What Is Indoor Air Quality (IAQ)?

In practice, indoor air quality means looking at the whole building, not reading one number off a meter. Our inspectors ask plain questions. 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 just into the attic?

Time is why it matters. Health Canada estimates that Canadians spend about 90% of their time indoors. That’s roughly 21 hours of every day, so the air inside your house is most of the air you’ll ever breathe.

Moisture, building materials, combustion, everyday products, people and outdoor air all affect the air inside a home. Moisture and materials are where we look first.

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

It depends on the pollutant. What a home makes itself (moisture, mould spores, cooking fumes, chemicals from products) builds up indoors because the air is trapped. Outside, the same things spread out.

Fine particles often go the other way. In Health Canada’s guidance on fine particles, indoor levels were generally lower than outdoor levels, except in homes with smokers.

Canadian winters tip the balance. We seal our homes tight against the cold for months, which saves heating money and also keeps in moisture and cooking fumes. A modern airtight house without good mechanical ventilation can feel stuffy by February, and better ventilation is the fix.

Wildfire smoke flips it again. Health Canada’s cleaner air guidance starts with closed doors and windows and good filtration.

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 at the house. The US EPA lists eye, nose and throat irritation, headaches, dizziness and fatigue as common short-term effects.

The house gives its own signs too:

• A musty or earthy smell, especially in the basement or a closet
• Condensation that runs down the windows and pools on the sill on winter mornings
• Black or grey spots on walls, ceilings or window frames
• Water stains, or a past flood or leak that was never fully dried

Some people feel it sooner. Health Canada names infants, children, seniors, pregnant people and people with respiratory conditions such as asthma as being at greater risk from mould. A test can’t diagnose anyone, so health worries belong with your doctor. Our checklist of signs your home needs air quality testing covers the building side.

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 targets, not laws a homeowner has to meet.

These are Health Canada’s guideline values for homes (page updated September 2025):

That last row is the one people don’t expect. Canada has no legal or health-based number for mould. In its mould guideline, Health Canada says the many mould species and the way people react differently rule out an exposure limit. So be wary of anyone who says a spore count passed or failed a Canadian limit.

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 monitors that track conditions over time, and with lab-analyzed samples that show what’s in the air at the moment of sampling.

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 goes further. 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.

So when is an air test worth it? Health Canada’s exception is concerns that stay unexplained after an inspection, especially after flooding, roof leaks or pipe leaks that may have reached wall cavities. A qualified professional may then recommend a non-viable air test, which is what a spore trap is. We’d also sample before buying a home or to confirm a cleanup worked, because you want a dated result to compare with. Those two are our judgment, not Health Canada’s.

Start with moisture, then ventilation, then the sources, then filtration. 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 is usually the simplest fix for a damp basement.

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 check it’s actually running.

Cut the sources. Skip scented candles and plug-in fresheners, since they add chemicals rather than remove odours. Store paint and solvents away from living space. Dry leaks fast: Health Canada says mould can begin to grow within 48 hours.

Filter what’s left. Change the furnace filter on schedule. Health Canada’s wildfire guidance recommends MERV 13 or higher for fine particles (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 says a professional is needed for mould that keeps coming back after cleaning, and for any single patch larger than 3 square metres (about one full 4-by-8 sheet 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 inspectors are IICRC certified, and an accredited third-party lab reads every sample. If the results say you don’t need to do anything, we’ll tell you that too.