Ontario Radon Map

The RadonTest.ca Ontario radon map — built from 7,734 measured long-term tests in Ontario, part of a 69,478-test national dataset — puts Ontario at 15.8% of homes at or above Health Canada’s 200 Bq/m³ action guideline, from 7,734 tests in this province, with a typical (geometric mean) level of 79 Bq/m³. Nationally the figure is 17.8%. The highest-reading area in Ontario is Lennox and Addington at 44.7% (38 tests; Lanark reads 42.1% on a larger 114-test sample); the lowest is Toronto at 1.3%. One Ontario home measured 32,321 Bq/m³ — over 161× the guideline, and the highest single reading anywhere in the national dataset. It was recorded in Thunder Bay.

See Ontario on the interactive map

Open the Canada Radon Map zoomed to Ontario → Type a postal code or town to find your area.

The underlying survey data is public; the RadonTest.ca Canada Radon Map is the free tool that makes it searchable — by province, city and postal code, in English and French, no signup.

Radon by area in Ontario

Every measured census division in Ontario, highest first. “Typical” is the geometric mean — the level a middling home in that area recorded.

Areas with few tests carry a wider margin of uncertainty — treat small-sample percentages as indicative rather than precise. A 0% reading means no home tested above 200 Bq/m³ among those measured there, not that the area is radon-free.

Radon in Ontario cities

Metro figures are census metropolitan areas; the census-division table above uses different boundaries. Where a name appears in both, the two numbers are measuring different areas — that is why they differ.

The same area at three different thresholds

Canada’s action guideline is 200 Bq/m³. The World Health Organization uses a stricter 100 Bq/m³ reference level, and the US EPA acts at 148 Bq/m³ (4 pCi/L). Measured against each, RadonTest.ca’s Ontario data reads:

No level of radon is completely risk-free — the risk rises with exposure, which is why the WHO line sits lower than Canada’s. Health Canada recommends acting within a year above 200 Bq/m³, sooner at higher levels.

Why this differs from older Health Canada figures

Health Canada’s 2009–11 national survey put Ontario at about 8.2% of homes at or above 200 Bq/m³, from 3,954 valid tests (2009–11 figures as re-tabulated by the 2024 survey team), with a typical level near 55 Bq/m³. This page uses the 2024 Cross-Canada Survey of Radon — 69,478 long-term tests, the largest residential radon dataset in Canada — which puts Ontario at 15.8% from 7,734 tests, typical level 79 Bq/m³.

Two things differ. First, the sampling: the 2009–11 survey recruited by random digit dialling across 121 health regions and deliberately over-sampled sparsely populated regions for geographic coverage, while the 2024 dataset aggregates long-term tests from homes whose occupants chose to test — which skews toward older housing, ground-contact basements and areas where radon is already a known concern. Second, the weighting: the 2009–11 figures quoted above are the raw, unweighted results. Because that survey deliberately over-sampled some regions, Health Canada also published population-weighted estimates, and its headline national result is 6.9%; the 11.3% raw figure comes from the 2024 survey team’s re-tabulation of the same dataset. None of these numbers is wrong — they answer different questions. For a population-representative estimate of Canadian housing, Health Canada’s weighted 6.9% is the right figure. For the largest body of actual measured results, use this page. Both are shown on the Canada Radon Map, and both can be switched between on the RadonTest.ca map. Full detail on methodology and sources.

Ontario: a low provincial average hiding some of Canada's worst counties

Ontario's 15.8% is below the national 17.8%, and that number is misleading in both directions. Eastern Ontario — Lennox and Addington at 44.7%, Lanark at 42.1%, Leeds and Grenville at 31.4%, Renfrew at 29.2% — sits on Precambrian Shield granites and is among the highest-radon country in Canada. Toronto, on deep glacial till and lakebed clays, reads 1.3%. Same province, a 34-fold difference.

Tarion and new homes — this is better coverage than most owners realise. Ontario’s new-home warranty explicitly names excessive radon as a covered defect under the seven-year Major Structural Defect warranty, with up to $50,000 of coverage for levels above Health Canada’s 200 Bq/m³ guideline. The practical condition is the test itself: Tarion requires a measurement over a minimum three-month period using a device or professional certified through the Canadian National Radon Proficiency Program (C-NRPP). If your home is Tarion-enrolled and within seven years of possession, that coverage may apply — check your status on Tarion’s MyHome portal.

