Radon in Ontario: Levels, Risk, Rules & What to Do

A note before you read. This is general consumer information for Ontario homeowners, buyers, sellers, landlords, and renters, drawn from publicly available Health Canada, Public Health Ontario, Cancer Care Ontario / Ontario Health, Tarion, Ontario government, municipal, and peer-reviewed sources. It is not medical, legal, or warranty advice. Radon results are reported against the Health Canada guideline of 200 Bq/m³; RadonTest.ca coordinates testing logistics and does not interpret individual results or provide health assessments.

Key facts: radon in Ontario at a glance

• Health Canada estimates 1 in 5 Canadian homes have high radon levels. In Ontario specifically, Health Canada's Cross-Canada Survey (2009–2013) found 8.2% of homes at or above the 200 Bq/m³ guideline; the more recent 2024 Evict Radon / University of Calgary study put Ontario at about 1 in 8 (12.4%) — a sign that measured prevalence has risen as more homes are tested.
• Radon is the number one cause of lung cancer in non-smokers and the second leading cause overall. Public Health Ontario researchers estimated radon is behind about 847 lung-cancer deaths in Ontario each year (2013 study).
• Levels vary enormously by region. Eastern Ontario (the Ottawa Valley, Kingston, Leeds-Grenville-Lanark) and parts of the north (the Thunder Bay area) are hotspots; the GTA is consistently the lowest. Any home can have high radon — the only way to know is to test.
• The Health Canada guideline is 200 Bq/m³ (annual average). If your long-term result is above it, Health Canada recommends taking corrective action within one year — sooner the higher the level.
• Ontario is the only province with a new-home warranty that covers radon. Tarion covers radon mitigation up to $50,000 for seven years from possession (for homes with an Agreement of Purchase and Sale signed on or after February 1, 2021).
• New Ontario homes now build in radon protection. Since the 2024 Building Code took effect (January 1, 2025), new low-rise homes province-wide must include a sub-slab depressurization rough-in.
• Testing takes a minimum of 91 days (a long-term test). Fixing a home typically costs around $2,700 and a system installs in about a day.

How much radon is in Ontario homes?

Radon is a naturally occurring radioactive gas with no colour, smell, or taste. It seeps up from the breakdown of uranium in soil and bedrock and accumulates indoors. Every Ontario home has some radon; what matters is how much.

Two large datasets describe Ontario, and they measure slightly different things, so it's worth keeping them straight:

• Health Canada's Cross-Canada Survey of Radon Concentrations in Homes (data collected 2009–2011, published 2012). Of roughly 3,900 valid Ontario results, 8.2% were at or above 200 Bq/m³ (7.3% in the 200–600 range and 0.9% above 600). On a population-weighted basis, Health Canada put Ontario at 4.6%. Cancer Care Ontario, restating the same survey, notes that 25.2% of Ontario homes had radon at or above 100 Bq/m³ — the level at which the World Health Organization recommends action.
• The 2024 Evict Radon National Study (University of Calgary, with Health Canada and CAREX Canada) — also published as the 2024 Cross-Canada Radon Survey. Built on more than 75,000 long-term readings, it placed Ontario at about 1 in 8 homes (12.4%) at or above 200 Bq/m³, with a weighted geometric-mean concentration of about 72 Bq/m³. Nationally, this study found nearly 1 in 5 homes (about 18%) above the guideline — more than double the 6.9% that Health Canada's own 2012 Cross-Canada Survey reported nationally.

The two numbers (8.2% in 2012, 12.4% in 2024) aren't contradictory — they come from different studies, years, and methods. The trend is the important part: as Canada tests more homes, the measured share above the guideline has climbed, and Health Canada now uses "1 in 5" as its national headline. (For how Ontario compares with the rest of the country, see our radon levels by province breakdown.)

What this means for you: provincial averages can't tell you about your house. Two homes next door to each other can read very differently depending on soil, foundation, and ventilation. A long-term test is the only way to know your own number.

