Almost every guide telling you how to improve indoor air quality is a list of tips in no particular order, and most of them are written for houses in the United States. Health Canada publishes its own list, it puts the items in a deliberate sequence, and the sequence is the useful part. This page follows that order and adds the Ontario rules that decide which of these your house already has. If you came here from a duct cleaning question, the evidence on that specific service is in our guide to air duct cleaning in Ontario.

Three steps to improve indoor air quality, and why the order matters

The order is not a stylistic choice, it is a cost and effectiveness ranking: removing a pollutant at its source costs nothing and works completely, ventilation dilutes what is left for the price of running a fan, and filtration is the most expensive of the three and only catches particles that reach the filter. Health Canada states the principle directly in its guidance on portable air cleaners: the best ways to improve indoor air are to remove the source of the contaminants and to ventilate with clean, outdoor air.

Most home air quality content inverts this. It opens with a device you can buy, because a device is a product and an open window is not. The table below is the order as published.

What good air looks like as a number

Canada publishes target values for indoor air, and knowing them turns a vague worry into something you can check. The list below is short because most indoor pollutants have no residential number attached, which is itself worth knowing before you buy a device that promises to score your air.

The two humidity figures are both Health Canada’s. We are not going to average them and present the result as a single fact. Aim for the overlap, roughly 35 to 50 per cent, and you satisfy both. If your windows fog on cold mornings you are above it, and our guide to an air conditioner that is not removing humidity covers the summer half of that problem.

Step one: reduce the source

Source reduction is the only step that removes a pollutant rather than moving it around, and Health Canada devotes more of its guidance to this than to ventilation and testing combined. The nine categories it names are smoking, the attached garage, cooking, mould and dampness and water leaks, heating appliances, renovations, household items and furniture, hobbies, and house cleaning and laundry.

Three of those are worth pulling out for an Ontario house. The attached garage is one: Health Canada advises against idling a vehicle or running gas-powered equipment in it, storing fuel containers in a detached shed instead, and keeping the door between the garage and the house closed and well sealed. Cooking is another, and the instruction is specific: use the range hood on its high setting and cook on the back burners, because that is where the hood actually captures. The third is heating, where the advice is a yearly inspection of every fuel-burning appliance plus at least one certified carbon monoxide alarm near sleeping areas.

Mould and dampness carry their own instruction, which is to fix moisture problems as soon as they appear, keep relative humidity in range, and dry a flooded home within 48 hours. In this region that usually means a basement, and it is a repair job rather than an air quality product.

Step two: ventilate, and the fan your house may already be required to have

Ventilation is the step where Ontario stops being generic, because the Ontario Building Code sets out what a house must have rather than what a homeowner might like to buy. Section 9.32 of the code requires ventilation in both seasons, and for the heating season it names three things a house needs: a principal ventilation exhaust fan, supplemental exhaust fans, and protection against depressurization.

The detail most homeowners have never been told is in clause 9.32.3.3: a range hood cannot serve as the principal ventilation fan. If you have been treating the kitchen hood as your whole-house ventilation, the code disagrees, and so does the physics. The principal fan is usually a dedicated exhaust fan or the exhaust side of a heat recovery ventilator, and its required airflow is set by the number of bedrooms.

The practical version: go and look. If there is a fan in the basement with two insulated ducts running to the outside, that is a heat recovery ventilator, and a surprising number of them are switched off because somebody found them noisy. Turning it back on is the cheapest air quality improvement available in a newer Ontario home, and it costs one trip down the stairs. If you cannot find one, our indoor air quality services page covers what can be added to an existing system.

Step three: filter, and where the MERV 13 advice goes wrong

Filtration is the third step because it only ever treats the symptom, and the advice circulating about it is half right. Health Canada does recommend a MERV rating of 13 or more to remove fine particulate, and it does recommend a MERV 8 pre-filter alongside it. What almost nobody repeats is the condition attached in the same document: the system has to be able to handle the pressure drop that those filters create.