On the building code: Ontario’s province-wide radon rough-in requirement (Article 9.13.4.3) arrived with the 2024 Ontario Building Code and applies to permits applied for on or after 1 January 2025. Before that it applied only in three designated areas. So a capped sub-slab pipe should be present in homes permitted from 2025 onward — which makes mitigation much cheaper — but not in most Ontario homes standing today.

Ontario has no mandatory radon testing in schools or daycares province-wide, and no disclosure requirement on resale. Several public health units — Ottawa, Kingston, Peterborough and Thunder Bay among them — run their own awareness or subsidised-kit programmes, which is why testing density is higher in exactly the counties where radon is worst.

A map cannot test your home

Every figure on the RadonTest.ca Ontario radon map is an area figure. Two identical houses on the same street in Ontario can test very differently, because radon depends on your foundation, your soil, your ventilation and how you live in the house. The map tells you the odds. Only a long-term test tells you your number.

Health Canada recommends a long-term test of at least 91 days, and you can start it any time of year. Ideally at least 91 of those days fall in the heating season (roughly October to April), and there is a simple rule for when they don’t: if your result comes back below 200 Bq/m³ on a mostly-summer test, Health Canada suggests re-testing next heating season, because a summer test can read low. If it comes back above 200, Health Canada says no re-test is needed — act on it. Find your own number — order a 91-day radon test kit →

What to do if your result is above 200 Bq/m³

Health Canada recommends taking corrective action within a year above 200 Bq/m³, and sooner the higher the reading. Most Canadian homes are fixed with sub-slab depressurisation — a pipe running from beneath the foundation slab to a fan that vents above the roofline, which, when properly installed, reduces levels by more than 80% in most homes (Health Canada); results vary by house. In Ontario homes permitted from 1 January 2025, a capped radon rough-in should already be under the slab, which usually reduces the job to connecting a fan to pipe that is already there.

Work should be done by a professional certified through the Canadian National Radon Proficiency Program (C-NRPP), and the home should be re-tested afterwards to confirm the fix worked. You can also look up radon by address or postal code anywhere in Canada on the RadonTest.ca map.

Ontario radon map — questions

How high is radon in Ontario?

RadonTest.ca’s Ontario radon map puts about 15.8% of Ontario homes at or above Health Canada’s 200 Bq/m³ guideline, from the 2024 Cross-Canada Survey of Radon (7,734 long-term tests), with a typical level of 79 Bq/m³. Against the WHO’s stricter 100 Bq/m³ reference level, 38.6% are over.

Which parts of Ontario have the most radon?

On the RadonTest.ca Ontario radon map the highest-reading areas are Lennox and Addington (44.7%), Kenora, Rainy River (43.8%), Lanark (42.1%), Thunder Bay (34.8%). The lowest is Toronto at 1.3%.

Is there a free radon map for Ontario?

Yes. The RadonTest.ca Canada Radon Map is free, needs no signup, and shows measured radon by census division and postal area for every part of Ontario.

Does a low reading on the map mean my house is fine?

No. On the RadonTest.ca Ontario radon map a low area share is the result most likely to talk you out of the test that would have caught yours. Homes above 200 Bq/m³ have been measured in every Canadian province, including in the lowest-reading Ontario areas.

Related

• Toronto radon map
• Ottawa radon map
• Radon in Ontario: levels, risk, rules and what to do
• Canada Radon Map
• Tarion radon warranty: how to claim (up to $50,000)
• What radon mitigation costs in Ontario
• Radon in Toronto
• Methodology and data sources

Recommended citation: Ontario Radon Map, RadonTest.ca, 2026. Data: 2024 Cross-Canada Survey of Radon (v1.1 (CCSR.24.1.1), ISSN 2818-8195). https://radontest.ca/pages/ontario-radon-map Data licensed CC BY 4.0 — attribution required: “RadonTest.ca Canada Radon Map”.