Radon by region in Ontario

Ontario's radon map is not uniform. The richest public dataset is Cancer Care Ontario's breakdown by public-health unit (from the 2009–2013 Health Canada survey), supplemented by more recent regional studies. Highlights:

A few regional stories stand out:

Eastern Ontario and the Ottawa Valley — the province's hotspot belt. A 2018–2019 KFL&A Public Health study of more than 1,000 homes around Kingston found 21% above 200 Bq/m³ and 52% above 100 — which the region's Medical Officer of Health described as "almost five times the provincial average." Leeds-Grenville-Lanark has the highest single health-unit reading in the province.

The north — high pockets on the Canadian Shield. The Thunder Bay area is a documented hotspot: a Thunder Bay District Health Unit study found 65% of homes in Oliver Paipoonge and 17% in Marathon above the guideline, and an earlier City of Thunder Bay study found 16% of homes elevated, with one ward as high as 43%.

Southwestern Ontario — surprisingly elevated pockets. Despite sitting on sedimentary rock rather than the Shield, Windsor-Essex and Chatham-Kent post some of the province's highest rates, a reminder that geology is local.

The GTA — consistently lowest, but never zero. In the 2024 Evict Radon city analysis, Toronto's metro average was just 43 Bq/m³, with about 1 in 22 homes above the guideline — yet the highest single Toronto reading in that study was over 1,000 Bq/m³. Low average risk is not no risk.

Radon by city across Ontario — we maintain a detailed local guide for each major community:

• Greater Toronto & the Golden Horseshoe: Toronto, Mississauga, Brampton, Vaughan, Markham, Oakville, Burlington, Milton, Ajax, Oshawa, Whitby, Hamilton, St. Catharines
• Eastern Ontario: Ottawa, Kingston
• Southwestern & central Ontario: London, Windsor, Kitchener-Waterloo, Cambridge, Guelph, Barrie
• Northern Ontario: Sudbury, Thunder Bay, Sault Ste. Marie

Why Ontario has radon: the geology

Radon levels track the amount of uranium and thorium in the ground beneath a home. Ontario's geology is essentially two stories:

• The Canadian Shield covers roughly 60% of the province (the north and the Grenville region of the southeast). This ancient igneous and metamorphic bedrock is uranium-bearing in places, which is why parts of the north — and the Shield-adjacent southeast — can run high.
• Southern Ontario's sedimentary lowlands, blanketed by thick glacial deposits from the last ice age. The heavy, low-uranium glacial cover across much of the GTA helps explain why Toronto and the inner 905 read so low.

The important nuance: Ontario radon is not simply "Shield = high, south = low." Some of the province's worst readings come from uranium-rich Ordovician shales in eastern Ontario — the Ottawa Valley, Kingston, and the Leeds-Grenville-Lanark corridor — even though that's sedimentary country, not Shield granite. Local bedrock and soil chemistry matter more than the broad map.

Elliot Lake — where Canada's residential radon story began. This northern Ontario town sits on naturally uranium-rich ground and has a long uranium-mining history. In the 1970s, a federal-provincial task force investigating high radon in Elliot Lake homes helped launch Canada's national attention to residential radon. According to the Canadian Nuclear Safety Commission, the elevated levels were attributed to natural radon from bedrock or to mine waste rock used in construction (not tailings), and many affected homes had continuously running radon fans installed. Elliot Lake is widely reported to have required radon mitigation in new homes through its local building code as early as the late 1970s — decades ahead of the rest of the province.

Is radon dangerous? The health risk in Ontario

Radon is classified by the International Agency for Research on Cancer as a Group 1 carcinogen — the highest category, the same as tobacco smoke and asbestos. Health Canada's current figures:

• Radon is the number one cause of lung cancer in non-smokers, and the second leading cause overall after smoking.
• About 16% of lung-cancer deaths in Canada are estimated to be radon-related — more than 3,000 deaths a year (Health Canada's fact sheet cites roughly 3,200).
• Health Canada estimates that a non-smoker exposed to high radon over a lifetime has roughly a 1 in 20 chance of developing lung cancer from it; for a smoker, the combined risk rises to about 1 in 3. Radon and tobacco smoke multiply each other's risk.