That condition matters in an Ontario basement. A furnace built around a one inch filter slot and a fixed-speed blower may not move enough air through a MERV 13 media, and the result is reduced airflow, a colder house at the far end of the duct run, and in the worst case a heat exchanger running hotter than it should. The right sequence is to check what the equipment is rated for before upgrading the filter, which is a five minute question for whoever services it. Our guide to how often to replace air filters covers the replacement interval separately.

The two hazards you cannot smell, see or filter

Radon and carbon monoxide sit outside the three steps because neither is caught by a filter and neither announces itself, so both are handled by measurement rather than by improvement. They are also the only two items on this page where getting it wrong has consequences beyond a stuffy room.

The Ontario radon figure deserves one honest caveat. A provincial average of 4.6 per cent hides a lot: the same survey found more than 10 per cent of homes above the guideline in 13 of Ontario’s 36 health regions. A provincial number tells you nothing about your street, which is exactly why the guidance is to test rather than to estimate.

Before you open the window: your AQHI community

Health Canada’s ventilation advice comes with a condition attached, which is to close the windows when outdoor air quality is poor, and that turns “open a window” into a decision that needs a number. Environment and Climate Change Canada publishes the Air Quality Health Index for named communities, and the west GTA is not evenly covered: four of the seven cities we serve have a community of their own on the list and three do not.

This is the part of the advice that changes by address rather than by province, and no American page and no national referral site will ever publish it. If you live in Bolton and you are reading an index measured in Brampton, that is worth knowing before you decide the outdoor air is fine.

Where air duct cleaning actually fits

Air duct cleaning is not on Health Canada’s list of ways to improve indoor air, and that absence is the honest headline of this page even though we sell the service. It is not the same as saying the service is worthless. It is saying that duct cleaning is a remedy for a specific condition rather than a general air quality improvement, and that treating it as the latter is how homeowners end up paying for something that changes nothing they can measure.

The conditions that do justify it, and the pricing and scam patterns to watch for in this region, are set out in our guide to air duct cleaning in Ontario, and the service itself is on our vent and air duct cleaning page. If you want the honest sequence: work through steps one and two on this page first, because they cost almost nothing, and only then ask whether the ducts are the problem.

The possible downside of asking us: we install and service equipment, so our answer to an air quality problem will often be equipment. If your issue turns out to be a source you can remove or a window you can open, that is a better fix than anything we can sell you, and we would rather tell you so on the estimate than after the invoice. We also do not measure radon and we are not a C-NRPP certified firm, so that one goes to someone else.

What should you fix first?

Two questions, and every answer below already appears in the tables above.

How we know this

Watch: the team behind the advice

These are company and furnace clips from our own channel. None of them demonstrates the guidance above, and none should be read as evidence for it.

Frequently asked questions

What is the most effective way to improve indoor air quality?

Removing the source of the pollutant, because it is the only action that eliminates rather than dilutes. Health Canada states that the best ways to improve indoor air are to remove the source of the contaminants and to ventilate with clean, outdoor air, and it puts source reduction first and gives it more space than anything else. Filtration comes third and only ever treats what the first two steps left behind.

What are the three most common ways to reduce pollutants in indoor air?

Source control, ventilation and air cleaning, in that order. Source control means stopping the pollutant entering the air, such as keeping the home smoke-free, not idling in an attached garage and fixing damp. Ventilation means diluting what remains with outdoor air. Air cleaning means filtering the particles that reach a filter, which excludes gases such as carbon dioxide and radon entirely.

What are the indoor air quality guidelines in Canada?

Health Canada publishes Residential Indoor Air Quality Guidelines for a short list of pollutants, and separate target conditions for cleaner air spaces: carbon dioxide below 1,000 ppm, carbon monoxide below 10 ppm, temperature below 26 C, relative humidity of 35 to 50 per cent, and fine particulate as low as possible. Radon is guided separately at 200 becquerels per cubic metre. Most household pollutants have no residential number at all.

How to purify air in home without an air purifier?

