Wildfire smoke and indoor air quality in Calgary

On July 24th of this year, Calgary recorded the worst air quality of any city in Canada. The Air Quality Health Index hit 10+ — the "very high risk" band, the top of the scale — as smoke from fires in British Columbia and Washington State settled over the city. For a couple of days Calgary was on the short list of the worst air on the planet.

That's not a freak event, it's a trend line. Between 1981 and 2010, Calgary averaged about 12 smoke hours per season. In 2023 the city logged 512. 2024 came in around 200. 2025 was mercifully quiet. The average since 1990 sits near 73 and is climbing, because the fires are burning bigger, hotter and earlier than they used to.

For a homeowner in Calgary, Airdrie, Okotoks or Cochrane, the practical takeaway is this: you now need a plan for two or three bad weeks a year, the same way you have a plan for -30°C. And the equipment that handles it is already half-installed in your basement.

The thing that matters in wildfire smoke is PM2.5 — particulate matter under 2.5 microns. Size is the whole problem. Particles that small slip past the nose and throat entirely, travel deep into the lungs, and at high concentrations cross into the bloodstream, where they drive inflammation that reaches the heart and vascular system. Health Canada's position is that fine particulate is the main health risk in wildfire smoke, and that there's no known safe exposure level.

Closing your windows helps, but it doesn't seal you in. A typical Calgary house exchanges its air with the outdoors continuously through gaps in the envelope, and PM2.5 rides in on that air. Indoor levels track outdoor levels with a lag — which is exactly why a house with good filtration behaves so differently from one without during the same advisory.

The good news: the EPA has found that upgrading to a higher-efficiency filter and running the system fan continuously can cut indoor PM2.5 roughly in half during a smoke event. That's a big result for a cheap change.

MERV is the rating scale for how much a filter captures. The cheap fibreglass panel at the hardware store is around MERV 4 — it exists to keep lint off your blower motor, not to clean your air. MERV 8 catches dust and pollen. MERV 13 is where smoke starts getting caught: roughly 90% capture in the 0.3–1.0 micron range, which is precisely where wildfire PM2.5 lives. It's the minimum the EPA recommends for smoke.

The objection I hear constantly is "won't that starve my furnace?" It's a fair question and the answer is nuanced. A restrictive filter does raise static pressure, and a blower fighting static pressure moves less air, runs hotter and dies younger. But the fix isn't a worse filter — it's more filter. A 4- or 5-inch media cabinet has several times the surface area of a 1-inch panel, so it captures more while restricting less. It also lasts far longer between changes.

Two operational notes worth more than the equipment itself. First, set your thermostat fan to ON / continuous during an advisory instead of AUTO. A filter only filters when air is moving through it, and in July your furnace might only fire the blower a few minutes an hour. Second, change filters every 2 to 4 weeks while smoke is active. Pull it and look — grey media means it's working, and means it's nearly done.

True HEPA captures 99.97% of particles at 0.3 microns. You generally can't put a true HEPA filter in a standard residential furnace — the pressure drop is more than a residential blower is built for, which is why bypass-HEPA units exist as a separate ducted appliance with their own fan.

For most homes, the higher-value move is simpler: a portable HEPA unit sized to the room you sleep in. You spend a third of your life in that room, and a portable unit runs continuously regardless of whether the furnace is calling. Match the unit's clean air delivery rate to the room's square footage, run it on a middle speed so you'll actually tolerate the noise overnight, and keep the bedroom door shut.

One caution on portable units: avoid anything marketed on ozone generation or "activated oxygen." Ozone is a lung irritant. You're trying to reduce the load on your respiratory system, not swap one irritant for another.

A heat recovery ventilator brings fresh outdoor air in and exhausts stale indoor air, recovering most of the heat in the exchange. In a modern tight Alberta home it's essential equipment — it's what keeps CO2 down and keeps humidity from condensing on your windows in February.

During a smoke event it's working against you. Its entire purpose is pulling outdoor air inside, and stock HRV filters are not smoke filters. So during an advisory: drop it to its lowest scheduled setting, or use recirculation / standby mode if your unit has one. Some units accept a MERV 13 cartridge on the supply side — worth asking about, because it lets you keep ventilating at a reduced rate instead of choosing between smoke and stale air.

Then flush the house once the plume moves off. Run the HRV up, open windows on a clear evening. A week of no ventilation in a tight house means CO2, humidity and cooking odours all stacking up, and that's its own comfort problem.

The same logic applies to your furnace's fresh-air intake and to opening windows for night cooling — a habit that works beautifully in Calgary's dry climate right up until the sky turns orange. On smoke nights, keep them shut and lean on mechanical cooling. If your house doesn't have any, that's the real gap; a heat pump cools in summer and heats in the shoulder seasons, and it lets you keep the windows closed without cooking.

I'll be blunt here because the marketing in this category is bad. A UV-C lamp mounted at the evaporator coil works by deactivating the DNA of living organisms — mould, bacteria, biofilm. Your AC coil is wet all summer and sits in the dark, which makes it the best mould habitat in your house. Keeping it clean is genuinely worth doing: it improves heat transfer, so the system runs more efficiently, and it stops that musty smell on startup.

What UV-C does not do is capture particles. Smoke PM2.5 flies through a UV beam untouched. The CDC's position on germicidal UV is that it's an adjunct measure — it cannot replace HEPA filtration. So if a contractor quotes you a UV light as your wildfire smoke solution, that's the moment to get a second opinion.

Buy UV for coil hygiene and efficiency. Buy filtration for smoke. Both can live in the same system; they're just answering different questions.

A whole-home humidifier does nothing for smoke. It's on this list because of what comes next. Alberta winter air is brutally dry, and a respiratory system that spent August irritated by particulate goes into a January heating season with no margin. Dry mucous membranes are worse at clearing irritants, which is why the sore-throat-and-nosebleed season lands harder on people who had a rough smoke summer.

Target 30–40% relative humidity in winter. Higher than that in a Calgary house and you'll get condensation on the windows and, eventually, mould in the window frames — which is a real problem, unlike the one you were solving. Bypass humidifiers are the budget option; steam units hold a setpoint far better in a cold, dry climate but need a dedicated circuit.

And service the thing. An unmaintained humidifier with a scaled-up pad is a microbial amplifier plumbed straight into your ductwork. If yours hasn't been touched in three years, that's a bigger air quality liability than anything the wildfires sent you.

We're not going to print a price list, because the honest answer is that it depends on what's already in your basement. A home with a 5-inch media cabinet already installed needs a $60 filter and a thermostat setting change. A home with a 1-inch slot, a marginal blower and no ventilation is a different conversation, and quoting it sight-unseen would be guessing.

What drives the number: whether your ductwork can accept a media cabinet without modification, what static pressure your blower is already fighting, whether you have an HRV at all, and whether a humidifier needs a new water line and circuit. Those are things we measure, not things we assume.

So here's the practical path. Use the instant estimate tool for a ballpark in a couple of minutes. If you've already got a quote from someone else, run it through our quote scanner — it'll tell you what's in it and what's missing. And if you want a real number, book an IAQ assessment and we'll measure the system before we price anything.