For Ontario specifically, Public Health Ontario researchers estimated (in a 2013 study) that 13.6% of the province's lung-cancer deaths — about 847 a year — are attributable to radon, roughly 84% of them in people who had smoked. Notably, that study found that bringing every home above the guideline down to background levels would prevent about 91 deaths a year, but acting at the lower 100 Bq/m³ level would prevent about 233 — because a large share of radon-related cancers come from the many homes sitting below the official guideline. (This estimate predates the 2024 survey showing prevalence has roughly doubled, so the current burden may be higher.)

Health Canada is careful to frame the risk proportionately, and so are we: there is no level of radon that is completely risk-free, the risk below the guideline is small, and it is ultimately each homeowner's choice what level of exposure they are willing to accept. Radon exposure is also long-term, not acute — there are no immediate symptoms. The point isn't alarm; it's that radon is a measurable, fixable risk, and testing is inexpensive.

The Health Canada guideline — and what your number means

Canada's guideline is 200 becquerels per cubic metre (Bq/m³), measured as an annual average in a normally occupied area of the home. It's a health-based guideline, not a hard legal limit for private homes. Health Canada's guidance:

• At or above 200 Bq/m³: take corrective action to reduce the level, within one year — and sooner the higher the result.
• Below 200 Bq/m³: no corrective action is recommended, though no level is risk-free; some Ontario households (especially where someone smokes) choose to act toward the WHO's more protective 100 Bq/m³ reference level. (See how the Health Canada, WHO, and US thresholds compare.)

Your result is a concentration, not a pass/fail — it exists on a spectrum. For a plain-language walkthrough of what different result ranges mean and what Health Canada recommends at each, see our guide on how to read your radon test results.

How to test for radon in Ontario

Testing is the only way to know your home's level, and it's straightforward:

• Use a long-term test of at least 91 days (three months). Health Canada recommends long-term testing because radon swings day to day and season to season; a 91+ day average is what reflects your real exposure. Short-term tests are only appropriate for checking a mitigation system, never for deciding whether to act.
• The heating season (roughly October–April) is ideal, because closed-up winter homes give a conservative reading — but you can start any time of year. A result at or above the guideline is worth acting on regardless of season; a low off-season result is worth confirming over a winter.
• Place the detector in the lowest lived-in level (usually the basement if it's used), in a room occupied more than four hours a day, about 30 cm off the floor and away from drafts, vents, and direct sun. Not the kitchen, bathroom, or laundry room.
• Use a C-NRPP-approved test. RadonTest.ca kits use an alpha-track detector analysed by a C-NRPP-certified laboratory. (For how long-term kits compare with continuous digital monitors, see our guide.)

Where to get your test: order a RadonTest.ca kit — the detector, analysis by a C-NRPP-certified laboratory (with an all-in-Canada analysis option), tracked shipping both ways, and your result delivered with clear Health Canada context are all included. One caution about cheaper retail kits: not every hardware-store kit is analysed by a C-NRPP-certified lab, which matters if you might ever file a Tarion warranty claim — see our comparison of hardware-store radon test kits.

When to retest. Health Canada recommends testing again after any renovation that affects your home's structure or ventilation (finishing a basement, a new furnace, adding a bathroom), after energy retrofits (new windows, insulation, air sealing), or after excavation near the foundation. If you install a mitigation system, retest every five years to confirm it's still working. There's no blanket "every five years" rule for an unmitigated home — the trigger is change.

For more, see when is radon testing season in Canada.

If your radon is high: mitigation in Ontario

A result above 200 Bq/m³ is a solvable problem (here's what to do if your radon is above 200 Bq/m³). Health Canada recommends hiring a C-NRPP-certified radon mitigation professional.

How it works. The standard, most effective method is active soil depressurization (ASD), also called sub-slab depressurization. A pipe is installed through the foundation slab and a continuously running fan draws radon from beneath the home and vents it safely outside, before it can enter your living space. In most homes this reduces radon by more than 80%, and a system can usually be installed in less than a day. A few important points a good contractor will handle: the fan must run continuously (never switched off); the installer should check that the system doesn't cause "back-drafting" of a furnace, water heater, or fireplace; and the work should be verified with a short-term test after activation and confirmed with a long-term test the following heating season — ideally not by the company that installed the system.

