Radon in the Lower Laurentians (Basses-Laurentides): Oka, Saint-Joseph-du-Lac, Mont-Laurier and the Risk Areas

The Laurentians (Laurentides) contain some of the best-documented radon hotspots in Canada. The Basses-Laurentides — Oka, Saint-Joseph-du-Lac and the Deux-Montagnes area — sit partly on the Oka carbonatite complex, a rare uranium- and thorium-bearing rock formation that has produced extraordinary indoor radon readings, while the MRC d'Antoine-Labelle and the Mont-Laurier area in the Hautes-Laurentides have also been flagged by public health authorities. That said, a region is a risk signal, not a result: the only way to know your home's level is a long-term radon test of at least 91 days.

See the Quebec radon map — measured results for every surveyed area, searchable by postal code.

Why does the Oka carbonatite make this a radon area?

Radon is a radioactive gas produced by the natural decay of uranium in soil and bedrock. The more uranium underfoot, the more radon can seep into homes through foundation cracks, sumps and service openings. What makes the Basses-Laurentides unusual is the rock itself.

Beneath Oka lies the Oka carbonatite complex, an alkaline igneous intrusion roughly 2 by 6 kilometres formed by an underground volcanic event about 95 million years ago — part of the same Monteregian episode that built features like Mont-Royal. Carbonatites are geologically rare, and the Oka body is, in the words of the Direction de santé publique (DSP) des Laurentides, "riche en uranium" — rich in uranium, and in places carrying high concentrations of radioelements including uranium and thorium (DSP des Laurentides / BAPE, 1998).

The complex is also why Oka has a mining history. From the 1960s into the 1970s, the St-Lawrence Columbium and Metals Corporation mined the deposit for niobium, hosted in the mineral pyrochlore. Pyrochlore-group minerals at Oka are enriched not only in niobium and titanium but also in uranium and thorium — analyses of the mine's slag have measured roughly 600 ppm uranium and 1,370 ppm thorium (ScienceDirect, 2024). It is the same uranium content, near the surface and over a populated area, that drives the radon. A geologically similar second carbonatite formation has also been identified in the Saint-André sector to the west, which is why the broader Argenteuil area shares the concern.

The decay chain is direct: uranium-238 in the rock breaks down through a series of steps to radium, which produces radon-222 — an invisible, odourless gas that migrates up through porous rock and fissures and accumulates in enclosed indoor air.

Which Laurentian towns are most at risk for radon?

According to the DSP des Laurentides, higher-than-average radon concentrations have been measured in homes in certain sectors of Oka, Saint-Joseph-du-Lac, Saint-André-d'Argenteuil and the MRC d'Antoine-Labelle, all located on uranium-rich geological formations. The public health authority lists the regions known to be most affected as the Basses-Laurentides (Oka, Saint-Joseph-du-Lac and Saint-André-d'Argenteuil), the Mont-Saint-Hilaire area, the MRC d'Antoine-Labelle and the Mont-Laurier region (Santé Québec — Laurentides).

Oka and Saint-Joseph-du-Lac. The numbers here are not typical. When the DSP screened homes on the Oka geological formation in the mid-1990s, the worst-affected sector — Mont-Saint-Pierre sud, in Oka-Paroisse — recorded a median of 482 Bq/m³ and a mean of 1,245 Bq/m³, with a maximum of 10,500 Bq/m³. Within that, a smaller "hot zone" of 46 homes showed a median of 1,000 Bq/m³, a mean of 1,776 Bq/m³, and 56% of homes above 800 Bq/m³. The report concluded that no other location in Quebec or Canada had been identified at that time with such a high proportion of homes at such high concentrations (DSP des Laurentides / BAPE, 1998). Those figures predate Canada's current 200 Bq/m³ guideline (the guideline was 800 Bq/m³ when the report was written), but they show how high local levels can run.

The Deux-Montagnes area and the lac des Deux-Montagnes shore. Demand for radon testing has surged across the western Laurentians in recent years, including Oka, Deux-Montagnes and Lachute. A St-Jérôme radon professional described one telling pattern to local radio: a home measuring 486 Bq/m³ while its immediate neighbour measured only 17 — because levels depend on the soil under each lot, how the house is built, and its year of construction (CIME / Cogeco, 2022). Communities along the lac des Deux-Montagnes such as Pointe-Calumet and Deux-Montagnes lie on or near the same geological belt, which is why testing is advised home by home rather than assumed to be below the guideline by address.

