Radon Mitigation Ontario 2026: Testing, 200 Bq/m3 Action Level, and Sub-Slab Depressurization Cost

What Radon Is and Why It Matters

Radon is a naturally occurring radioactive gas produced by the slow decay of uranium in soil and rock. It seeps up through the ground everywhere in Canada, enters buildings through cracks, service penetrations, and the porous foundation itself, and then accumulates indoors (especially in basements and lower levels) if the building is well sealed against outside air. Outdoors, it disperses to near-zero concentrations almost immediately. Indoors, it can build up to levels that materially raise your risk of lung cancer.[1]

Health Canada estimates radon is responsible for about 16 percent of lung cancer deaths in Canada, making it the second leading cause of lung cancer after smoking and the number one cause for non-smokers. The Canadian Cancer Society reinforces this: radon exposure is a modifiable risk factor, and testing plus mitigation meaningfully lowers long-term cancer risk for the household.[6] The Canadian Lung Association provides consumer-facing guidance on the same point.[5]

Ontario's geology is not the worst in Canada for radon (the Prairies and parts of Atlantic Canada have higher average readings), but there is no safe zone. Homes in every Ontario county have tested above the action level. Two houses on the same street can test very differently because radon entry is driven by building factors (foundation type, crack pattern, air pressure dynamics) as much as by the soil underneath. The only way to know your level is to test your home.

The Health Canada 200 Bq/m3 Action Level

The Canadian guideline for indoor radon in dwellings is 200 becquerels per cubic metre (200 Bq/m3), averaged over a minimum three-month measurement during the heating season. That number, and the measurement protocol that backs it up, comes directly from Health Canada's Guide for Radon Measurements in Residential Dwellings.[2]

Under the guideline:

• Below 200 Bq/m3: no remedial action required. Retest every five years or after major renovations that change the building envelope.
• 200 to 600 Bq/m3: Health Canada recommends mitigation within two years.
• Above 600 Bq/m3: mitigate within one year.

The World Health Organization uses a lower reference level of 100 Bq/m3, and some homeowners choose to mitigate at lower levels as a precaution. That is a reasonable personal choice, but the regulatory and insurance-relevant number in Canada is 200. Public Health Ontario's own radon guidance refers back to the same Health Canada guideline for Ontario residents.[7]

Ontario's Building Code has required soil-gas control measures in new-home construction since the 1990s (Article 9.13 of the Building Code Regulation), including a rough-in for a future active radon system in new basements. These measures reduce but do not eliminate radon ingress, and they do not apply retroactively to existing homes. Even a newer home can test above 200 Bq/m3.[8]

DIY Test Kit vs Professional Long-Term Test

The first step is always a long-term measurement. Radon varies day to day and season to season (it is usually higher in winter when houses are closed up), so a short one-week snapshot can mislead in either direction. The Canadian guideline is written around a minimum 91-day test during the heating season for a reason.[2]

DIY alpha-track kits ($30 to $50)

An alpha-track kit is a small plastic chamber with a strip of film inside. You place it on the lowest lived-in level of the house (usually the basement or lowest bedroom), leave it there for at least 91 days, then seal it up and mail it back to the certified lab printed on the package. Four to six weeks later you get a written report in Bq/m3. The Take Action on Radon program, a Health Canada and Canadian Cancer Society partnership, sells kits at cost and publishes consumer-facing instructions on how to deploy them.[3]

For a homeowner doing baseline testing on a single-family home, this is the right spend. The kit is accurate, the lab is certified, and the $30 to $50 cost makes it an obvious starting point before any discussion of mitigation.

Professional long-term test ($150 to $400)

A C-NRPP certified measurement professional will bring in the same style of long-term device (or sometimes a continuous radon monitor for 7 to 10 days if the timeline is compressed) and provide a written report with their certification number on it. The extra cost pays for independence, documentation, and the professional's judgment on placement in complex layouts. Use this path for:

• Real-estate transactions where a buyer wants a third-party report.
• Homes with multiple suites, walkout basements, or additions on different footings.
• Follow-up confirmation after a mitigation system is installed.
• Situations where you want the report signed by a certified professional for insurance or legal reasons.

Find a C-NRPP measurement professional through the public directory at c-nrpp.ca. Verify their certificate is current before signing anything.[4]

Sub-Slab Depressurization: Cost and Process

If your long-term test comes back above 200 Bq/m3, the standard fix for a typical Ontario home with a full basement is active sub-slab depressurization. It is the most effective radon mitigation technique, the one C-NRPP trains against, and the one a certified mitigator will recommend nine times out of ten.

Installed cost: $2,500 to $4,500 for a standard single-family home in Ontario, including the post-installation confirmation test. Homes with unusual footprints (multiple slabs on different levels, large square footage, stone or rubble foundations) can run higher.

What the installer actually does

1. Site assessment: the mitigator inspects the basement slab, identifies the best suction point, maps a pipe route up through the house, and picks an exterior or attic location for the fan.
2. Suction pit: a 10 to 15 cm hole is cut through the basement floor. A small amount of gravel below the slab is removed to create a suction cavity.
3. Pipe run: a 4-inch (100 mm) PVC pipe is sealed to the slab, routed up through the house (typically through a closet, garage, or exterior chase), and terminated above the roofline at least two feet above the highest point within 10 feet.
4. Fan: a continuous-duty in-line radon fan is installed on the pipe in an unconditioned space (attic or exterior). The fan runs 24/7 and costs about $50 to $120 a year in electricity.
5. Sealing: any remaining visible cracks, sump pit covers, and service penetrations are sealed to improve system efficiency.
6. Post-mitigation test: a short-term continuous radon monitor is deployed for 48 to 96 hours immediately after installation to confirm the system is working, and a long-term test is recommended within six months to confirm sustained performance.

