Radon in Toronto: What GTA Homeowners Need to Know

Toronto has lower radon levels than most major Canadian cities. The GTA average sits around 43 Bq/m³, and about 1 in 22 homes in the metro area exceeds Health Canada's action level of 200 Bq/m³. Compare that to Edmonton at 1 in 6 or Winnipeg at 1 in 5, and Toronto looks relatively safe.

The problem with that comparison is that Toronto has 6 million people. At 4.5%, that's still roughly 120,000 homes with elevated radon. And the city's own public health office estimates that seven per cent of Toronto buildings have radon at levels that may pose a health risk, and that about 10 deaths per year could be prevented if those levels were reduced. Low risk relative to Calgary or Winnipeg is not the same as no risk.

Why Toronto runs lower than other Canadian cities

The short answer is geology. Southern Ontario sits primarily on limestone and shale bedrock, which contains lower uranium concentrations than the granite of the Canadian Shield or the glacial sediments of the Prairies. Less uranium in the ground means less radon gas produced.

The GTA also sits on deep lake clays deposited by glacial Lake Iroquois, the predecessor to Lake Ontario. These dense clay layers absorb and slow the upward movement of soil gas, reducing how much radon migrates into homes compared to the coarser, more permeable soils of Manitoba and Alberta.

This is why Vancouver runs even lower — coastal geology and river delta deposits produce very little radon — while Winnipeg and Edmonton run much higher. Toronto lands somewhere in the middle of the Canadian spectrum.

Where Toronto radon risk is highest

Within the GTA, risk is not uniform. A few factors push certain homes and areas higher:

Older homes with basements. Pre-1980 construction in the inner suburbs, Scarborough, Etobicoke, North York, and East York typically has more foundation gaps, older poured concrete or block construction, and less sealed entry points. These homes accumulate radon more easily than tightly built newer ones.

Areas with different underlying geology. Parts of the GTA sit closer to geological transitions. Homes in Scarborough and Durham Region, where the Canadian Shield influence is more pronounced, can test higher than the city average.

Construction disturbance. Toronto has seen decades of heavy construction. SafeAir, a Toronto-based radon testing company, has documented cases where nearby excavation and soil disruption temporarily spikes radon in neighbouring homes. This is worth keeping in mind in any rapidly developing neighbourhood.

Below-grade living spaces. Radon accumulates at its highest concentrations in the lowest parts of a home. A basement apartment or home office below grade will have meaningfully higher exposure than an upper floor of the same house.

None of this means your specific home is elevated. It means testing is the only way to know.

New construction and condos in Toronto

Toronto's construction boom has produced hundreds of thousands of new units, many of them condos and stacked townhomes. The radon picture for new construction in Toronto is generally better than for older homes, for a few reasons.

Modern building codes include provisions for radon-resistant construction in new builds, including sub-slab rough-ins and vapour barriers. Toronto's lower-radon geology also means new construction starts from a lower baseline.

For condo units, floor level matters. Ground-floor and basement units have the most exposure. Mid-rise and high-rise units above the third floor have very low risk and testing is generally not necessary.

If you're in a new Toronto home, Health Canada recommends testing 1 to 2 years after move-in to get an accurate baseline once the home has settled into normal living patterns.

For more on condos and radon: Radon in condos and new builds.

How to test your Toronto home

The process is the same as anywhere in Canada. A long-term alpha track test, placed in the lowest livable area of your home for at least 91 days, gives you the most reliable result. Run it during heating season, October through April, when windows are closed and radon concentrations are highest.

Option 1: Mail-in long-term test kit (~$30 to $60)

Buy a kit, place it in your basement or lowest regularly used floor, leave it for 91 days, mail it to the lab. Results arrive within a couple of weeks. This is the method Health Canada recommends for mitigation decisions.

Option 2: Electronic continuous monitor (~$160 to $350)

A digital monitor gives you ongoing readings and a long-term average. C-NRPP approved models include the Airthings Corentium Home, Aranet RN+, and Ecosense RadonEye. Leave it in place for at least 90 days before relying on the long-term reading.

For placement guidance and timing: How to test your home for radon.

What to do with your result

Radon level	What Health Canada recommends
Under 100 Bq/m³	No action; retest every 2 to 5 years
100 to 200 Bq/m³	Consider mitigation; confirm with a long-term test
200 to 600 Bq/m³	Mitigate within 2 years
Above 600 Bq/m³	Mitigate within 1 year

Given Toronto's average of 43 Bq/m³, most homeowners will test well below the action level. But "most" is not all, and a result between 100 and 200 Bq/m³ is common enough to be worth thinking about, especially for older homes with finished basements where people spend regular time.

If your result is at or above 200 Bq/m³, sub-slab depressurization is the standard fix. A certified contractor drills through your foundation slab, routes a pipe to a continuously running fan, and vents radon outside before it can build up indoors. Most systems are installed in a day and reduce indoor radon by 80 to 99%.

For a plain-language read on what your number means: How to interpret your radon test results.

What radon mitigation costs in Toronto

Toronto sits at the higher end of the national range, reflecting the city's labour costs and the complexity that finished basements and older foundations can add.

Standard sub-slab depressurization: $2,000 to $3,500
Complex configurations (finished basement, multiple suction points, difficult pipe routing): $3,500 to $5,000
Post-mitigation re-test: $30 to $60, done 30 to 90 days after installation

Get quotes from at least two certified contractors. Any legitimate quote should include post-installation testing or clear direction on how to arrange it yourself.

For a full cost breakdown: How much does radon testing and mitigation cost in Canada?

Radon in Toronto real estate

Toronto's real estate market is among the most active in the country. Radon testing in purchase transactions is growing, though it's not yet as common here as in higher-risk cities like Calgary or Winnipeg.

For a Toronto buyer, including a radon test condition makes sense for any home with a finished basement, particularly pre-1980 construction in the inner suburbs. Short closing timelines call for professional testing with continuous monitors, which produce results in 48 to 96 hours rather than 91 days.

Sellers who have already tested and show a clean result are in a good position in a market where informed buyers are starting to ask. A mitigated home with documentation is easier to sell, not harder.

For more on handling radon in a transaction: Radon and real estate in Canada.

Find a certified radon professional in Toronto

All professionals listed on RadonFinder.ca hold active C-NRPP certification, the standard recognized by Health Canada for radon measurement and mitigation in Canada.

Browse certified radon professionals serving Toronto

When contacting a contractor, ask whether they're certified for both testing and mitigation (separate credentials), whether post-mitigation testing is included in the quote, and what warranty they offer on the system.

The bottom line

Toronto's radon risk is real but modest compared to most Canadian cities. The average GTA home tests below the action level, and the odds of finding an elevated result are lower here than in Calgary, Edmonton, Ottawa, or Winnipeg. But low odds are not zero odds, especially in older homes with finished basements. A long-term test kit costs $30 to $60 and takes 91 days. For the cost of a dinner out, you'll know where you stand.