What Is Indoor Air Quality (IAQ)?

Indoor air quality is the condition of the air inside a home, school or workplace, judged by how it affects the health and comfort of the people breathing it. IAQ stands for exactly that: indoor air quality.

It matters more than most people think. Health Canada estimates that Canadians spend about 90% of their time indoors. So the air inside your house isn’t a side issue. It’s most of the air you’ll ever breathe.

When our inspectors assess IAQ, we don’t look at one number. We read the building. Is there more mould in the air inside than outside? Is the relative humidity sitting at 65% in a basement that should be closer to 45%? Does the bathroom fan actually push air outdoors, or into the attic (we find that one more often than you’d expect)? After 15,000+ inspections since 2005 across Ontario and Quebec, we’ve learned that the air almost always tells you about the house. Fix the house and the air usually follows.

Most indoor air problems start inside the building, not outside it. Outdoor air plays a part, but the biggest swings we measure come from moisture, materials and daily habits.

Want the details on each one? Our guide to common indoor air pollutants covers them one by one.

Often, yes. Indoor air can hold more of certain pollutants than the air outside, because a home traps what it produces. Outdoor air disperses. Indoor air builds up.

Canadian winters make this worse. We seal our homes tight against the cold for five or six months, which saves heating money but also keeps in moisture, cooking fumes and everything else. A modern airtight house without good mechanical ventilation can feel stuffy by February. That’s not a defect in the house. It’s a ventilation problem.

There’s a flip side, though. On a heavy wildfire smoke day, a closed-up home with a good furnace filter is usually the cleaner place to be. So “indoor is worse” isn’t a rule. It depends on the day and the house.

The clearest sign is symptoms that improve when you leave home. A cough, headache or stuffy nose that fades at the office and comes back each evening points straight at the house.

Watch for these too:

• Itchy eyes, sneezing or a sore throat that won’t clear up
• Headaches, tiredness or trouble focusing, mostly at home
• Asthma or allergies flaring more than usual
• A musty or earthy smell, especially in the basement or a closet
• Condensation that runs down the windows and pools on the sill every winter morning
• Black or grey spots on walls, ceilings or window frames
• Dust that comes back a day after you clean

Some people feel poor air much sooner than others. Babies and young children breathe more air for their body size. Older adults, pregnant people, anyone with asthma or a lung condition, and people with weakened immune systems are all at higher risk. If someone in your home falls into one of those groups, it’s worth taking the signs seriously sooner. Our checklist of signs your home needs air quality testing goes deeper.

Canada’s main benchmark for homes is Health Canada’s Residential Indoor Air Quality Guidelines. They set recommended exposure limits for common indoor pollutants. They’re voluntary guidance, issued under the Canadian Environmental Protection Act, so they aren’t laws a homeowner has to meet.

Here are the guideline values for the pollutants we hear about most (Health Canada, updated September 2025):

That last row surprises people. There’s no legal or health-based number for mould in Canada. In its mould guideline, Health Canada says the number of mould species and the way people react differently make a safe limit impossible to set. Its advice is simple: control humidity, repair water damage quickly and clean any mould you find.

Workplaces are a separate case. Offices and commercial buildings fall under provincial workplace health and safety rules in Ontario and Quebec, which is a topic for our commercial air quality testing page.

IAQ is measured two ways: with continuous monitors that track conditions over time, and with lab-analyzed samples that identify exactly what’s in the air.

A home monitor (they run about $200 to $400) is good at trends. It’ll show your humidity climbing every time someone showers, or carbon dioxide rising in a closed bedroom overnight. It can’t tell you whether you have mould, which kind, or where it’s coming from.

Professional sampling answers those questions. For mould, our inspectors use a calibrated pump to pull a set volume of air through a spore-trap cassette, take a matching outdoor sample the same day, and send both to an accredited lab. The results come back as spores per cubic metre. For suspected asbestos, we sample materials or air and the lab reads them under a microscope. The full step-by-step is on our inspection process page.

Start with moisture, then ventilation, then the sources, then filtration. That order matters. A $600 air purifier won’t fix a basement that sits at 70% humidity.

Keep humidity between 30% and 50%. That’s Health Canada’s recommended range. A $20 hygrometer tells you where you stand. In a cold Ontario or Quebec January, aim for the lower end. If water streams down your windows each morning, the air is too wet for the outside temperature. In summer, a dehumidifier in the basement does more for most homes than anything else on this list.

Move the air out. Run the bathroom fan during every shower and for 20 minutes after. Use the range hood whenever you cook, ideally one that vents outside instead of just recirculating. If your home has an HRV (heat recovery ventilator), have it cleaned and make sure it’s actually running. We find them switched off more often than you’d think.

Cut the sources. Skip scented candles and plug-in fresheners, since they add chemicals rather than remove odours. Store paint and solvents in the garage, not the basement. Fix leaks within a day or two, before mould gets a foothold.

Filter what’s left. Change the furnace filter on schedule. A MERV 11 to 13 filter catches far more fine dust than the cheap fibreglass kind (check your furnace can handle it). A HEPA purifier helps in a bedroom, especially for allergies or during smoke season.

And houseplants? We like them. They just don’t clean the air in any way you’d notice. A 2019 Drexel University review found you’d need somewhere between 10 and 1,000 plants per square metre of floor to match what ordinary ventilation already does. Open a window instead.

Call an inspector when the problem has a cause you can’t find or fix yourself. That usually means a musty smell with no visible source, symptoms that clear up when you’re away from home, water damage behind walls, or plans to renovate an older home that might contain asbestos.

Health Canada also suggests professional help for any single patch of mould larger than about three square metres (roughly a sheet and a half of drywall).

If that sounds like your home, our residential air quality testing covers the full inspection, and we offer focused mould testing and asbestos testing across Ontario and Quebec. Our IICRC-certified inspectors use an independent accredited lab, so the report reflects what’s in your air and nothing else. And if the results say you don’t need to do anything, we’ll tell you that too.