A correctly installed sub-slab depressurization system will typically drop basement radon by 80 to 99 percent. A home testing at 500 Bq/m3 commonly ends up under 50 Bq/m3 after mitigation.

HRV-Based Dilution: When It Makes Sense

Heat recovery ventilators and energy recovery ventilators move stale indoor air out and fresh outdoor air in while recovering most of the heat (or cool) from the exhaust stream. By introducing more fresh air, they dilute indoor radon. HRV-based radon reduction costs $4,000 to $8,000 installed in Ontario, and it is the right answer in a narrow set of cases:

• Slab-on-grade homes with no accessible sub-slab footprint for a suction pit.
• Tight newer homes with very low natural ventilation rates, where additional fresh air will meaningfully dilute radon and improve indoor air quality generally.
• Marginal starting levels (say, 220 to 300 Bq/m3) where the dilution effect plus modest crack sealing is enough to get under 200 Bq/m3.

For a typical Ontario basement testing at 400 Bq/m3 or higher, sub-slab depressurization is almost always the better spend: cheaper, more reliable, and specifically engineered for the problem. An HRV has other benefits (fresh-air ventilation, humidity control) that can justify the cost on its own merits, but it should not be sold to you as a radon fix when a depressurization system would do the job for half the price.

Crack Sealing Alone Rarely Works

There is a persistent belief that sealing visible basement cracks, sealing the sump pit, and caulking around pipe penetrations will solve a radon problem. It will not, at least not on its own. Field data collected by C-NRPP mitigators and published by Take Action on Radon consistently shows that sealing alone reduces radon levels by 10 to 30 percent, which is nowhere near enough to move a 400 Bq/m3 home below the 200 Bq/m3 action level.[3]

Radon enters through the entire foundation footprint, not just the visible cracks. The slab itself is porous to soil gas. Sealing visible entry points reduces the easy pathways but does not remove the pressure differential that is pulling soil gas into the home in the first place. That is what an active depressurization system fixes, and why it works when sealing does not.

That said, crack sealing is still worth doing as a companion measure. A $400 sealing pass before the mitigator arrives improves the efficiency of the eventual depressurization system and is a reasonable cheap first step if you are waiting a month for an installer. Just do not let anyone sell you sealing alone as a complete fix.

C-NRPP Certified Contractor Verification

The Canadian National Radon Proficiency Program (C-NRPP) is the certification body for radon measurement and mitigation professionals in Canada. C-NRPP certification is what Health Canada and Take Action on Radon point consumers to, and it is the credential Ontario mitigators need to operate credibly.[4]

There are two relevant C-NRPP certifications:

• Radon Measurement Professional: certified to conduct and interpret radon tests, including real-estate transactional testing.
• Radon Mitigation Professional: certified to design and install mitigation systems, including sub-slab depressurization.

For a mitigation project, you want someone with the Mitigation credential. Some contractors hold both. Before signing any contract:

1. Look the contractor up in the C-NRPP public directory at c-nrpp.ca. The directory is searchable by name, company, and province. If they are not in the directory, they are not certified, full stop.
2. Confirm the certificate is current and the scope matches the work (measurement vs mitigation).
3. Ask for two references from Ontario installations in the past 12 months, including the post-mitigation test results. A good mitigator will have dozens.
4. Get a written scope that specifies the fan model, the pipe route, the termination location, the post-mitigation test method, and a warranty of at least five years on the system itself.

The flip side is that radon mitigation is not a highly competitive market in most Ontario regions, which means pricing can vary significantly between certified installers. Get two or three quotes. The price spread on the same scope is often 20 to 30 percent.

Real-Estate Disclosure and Insurance

Radon is increasingly showing up in Ontario real-estate transactions. Buyers' agents are asking. Some home inspectors now offer a radon add-on. The legal position in Ontario is nuanced: there is no specific statutory requirement on the standard OREA Seller Property Information Statement to disclose indoor radon levels, but sellers have a general duty not to misrepresent known latent defects. If you have tested and know you are above 200 Bq/m3, a misleading answer to a direct question from a buyer is a legal risk.

The practical takeaway for sellers: test before listing. Mitigate if needed. Keep the C-NRPP mitigator's paperwork and the post-mitigation confirmation test. A mitigated home with a documented low result is a stronger sell than an untested home, and it eliminates the negotiating leverage a buyer gets from commissioning their own radon test during the conditional period.

For buyers: if a home has not been tested, a short-term continuous radon monitor deployed during the conditional period is a reasonable $150 to $300 expense. A result above 200 Bq/m3 is a legitimate basis to negotiate a mitigation credit (typically $3,000 to $5,000 to cover a sub-slab depressurization system with margin).

Related Guides

• Indoor Air Quality Ontario 2026
• Basement Waterproofing Cost Ontario 2026
• Foundation Repair Cost Ontario 2026

Frequently Asked Questions