Radon Map of Ontario: Where the Hot Spots Actually Are
Wellington County. Kingston. Ottawa.
Three Ontario regions where 25 to 50 percent of tested homes sit above the Health Canada action level of 200 Bq/m³.
Most homeowners assume Ontario is a low-radon province. Across the whole province, the average reading is around 92 Bq/m³.
This post maps Ontario by real radon data, region by region. We’ll show you the actual numbers, the surprising places, and the reason a map alone can’t tell you if your home is safe.
The short version:
- Ontario’s province-wide average is about 92 Bq/m³, but regional averages swing from 40 to 300+ Bq/m³
- The worst regions in Ontario are Wellington County, Kingston, the Ottawa Valley, and parts of Northern Ontario
- The GTA averages lower than the province, but homes inside the GTA still test high all the time
- A high-radon area doesn’t mean your home is high. A low-radon area doesn’t mean your home is low.
- Every Ontario home should test, no matter what the map says
- Tests run $40 DIY to $99 with a pro. There’s no excuse not to know your number.
Ontario radon data: where it actually comes from
Two main data sets shape what we know about Ontario radon.
The first is the Health Canada Cross-Canada Survey of Radon Concentrations in Homes, finalized in 2012.
The second is Take Action on Radon, the national outreach program run by CARST.
A third source, the Evict Radon study, runs ongoing public tests and now has over 60,000 Canadian homes in its database.
Put these together and you get a real picture. Not a colour-coded heat map, but real percent-of-homes-above-guideline numbers by region. For the bigger national picture, see our radon map of Canada, which breaks the same data down province by province.
Ontario’s worst radon regions
These are the parts of Ontario where the published data shows the highest share of homes above the 200 Bq/m³ guideline.
1. Kingston and eastern Ontario (25 to 50% of homes above guideline)
Kingston sits on the southern edge of the Canadian Shield. The bedrock is granite and gneiss, both rich in uranium. Average tested homes run above 130 Bq/m³, with around 1 in 4 homes above the guideline.
The whole Kingston-to-Ottawa corridor follows the same pattern. Brockville, Smiths Falls, Perth, and the smaller Lanark and Frontenac County towns all sit on the same rock.
2. Ottawa Valley (highest big-city average in Canada)
Ottawa has the highest big-city average radon in the country. The reason is geology. The Ottawa River sits in a rift valley, with the Canadian Shield to the north and the Champlain Sea sediments under most of the city.
The Champlain Sea was a saltwater inland sea that covered the Ottawa Valley after the last ice age. Its sediments are permeable and uranium-bearing. When you build a house with a basement into that ground, you get a gas path.
Ottawa neighbourhoods are not all equal, but a 2019 study by the University of Calgary found that Ottawa homes averaged over 300 Bq/m³ in some samples.
3. Wellington County (around 1 in 10 homes above guideline)
Wellington County is where we live and work. The data has been gathered through Take Action on Radon’s 100 Kit Challenge and through our own customer base.
Guelph homes average around 100 Bq/m³, which is above the provincial average. About 1 in 10 Guelph-area homes test above the 200 Bq/m³ guideline. The underlying rock is dolostone, which can carry uranium, plus glacial deposits dumped during the last ice age.
For more on the local context, see our radon mitigation in Guelph guide.
4. Northern Ontario (variable, but high in pockets)
Northern Ontario sits almost entirely on the Canadian Shield. The Shield is the largest exposed granite outcrop in the world. It’s loaded with trace uranium.
Sudbury, Timmins, North Bay, Thunder Bay, and the smaller Shield towns all see elevated radon. The 100 Kit Challenge has tested in several Northern cities. Results have shown 15 to 30 percent of homes above guideline, similar to Kingston.
Regional breakdown: the GTA and surrounding Southern Ontario
The GTA averages lower than the province. The bedrock under most of the GTA is shale and limestone, which carry less uranium than granite. The glacial deposits on top vary.