Mont-Laurier and the MRC d'Antoine-Labelle. Further north in the Hautes-Laurentides, the Mont-Laurier area and the MRC d'Antoine-Labelle are also flagged by the DSP as uranium-rich and radon-prone, and in the 2000s certain municipalities there adopted special protective measures regarding radon exposure (Santé Québec — Laurentides). Region-wide measured percentages specific to Antoine-Labelle are not separately published, so the prudent reading is that the area carries elevated geological potential — and individual homes should be tested.

One important caution: high-radon and low-radon homes sit side by side here, and the geological maps describe potential, not a measured value for your house. No part of Quebec is radon-free, and no address is automatically exempt.

For the wider provincial picture, see our Quebec radon map and risk regions and the pillar guide to radon in Quebec.

How does the regional data fit the broader Quebec picture?

The Laurentians are not an island of risk in an otherwise clean province. The 2024 Cross-Canada Radon Report (Evict Radon) found that nearly 18% of Canadian homes now measure at or above 200 Bq/m³ — more than double the share recorded a decade earlier. In Central Canada (Ontario and Quebec) about 1 in 6 properties are at or above the guideline, and in the Montréal Metro Area specifically, 1 in 5 properties exceed 200 Bq/m³, with an area average of 82.4 Bq/m³ (Evict Radon, 2024). The neighbouring Ottawa–Gatineau region sits at 1 in 6.

Evict Radon does not publish a separate figure for the Laurentides, so we can't quote a regional percentage from that study — but Montreal's metro data is the closest large-sample benchmark, and the Laurentians' uranium geology means local levels can run well above it. Radon is also the leading cause of lung cancer in non-smokers and is linked to about 1 in 6 lung-cancer cases in Canada, which is why the guideline exists at all.

Are there municipal radon test kit programs in the Laurentians?

Yes. Several Laurentian municipalities have run subsidized or free testing campaigns, often tied to the national Take Action on Radon "100 Radon Test Kit Challenge" (Défi 100 trousses) and the provincial "Villes et municipalités contre le radon" campaign.

The clearest documented example is Sainte-Thérèse, which hosted the 8th provincial "Villes et municipalités contre le radon" campaign and has offered residents a certified test kit at a reduced price through a partnership with the Quebec Lung Association. In an earlier Sainte-Thérèse campaign, 207 homes were tested and 35 of them — about 16.9% — measured above 200 Bq/m³, with another quarter between 100 and 200 (Ville de Sainte-Thérèse). Neighbouring municipalities such as Boisbriand have launched similar "Occupe-toi du radon" programs.

These programs are excellent for awareness, but they are seasonal and capped — Sainte-Thérèse's discount code, for instance, expired once the participant limit was reached. If a local program isn't running when you need it, you can test on your own schedule with a managed long-term kit and still get a lab-grade result.

How do you test for radon in the Laurentians?

Radon levels swing with the seasons, the weather and how a house is run, so a single short snapshot can mislead. Health Canada recommends a long-term test of at least 91 days, ideally during the heating season (roughly October to April) when homes are closed up and levels are typically highest (Health Canada radon guideline).

The practical steps:

1. Place an alpha-track detector in the lowest level you regularly occupy — a finished basement, family room or main-floor bedroom — away from drafts, exterior walls and humidity.
2. Leave it undisturbed for 91 or more days.
3. Return it to a C-NRPP-certified laboratory for analysis, and read your annual-average result in Bq/m³.

RadonTest.ca's managed kit is built for exactly this: an alpha-track detector for a 91+ day measurement, analyzed by a C-NRPP-certified Canadian lab, with prepaid return shipping included and the option to have your test analyzed entirely in Canada. There's nothing to send across a border and no add-on lab fee. (Note that "C-NRPP-certified lab" refers to the analysis; no radon kit is "Health Canada approved.") See exactly how the test works.

What should you do if your radon level is above 200 Bq/m³?

200 Bq/m³ is Health Canada's action guideline, not a safety line — there is no radon level considered risk-free, and the goal is always to get as low as practical. If your long-term test comes back at or above 200 Bq/m³, Health Canada recommends taking corrective action to reduce the level within one year, and sooner if the reading is high (Health Canada).

The proven fix is active soil depressurization — a certified mitigator installs a sealed pipe and a quiet continuous fan that draws radon from beneath the foundation and vents it safely above the roofline. Done by a C-NRPP-certified professional, mitigation routinely cuts indoor radon by 80–90% or more. For typical pricing, see our guide to radon mitigation cost in Canada, and for the full step-by-step, read what to do if your radon level is above 200 Bq/m³. After any mitigation, always retest to confirm the work succeeded.

One Laurentian wrinkle worth knowing: in Oka's most extreme zones, the 1998 DSP report found that standard mitigation could not always guarantee a reduction below the (then) 800 Bq/m³ threshold, and that design choices at the construction stage mattered most. That is unusual and reflects the area's exceptional geology — but it underscores why you start with an accurate long-term measurement before deciding on a fix.