What it costs in Ontario. Industry data compiled by Take Action on Radon (hundreds of systems installed 2013–2021) puts the average mitigation cost at about $2,700, with most jobs falling in the low-thousands and complex installations ranging higher (up to around $11,000 for difficult buildings). Health Canada's own estimate for a typical sub-slab system is $2,000–$3,000. Running the fan costs roughly $50–$75 a year in electricity, and the fan itself (about $200–$300) lasts 5–10+ years.

Find a certified professional. The Canadian National Radon Proficiency Program (C-NRPP) — run by the Canadian Association of Radon Scientists and Technologists with Health Canada oversight — certifies radon measurement professionals, mitigation professionals, and labs. Health Canada recommends using a C-NRPP-certified contractor. Search the directory at c-nrpp.ca/find-a-professional. RadonTest.ca does not perform mitigation or recommend specific companies.

See also: how much radon mitigation costs in Ontario, radon mitigation cost across Canada, and how to choose a licensed radon mitigator.

Ontario's radon rules and protections

This is where Ontario differs from every other province — it has the strongest mix of new-home warranty, building-code, and municipal radon rules in Canada.

Tarion new-home warranty (up to $50,000)

Ontario's statutory new-home warranty, administered by Tarion, covers the cost of radon mitigation — the only such warranty in Canada. Key points, per Tarion:

• Coverage of up to $50,000 for homes with an Agreement of Purchase and Sale signed on or after February 1, 2021 (earlier agreements carry a lower cap — confirm your situation with Tarion).
• The warranty runs for seven years from the original possession date, and travels with the home to resale buyers within that window.
• It applies if the home is less than seven years old and a long-term radon test of at least three months shows a level above 200 Bq/m³.
• To claim, you report the result to your builder and Tarion on the applicable warranty form; the builder is required to take corrective action, and Tarion can step in if the builder doesn't.

Because eligibility depends on your possession date and warranty year, confirm your coverage directly with Tarion. RadonTest.ca is not affiliated with Tarion. For a step-by-step walkthrough, see our Tarion radon warranty claim guide.

The Ontario Building Code (new construction)

Under the 2024 Ontario Building Code (in force January 1, 2025), all new low-rise (Part 9) residential buildings province-wide must be built with a sub-slab depressurization rough-in and soil-gas barrier — a capped pipe and sealed slab that let an active fan be added later without major work. This is a significant expansion: before 2024, prescriptive radon control in the Building Code applied only to a handful of "designated areas." A rough-in is not an active system — if a later test shows elevated radon, a fan is added to complete it. The Building Code governs new construction only and does not require anything of existing homes. (The National Building Code of Canada's 2025 edition carries matching radon provisions.) More detail: Canadian building codes and radon.

Municipal programs

Several Ontario municipalities go further than the provincial baseline:

• Guelph runs the strictest program in the province. For building permits applied for after December 31, 2024, new Part 9 homes must have a full active sub-slab depressurization system installed (not just a rough-in), and larger/commercial buildings face mandatory long-term testing, a city radon-mitigation certification form, and multiple inspections before a permit can close.
• Hamilton requires new low-rise construction to incorporate the Building Code's radon measures, with testing required depending on the option chosen.
• Kingston restates the Building Code's soil-gas requirements for new builds; the surrounding KFL&A region has been a focus of local radon study (the source of the 21%-above-guideline finding noted earlier).
• Ottawa publishes radon/soil-gas advisories for builders of small and large buildings.

Selling or buying a home: disclosure in Ontario

Radon disclosure is not mandatory in Ontario. The Seller Property Information Statement (OREA Form 220) is voluntary, and real-estate transactions are generally governed by "buyer beware" (caveat emptor). However, a known elevated radon level is the kind of hidden ("latent") defect that a seller who is aware of it should disclose, and real-estate professionals carry a higher ethical disclosure duty than a private seller. Practical takeaway: if you've tested and know your level is high, that knowledge should be disclosed; if you're buying, make radon part of your due diligence. See radon when buying or selling a home in Canada. Renting, or renting out a property? See our guides for Ontario landlords and renters.