Use the two steps that come before filtration. Remove sources by keeping the home smoke-free, cooking on back burners with the range hood on high, storing fuel and running engines outside an attached garage, and fixing damp within 48 hours. Then ventilate: open interior doors, run bathroom and kitchen fans, keep vents clear, and open windows when outdoor air quality is good.

How to reduce indoor humidity with HVAC?

In summer the air conditioner is the dehumidifier, and it only removes moisture while the compressor runs, so an oversized unit that satisfies the thermostat in short bursts leaves the house cool and damp. Run the fan on auto rather than on, because a fan running after the coil stops re-evaporates water back into the air. In winter, reduce the humidifier setting instead and use exhaust fans, and target 35 to 50 per cent either way.

Can you run AC with poor air quality?

Yes, and during a smoke event it is usually the better option, because a central system with the windows closed lets you filter the air you are recirculating. Health Canada’s guidance for smoke events is to switch to recirculation while outdoor air is poor, then draw fresh air back in once conditions improve so that carbon dioxide does not build up. A window air conditioner with an outdoor air vent should have that vent closed.

The short version

Improve indoor air quality in the order the evidence supports. Reduce the source, because that is the only step that removes anything and it usually costs nothing. Ventilate second, and in an Ontario house that starts with finding out whether you already have a principal ventilation fan or a heat recovery ventilator, and whether it is switched on. Filter third, after confirming your system can handle the filter you are about to fit.

Then handle the two that sit outside the sequence. Put a long-term radon test on the lowest lived-in level for at least 91 days through a heating season, and make sure there is a certified carbon monoxide alarm near where people sleep, which Ontario law requires anyway.

Air duct cleaning is not on Health Canada’s list, and we would rather tell you that than sell it to you as an air quality upgrade. If after the free steps you still want equipment, call 905-564-8545 or book a free in-home estimate and we will tell you what would actually change the number.

Sources and references

• Health Canada, Improve indoor air quality in your home. The nine source categories, the six ventilation actions, the two tests, the 30 to 50 per cent humidity range and the 48 hour drying instruction. Date modified November 26, 2024. Read September 7, 2026.
• Health Canada, Guidance for Cleaner Air Spaces during Wildfire Smoke Events. MERV 13 or more with a MERV 8 pre-filter, the pressure drop condition, two to three room volumes per hour, and the target conditions for carbon dioxide, carbon monoxide, temperature and humidity. Date modified November 13, 2020. Read September 7, 2026.
• Health Canada, Using a portable air cleaner to improve indoor air. HEPA filtration, the clean air delivery rate rule of at least two thirds of the room area, the warning on ozone-producing devices, and the statement that the best ways to improve indoor air are to remove the source and to ventilate. Date modified December 24, 2021. Read September 7, 2026.
• Health Canada, Guide for radon measurements in residential dwellings. The minimum 91 day test through the heating season, the lowest lived-in level, and the 0.3 to 2.5 metre breathing zone placement. Date modified May 22, 2026. Read September 7, 2026.
• Health Canada, Cross-Canada Survey of Radon Concentrations in Homes. The 4.6 per cent Ontario figure against 6.9 per cent nationally, and the finding that 13 of Ontario’s 36 health regions exceeded 10 per cent. Published March 2012. Read September 7, 2026.
• Environment and Climate Change Canada, Air Quality Health Index, Ontario summary. The list of named Ontario communities, including Mississauga, Brampton, Milton, Oakville, Burlington and Toronto, and the absence of Etobicoke, Georgetown, Caledon and Bolton. Read September 7, 2026.
• Heating, Refrigeration and Air Conditioning Institute of Canada, What’s New in Section 9.32. The heating season requirement for a principal ventilation exhaust fan, supplemental exhaust and depressurization protection, the rule that a range hood cannot be a principal fan under 9.32.3.3, and the heat recovery ventilator condition in 9.32.3.12. The 2024 Ontario Building Code took effect January 1, 2025. Read September 7, 2026.