Whether you're in Oka, Saint-Joseph-du-Lac, Deux-Montagnes, Sainte-Thérèse, Saint-Jérôme or Mont-Laurier, the first move is the same — get a real number for your own home.

Order your radon test kit → — all-in, analyzed in Canada, prepaid return shipping.

If you're closer to the regional hub, you can also read our local guide to radon in Saint-Jérôme.

Frequently Asked Questions

Is the Laurentians a high-radon region? Parts of it are. The Basses-Laurentides — Oka, Saint-Joseph-du-Lac and the Deux-Montagnes area — sit on the uranium- and thorium-bearing Oka carbonatite complex, where public health screening has measured some of the highest indoor radon levels documented in Canada, and the Mont-Laurier area and MRC d'Antoine-Labelle are also flagged. But radon varies house by house, so a region's geology is a reason to test, not a verdict on your home. Only a long-term test of 91 days or more reveals your actual level.

Why is Oka specifically a radon hotspot? Oka sits on a rare carbonatite intrusion that is rich in uranium and thorium — the same deposit that was mined for niobium by the St-Lawrence Columbium and Metals Corporation in the 1960s and 1970s. Uranium near the surface decays into radon gas, which seeps into homes. In the worst-affected Oka sector, the DSP des Laurentides measured a median around 482 Bq/m³ and a maximum of 10,500 Bq/m³ in the 1990s.

What is the radon guideline in Canada? Health Canada's action guideline is 200 Bq/m³ (becquerels per cubic metre) as an annual average. If your home is at or above 200, Health Canada recommends fixing it within one year, and sooner if the level is high. There is no radon level considered completely risk-free, so reducing exposure as much as practical is always the goal.

Can I rely on a radon map to know if my home is below the guideline? No. Maps such as the Quebec radon-potential maps show geological likelihood across an area, not the level inside any specific house. In the Laurentians, a home reading 486 Bq/m³ has been recorded next door to one reading 17. No region of Quebec is radon-free, so testing is recommended on a case-by-case basis regardless of what a map shows.

When should I test for radon? A long-term test of at least 91 days is recommended, ideally during the heating season (October to April) when homes are closed and radon levels are typically highest. Place an alpha-track detector in the lowest level you regularly occupy and have it analyzed by a C-NRPP-certified lab.

Are there free or subsidized radon test kits in the Laurentians? At times, yes. Municipalities including Sainte-Thérèse and Boisbriand have run subsidized or free campaigns tied to Take Action on Radon and the Quebec Lung Association. These are seasonal and limited in quantity, so if no program is currently open, a managed long-term test kit lets you test on your own schedule with the same lab-grade analysis.

Sources

• DSP des Laurentides — Le radon à Oka, public health intervention report (1998), via BAPE archives — primary public-health report; Oka carbonatite geology, the St-Lawrence Columbium niobium mine, uranium/thorium content, and the measured indoor radon levels (Mont-Saint-Pierre sud median 482 / max 10,500 Bq/m³; "hot zone" median 1,000 Bq/m³).
• Santé Québec — Laurentides (DSP), radon page — regional health authority's list of most-affected areas (Basses-Laurentides: Oka, Saint-Joseph-du-Lac, Saint-André-d'Argenteuil; MRC d'Antoine-Labelle; Mont-Laurier), testing advice and the 200 Bq/m³ guideline.
• ScienceDirect — Geoenvironmental characteristics of ferroniobium slag from the St-Lawrence Columbium mine, Oka (2024) — peer-reviewed analysis confirming the uranium (~600 ppm) and thorium (~1,370 ppm) content tied to pyrochlore in the Oka deposit.
• Evict Radon — Cross-Canada Radon Report: radon in Canada's six largest cities (2024) — national and metro context: ~18% of Canadian homes above 200 Bq/m³; Montréal Metro average 82.4 Bq/m³ with 1 in 5 above 200; Ottawa–Gatineau 1 in 6.
• CIME 103.9 (Cogeco) — Fortes concentrations de radon dans l'ouest des Laurentides (2022) — regional reporting on surging testing demand in Oka, Deux-Montagnes and Lachute, and the 486-vs-17 Bq/m³ neighbour example illustrating home-by-home variability.
• Ville de Sainte-Thérèse — radon screening campaign — documented municipal kit program with the Quebec Lung Association; earlier campaign tested 207 homes, ~16.9% above 200 Bq/m³.
• Health Canada — Radon: the Canadian guideline — 200 Bq/m³ action guideline, correct action within one year (sooner if high), no risk-free level, and the long-term 91+ day testing recommendation.