Radon in Ontario workplaces

Ontario has no specific binding radon exposure limit for provincially regulated workplaces; radon is addressed through the Occupational Health and Safety Act's general duty to take "every precaution reasonable," supported by Health Canada's NORM guidance (which treats 200 Bq/m³ as the working level and sets management tiers above it). Underground mines are covered separately. Separately, federally regulated workplaces in Ontario (banks, telecom, interprovincial transport, air transport, federal operations) fall under a new federal rule, SOR/2026-10, which sets a binding workplace limit of 200 Bq/m³ (replacing the old 800 Bq/m³ standard) and comes into force around February 2027.

Financial help in Ontario

• New homes: the Tarion warranty above is the main route — it can cover the full mitigation cost (up to $50,000) within seven years of possession.
• There is no Ontario provincial radon grant or tax credit. Unlike Saskatchewan (which had a now-expired renovation tax credit) or Manitoba (on-bill financing through Manitoba Hydro), Ontario does not offer a dedicated provincial radon subsidy.
• Lungs Matter (Canadian Lung Association) offers up to $1,500 toward home radon mitigation, prioritizing people diagnosed with lung cancer and lower-income households.
• Ontario Renovates (forgivable loans for low-to-moderate-income homeowners, delivered by municipalities/regions) may cover radon mitigation depending on the local administrator — check with your municipality.
• There is currently no open federal radon-mitigation grant for the general public.

Frequently asked questions

What is a risk-free radon level in Ontario? There is no completely risk-free level — Health Canada's guideline is 200 Bq/m³, the level above which it recommends taking action within a year. The risk below the guideline is small but not zero, and the WHO references a more protective 100 Bq/m³.

How common is high radon in Ontario? Health Canada's 2009–2013 survey found 8.2% of Ontario homes above the guideline; the 2024 Evict Radon / University of Calgary study put it at about 1 in 8. Nationally, Health Canada now says 1 in 5 homes have high radon.

Where is radon worst in Ontario? Eastern Ontario (the Ottawa Valley, Kingston, Leeds-Grenville-Lanark) and parts of the north such as the Thunder Bay area are the highest; pockets of the southwest (Windsor-Essex, Chatham-Kent) are also elevated. The GTA is lowest on average. But any individual home can be high — testing is the only way to know.

Does Tarion cover radon in Ontario? Yes. For new homes with an Agreement of Purchase and Sale signed on or after February 1, 2021, Tarion covers radon mitigation up to $50,000 for seven years from possession, if a long-term (91+ day) test shows a level above 200 Bq/m³. Confirm your coverage with Tarion.

Do I have to disclose radon when selling a home in Ontario? There's no mandatory radon disclosure law, but if you know your level is high, that knowledge should be disclosed as a latent defect. The seller information statement is voluntary.

How much does radon mitigation cost in Ontario? About $2,700 on average, with most jobs in the low thousands and complex buildings costing more. Health Canada estimates $2,000–$3,000 for a typical system.

Do new Ontario homes have radon protection built in? Yes — since January 1, 2025, the Ontario Building Code requires a sub-slab depressurization rough-in in new low-rise homes province-wide. A rough-in lets a fan be added later if testing shows elevated radon; it isn't an active system on its own.

How long does a radon test take? At least 91 days (three months) for a valid long-term result. Short-term tests aren't suitable for deciding whether to act.

Is a RadonTest.ca result valid for a Tarion claim? Tarion accepts a long-term test of at least three months from a C-NRPP-certified lab. RadonTest.ca kits are analysed by a C-NRPP-certified lab, so they meet that requirement (the result must be above 200 Bq/m³ and the home within its 7-year window). Confirm details with Tarion.

Test your Ontario home

Radon is invisible, common in parts of Ontario, and fixable. The only way to know your home's level is a long-term test.

Order your RadonTest.ca kit → — lab analysis by a C-NRPP-certified laboratory (with an all-in-Canada analysis option), tracked shipping both ways, and your result delivered with clear Health Canada context.