But the GTA still has high-radon homes. Tens of thousands of them. Here’s the regional picture.
| Region | Avg radon (Bq/m³) | % above guideline | Notes |
|---|---|---|---|
| Toronto (City of) | ~43 | ~3 to 5% | Lowest big-city avg in Ontario. Slab-on-grade condos and basement units run higher. |
| Peel Region (Mississauga, Brampton, Caledon) | ~60 to 80 | ~5 to 8% | Newer subdivisions test higher than the average. See our Mississauga testing guide. |
| York Region (Vaughan, Markham, Richmond Hill) | ~60 to 80 | ~5 to 8% | High concentration of post-2021 Tarion-eligible builds. |
| Halton Region (Oakville, Burlington, Milton, Halton Hills) | ~70 to 90 | ~8 to 10% | Escarpment dolostone runs higher. |
| Durham Region (Pickering, Ajax, Whitby, Oshawa) | ~50 to 70 | ~5 to 7% | Lakeshore lower, north and east higher. |
| Hamilton-Niagara | ~80 to 100 | ~10% | Niagara Escarpment dolostone, similar to Halton. |
| Wellington (Guelph, Fergus, Centre Wellington) | ~100 | ~10% | One of the higher-risk Southern Ontario regions. |
| Waterloo Region (Kitchener, Waterloo, Cambridge) | ~80 to 100 | ~8 to 10% | Mixed bedrock, glacial till. Several streets test very high. |
The full city-by-city averages are on our areas page. The Halton and Wellington numbers in particular surprise people.
Why a radon map alone can’t predict your home
Here’s the part most websites won’t tell you. A radon map is useful, but it cannot predict your individual home’s reading.
The reason is simple. Radon levels vary 10x or more between neighbouring homes on the same street.
Same builder. Same year. Same soil. Different readings.
Three things drive that variation, and a map can’t see any of them.
Foundation condition. Cracks, sump pits, and slab penetrations are unique to each home. A neighbour with sealed everything reads lower than a neighbour with a hairline-cracked slab.
Ventilation. A home with an HRV running 24/7 dilutes radon. A tight, no-mechanical-ventilation home concentrates it.
Pressure dynamics. Stack effect strength varies with home height, leakiness, and furnace placement. Some homes pull harder on the soil than others.
A 2020 review of Canadian radon variability found that two homes on the same street can read 50 Bq/m³ and 800 Bq/m³, with no obvious surface difference between them.
That’s why every Ontario home should test, even if your municipality looks low on the map.
The map tells you the odds. The test tells you the answer. A 5 percent regional rate means 1 in 20 homes in your area is above guideline. You don’t know if you’re the 1.
Ontario radon hot-spot list: high-risk municipalities
Take Action on Radon publishes 100 Kit Challenge results by city. These are the municipalities where the most recent published data showed the highest share of homes above guideline.
- Kingston: ~25% above 200 Bq/m³
- Greater Sudbury: ~22% above
- Timmins: ~20% above
- North Bay: ~18% above
- Ottawa: ~15 to 20% above (varies by neighbourhood)
- Guelph: ~10% above
- Kitchener-Waterloo: ~8 to 10% above
- Hamilton: ~10% above
- Halton Hills: ~8 to 10% above
The cities that surprise people are usually Halton Hills, Hamilton, and Kitchener-Waterloo. People expect Ottawa to be high. They don’t expect Hamilton to land at 10%.
Why the data keeps moving (and why some maps are wrong)
If you Google “Ontario radon map,” you’ll see colour-coded maps from a few different sources. They don’t all agree.
There are three reasons for the disagreement.
Sample size. Some municipalities have 30 tested homes in the data set. Others have 3,000. The 30-home cities have noisy averages.
Sampling method. The 100 Kit Challenge uses 91-day winter tests, which give the most conservative reading. Some older maps use short-term summer tests, which underread by 2 to 3 times.
Year of testing. Tighter modern homes trap more radon. A map built on 1990s data underrepresents radon in subdivisions built after 2000.
The best current data sources are the Cross-Canada Survey (Health Canada),
DIY map check vs paying for a pro test
The honest answer for most Ontario homeowners is: look at the map for context, but test your own home.
