals and those with respiratory or cardiovascular conditions like asthma.
'There's a lot of epidemiology research that says (pollutants released by wildfires) is a major killer,' Brook explained. 'It's one of the most
significant causes of premature mortality
in the world.'
And yet, our building codes and ventilation systems generally aren't designed to prioritize smoke and other pollutants, experts say. As the
climate crisis
makes wildfires a more common occurrence, policymakers are being urged to consider long-term solutions to a worsening issue.
There are three main ways smoke (and other air pollutants) can infiltrate our homes, according to Amy Li, an assistant professor of civil and environmental engineering at the University of Waterloo.
The first is through any large openings, like a door left ajar or an opened window. 'This is why we often have the recommendation to close all windows and doors when we know the outdoor air is not good,' she explained.
The second pathway is less visible — leaks and cracks in a home's enclosure through which pollutants can enter. 'Buildings are not like a spaceship,' Li said. 'There will always be some amount (of pollutants) getting into the building.'
Health experts are not currently concerned about air quality in Toronto but warn prolonged exposure to moderate risk air may threaten health.
Health experts are not currently concerned about air quality in Toronto but warn prolonged exposure to moderate risk air may threaten health.
The third is through mechanical ventilation systems that funnel outside air indoors, said Jeffrey Siegel, a professor of civil engineering at the University of Toronto. This sort of system is present in 'most non-residential buildings,' he said.
It's why large commercial and institutional buildings, like schools, offices and community centres, are often more vulnerable to smoke, Li added, although it varies from building to building.
Older buildings also tend to have more cracks for pollutants to seep through and ventilation systems less able to keep smoke out, Li and Siegel explained. Meanwhile, single family homes and townhouses tend to be 'leakier' compared to highrise apartment complexes, Li said.
Buildings without air conditioning are also at greater risk if windows are kept open to regulate high temperatures, Siegel added.
The first rule of thumb is to close any doors and windows when it's smoky out. You might even consider taping windows shut if leaks are present, Li said. 'That's not as protective as would be ideal because of the leaks in buildings … but it will help,' Siegel added.
But this may not be an option if your residence has no air conditioning and you're weathering a period of extreme heat: 'The health effects of extreme heat can be more serious than the effects of wildfire smoke,' Siegel said.
If your home has a central forced air system with a MERV 11-rated air filter or better, Siegel recommends keeping your fan going continuously if possible. MERV, or 'minimum efficiency reporting values,' measures how efficiently a filter can capture particles; a MERV 13 to 15 rated filter is preferred, Siegel said.
Unlike many commercial buildings, most residences in Canada recirculate the indoor air instead of pumping in fresh air from the outside, Li said.
TORONTO - The Canadian Ophthalmological Society is urging people to take care of their eyes as wildfires burn across Manitoba and Saskatchewan.
TORONTO - The Canadian Ophthalmological Society is urging people to take care of their eyes as wildfires burn across Manitoba and Saskatchewan.
Siegel also recommended running portable air filters in crowded spaces, especially in areas with susceptible individuals.
The Star previously outlined how to make an affordable filter
using a fan, air filters, duct tape and cardboard.
If possible, Li recommends upgrading your home's filtration system. Consult with a professional first, especially if your home's ventilation system is old, as a high-efficiency filter might overwhelm the system and lead to drops in pressure, she said.
Finally, those at high risk can consider wearing a well-fitted respirator or mask, like an N95 mask, while indoors when it's especially smoky, Siegel explained.
Until recently, building filtration systems have placed less emphasis on keeping wildfire smoke and other environmental pollutants out, Brook noted. And while the COVID-19 pandemic saw a shift toward improving HVAC systems in buildings like schools,
experts and advocates have noted it's not sufficient
to keep up with the increasing prevalence of wildfires.
Almost any upgrades to a building will also make it more resistant to wildfire smoke, Siegel said, from installing new windows to heating, ventilation and air-conditioning retrofits.
The problem is many of the buildings where these improvements are needed most, like schools, are unable to implement them due to budget and funding limitations, he continued.
For urbanites, there's a new way to mark the calendar: the day the smoke reaches the city.
For urbanites, there's a new way to mark the calendar: the day the smoke reaches the city.
'It's an important area that we should be thinking about for investment, because the issue isn't just wildfire smoke,' Siegel said. 'We have increasing ambient air pollution just because of urbanization. We have a lot more construction, a lot more traffic, a lot of major roadways with schools and other buildings near them.'
There's another hurdle. Brook noted that improved ventilation, air-conditioning and filtration systems may end up contributing to the climate crisis.
'If you just start expending more energy to make the indoor air clean, it can becomes a vicious cycle of more emissions if our energy is coming from dirty sources,' he said.
'We have to think, what's our long-term strategy?' Brook continued. 'Why have we stopped talking about how we're going to meet carbon emission targets? How are we going to upgrade our housing and clean air while not increasing our energy usages?'