Sources

1. Health Canada — Radon guideline (corrective action within 1 year), modified 2025-09-24. https://radontest.ca/links/hc-guideline
2. Health Canada — Radon: What you need to know ("1 in 5 homes," 16% of lung cancers, ~3,200 deaths), 2025 edition. https://radontest.ca/links/hc-what-you-need-to-know
3. Health Canada — Cross-Canada Survey of Radon Concentrations in Homes, Final Report (Ontario 8.2%), 2012. https://www.canada.ca/en/health-canada/services/environmental-workplace-health/reports-publications/radiation/cross-canada-survey-radon-concentrations-homes-final-report-health-canada-2012.html
4. Health Canada — Reducing radon levels in your home (>80% reduction; C-NRPP), modified 2025-10-09. https://radontest.ca/links/hc-mitigation-guide
5. Health Canada — Guide for radon measurements in homes (91-day test; retest triggers; 5-year retest for mitigated homes), modified 2025-12-22. https://radontest.ca/links/hc-measurements-guide
6. Health Canada — Radon Reduction Guide for Canadians (ASD mechanism; back-draft; operating cost), modified 2025-09-24. https://www.canada.ca/en/health-canada/services/environmental-workplace-health/reports-publications/radiation/radon-reduction-guide-canadians-health-canada.html
7. Cancer Care Ontario — Risk of Residential Radon Exposure Varies Geographically (Ontario public-health-unit breakdown), 2017. https://www.cancercareontario.ca/en/cancer-facts/risk-residential-radon-exposure-varies-geographically
8. Peterson E. et al. (Public Health Ontario) — Lung cancer risk from radon in Ontario, Canada, Cancer Causes & Control, 2013 (847 deaths/yr; 13.6%). https://pubmed.ncbi.nlm.nih.gov/23982909/
9. Evict Radon National Study / University of Calgary — 2024 Cross-Canada Survey of Radon (Ontario ~12.4%; national ~18%; Toronto/six-cities data). https://evictradon.org/radon-research-series-radon-levels-across-canadian-regions/ and https://evictradon.org/radon-research-series-radon-levels-in-canadas-six-largest-cities/
10. KFL&A Public Health — High levels of radon found in KFL&A (21% above guideline), 2019. https://www.kflaph.ca/en/news/high-levels-of-radon-found-in-kfla.aspx
11. Thunder Bay District Health Unit — Prevalence of High Residential Radon (Oliver Paipoonge 65%), 2018. https://www.tbdhu.com/news/high-levels-of-radon-oliver-paipoonge-and-marathon
12. Tarion — Eight quick facts about radon ($50,000; 7 years; 200 Bq/m³; 3-month test). https://www.tarion.com/media/eight-quick-facts-about-radon
13. Ontario — 2024 Building Code / O. Reg. 163/24 (sub-slab rough-in, in force Jan 1 2025); Radon in the workplace. https://www.ontario.ca/page/radon-workplace
14. City of Guelph — Radon Gas Mitigation Program. https://guelph.ca/city-government/building-permits-inspections/residential-building-permits/radon/
15. Canada Gazette Part II — SOR/2026-10 (federal workplace 200 Bq/m³, in force ~Feb 2027), 2026-02-11. https://gazette.gc.ca/rp-pr/p2/2026/2026-02-11/html/sor-dors10-eng.html
16. Take Action on Radon — Reducing radon (cost data; ASD explainer; C-NRPP governance). https://takeactiononradon.ca/protect/reducing-radon/
17. Canadian Nuclear Safety Commission — The natural occurrence of radon in homes (Elliot Lake). https://www.cnsc-ccsn.gc.ca/eng/resources/news-room/feature-articles/the-natural-occurrence-of-radon-in-homes/

Lab analysis is performed independently by a C-NRPP-certified laboratory. Results are reported against the Health Canada guideline of 200 Bq/m³. RadonTest.ca coordinates kit logistics and sample submission only — it does not interpret or modify lab results and does not provide medical, legal, or warranty advice. Information attributed to Health Canada, Tarion, and others is summarized from the public sources listed above; confirm time-sensitive details (warranty coverage, building-code requirements, program availability) with the responsible body.

Regional radon testing guides

Testing your home in a specific part of Ontario? These local guides cover the communities we serve:

• Radon testing in Waterloo Region — Kitchener, Cambridge and Guelph
• Radon testing in Peel Region — Mississauga and Brampton
• Radon testing in Halton Region — Oakville, Burlington and Milton
• Radon testing in the Niagara Region — St. Catharines and Niagara Falls