Here’s the practical breakdown.
| Step | DIY is fine | Hire a pro |
|---|---|---|
| Check your municipality on a map | Yes, free | Not needed |
| Read the published 100 Kit Challenge data | Yes, free at takeactiononradon.ca | Not needed |
| Order a 91-day long-term test kit | Yes, $35 to $60 at Canadian Tire or direct | Only for a Tarion claim or real estate timeline |
| Place the kit correctly | Yes, follow the placement card | If your home is unusual (multiple slabs, large footprint) |
| Read the result | Yes, the lab returns a Bq/m³ number | If you’re between 100 and 200 and unsure what to do |
| Run a 3 to 7 day pro test for a real estate closing | No, timing-sensitive | Yes, $99 short-term or $399 real estate test |
The DIY kit at Canadian Tire is enough for most homeowners. We say that in writing on our services page. We’d rather you spend $40 with a good kit than $99 with us if you don’t need us.
A pro test makes sense when you need fast turnaround (real estate), defensible documentation (Tarion warranty claim), or a sanity check on a DIY result between 100 and 200 Bq/m³.
What to do once you know your number
Three reading bands. Three actions.
Under 100 Bq/m³. Low risk. Retest in 5 years. Test sooner if you do a major renovation, finish a basement, or change the furnace setup.
100 to 200 Bq/m³. Borderline. Health Canada recommends taking action within 2 years.
Run a long-term winter retest first to confirm. If confirmed, consider mitigation.
Above 200 Bq/m³. Plan to mitigate within 12 months. Above 600, within 6 months. Sub-slab depressurization drops most homes by 80 to 99 percent.
If your home was built or sold after February 1, 2021, you may have a Tarion warranty claim instead of an out-of-pocket bill. That’s up to $50,000 of builder-paid mitigation. We help test customers with the paperwork for free.
For the full health and policy context, see what is radon (Ontario guide). The Government of Canada also publishes a plain-English radon overview worth reading.
Where Breathe Radon Free fits
We’re an Ontario-based testing service, owner-led, based in Guelph. We serve the Greater Toronto Area plus surrounding Southern Ontario. We publish our prices. We tell you when you don’t need us.
Our published pricing:
- Residential test: $99 flat. 3 to 7 days, written report.
- Real estate transaction test: $399. Faster turnaround for closing timelines.
- Long-term test: $299. 91-day for ongoing residential.
- Mitigation: $2,800 to $3,800. Full system, post-mitigation verification, 5-year workmanship warranty.
- Tarion claim help: free for test customers.
If your test reads under 100 Bq/m³, we’ll say so and not push you. The point of a radon map is to start a conversation. The point of a test is to finish it.
Get a $99 radon test booked in 60 seconds.
Frequently asked questions
Is there an official radon map of Ontario?
There’s no single official map. Health Canada’s 2012 Cross-Canada Survey has Ontario regional averages, Take Action on Radon publishes city-by-city results from the 100 Kit Challenge, and Evict Radon runs a growing public database. Together they give a real picture.
What’s the highest-radon city in Ontario?
Kingston and Ottawa are the two most commonly cited. Ottawa has the highest big-city average in Canada. Kingston has the highest share of homes above the 200 Bq/m³ guideline in published data. Several Northern Ontario cities (Sudbury, Timmins) also rank near the top.
Does Toronto have a radon problem?
The City of Toronto averages around 43 Bq/m³, which is well below the action level. But individual Toronto homes test high all the time. Basement-level condo units, slab-on-grade builds, and homes in Cabbagetown and Riverdale with pre-war stone foundations can read above 200 Bq/m³.
If my neighbour tested low, am I safe?
No. Radon varies 10x between neighbouring homes on the same street, even with the same builder, same year, and same soil. The differences are in cracks, ventilation, and stack effect. Test your own home.
Is the radon map enough to decide if I need to test?
No. The map gives you the odds for your area, but it can’t tell you your own number. Even in low-risk areas, about 1 in 20 homes is above guideline. The only way to know your home is to test.
Where can I find local Ontario radon data for my city?
Start with our city pages for the GTA and surrounding Southern Ontario. Each page has the population, average radon level, and percent of homes above guideline for that municipality. For other Ontario cities, check the 100 Kit Challenge results at takeactiononradon.ca.
Get a $99 radon test for your home →
Authored by the Breathe Radon Free Team. Every claim is backed by a primary source. Full bibliography at /sources.
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