Radon in the Montérégie: Mont-Saint-Hilaire, Beloeil, and the Risk Areas

Is the Montérégie a high-radon area? The Montérégie — Greater Montréal's South Shore and the area around the Monteregian Hills — carries a real, documented radon signal. In Beloeil, 23% of homes that took part in a community kit program measured above Health Canada's 200 Bq/m³ guideline, and across the wider Montréal metropolitan area roughly 1 in 5 homes test above that level. But a regional pattern is not a verdict on your house: radon varies dramatically from one home to the next, and the only way to know your level is a long-term test of at least 91 days. This guide covers the geology, the towns with the strongest signal, Beloeil's kit program, and exactly how to test.

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

Why might the Monteregian Hills matter for radon?

The Montérégie takes its name from the Monteregian Hills (collines montérégiennes) — a chain of isolated, forested summits that rise out of the flat St. Lawrence Lowlands, including Mont-Saint-Hilaire, Mont-Saint-Bruno, Mont Rougemont, Mont Saint-Grégoire and Mont Yamaska. Geologically, these are not ordinary hills. Each is an eroded remnant of a Cretaceous (roughly 125-million-year-old) intrusive igneous body — magma that crystallized at depth and was later exposed by erosion. Many of them are alkaline intrusions rich in unusual minerals.

Mont-Saint-Hilaire is the textbook example. Its core is nepheline syenite, an alkaline rock enriched in titanium, zirconium and rare-earth elements, and the mountain is world-famous among mineralogists for hosting hundreds of rare mineral species. Alkaline intrusions of this kind can also concentrate uranium and thorium — and because radon gas is produced by the radioactive decay of uranium in rock and soil, that geology is a plausible reason to pay attention here.

It is important to be precise about how strong this link is. The clearest published radon evidence for Mont-Saint-Hilaire comes from groundwater: a study of radon in St. Lawrence Lowlands well water measured activities as high as 310 Bq/L in wells associated with the Mont-Saint-Hilaire intrusion, and described the area as "known for its high level of indoor radon." Near the mountain, radon dissolved in well water can also contribute a larger share of a household's radiation dose than is typical elsewhere in the lowlands.

So the honest summary is this: the Monteregian Hills' alkaline igneous geology is a legitimate reason the Montérégie is worth taking seriously for radon, and Mont-Saint-Hilaire in particular has a documented reputation for elevated indoor radon. But geology only shifts the odds — it does not set your number. Two houses on the same street, on the same rock, can read completely differently depending on foundation cracks, sump pits, ventilation and construction. Geology is a reason to test, never a substitute for testing.

Which Montérégie towns show the strongest radon signal?

There is no single "Montérégie radon map" that grades every street, but several reliable signals point to where attention is warranted.

Mont-Saint-Hilaire and the Vallée-du-Richelieu. As described above, the Mont-Saint-Hilaire intrusion is documented as a local hotspot, with elevated radon in groundwater and a noted reputation for high indoor radon. Neighbouring communities along the Richelieu — including Beloeil, Otterburn Park, McMasterville and Saint-Mathieu-de-Beloeil — sit in the same geological neighbourhood and are where the most active municipal radon work in the region has happened.

Beloeil. The standout data point. In a community testing program, 23% of participating Beloeil homes measured above 200 Bq/m³ — more than the provincial average (see the table below). Beloeil has since become one of the most radon-active municipalities in Québec, with a dosimeter rebate and a building-code requirement for new homes.

The wider South Shore (Rive-Sud) and Greater Montréal. The Evict Radon 2024 Cross-Canada Survey reports that the Montréal metropolitan area averages 82.4 Bq/m³, with roughly 1 in 5 homes above the 200 Bq/m³ guideline — well above Toronto (1 in 22) and Vancouver (1 in 113). The survey does not publish a Montérégie-only figure, so the Montréal-metro number is the closest reliable proxy for the densely populated parts of the South Shore.

The rest of the Montérégie. The region is large, stretching from the river south to the U.S. border and east toward the Appalachian foothills. RadonTest already covers two Montérégie cities in depth — see Radon in Saint-Hyacinthe and Radon in Saint-Jean-sur-Richelieu — and for the province-wide picture and how the Montérégie fits into it, see Radon in Quebec and the Quebec radon map and risk regions.

What did Beloeil's free radon kit program find?

Beloeil is one of the clearest Canadian examples of a municipality taking radon seriously, and its data is the most concrete reason to test if you live nearby.

The city took part in the "Défi 100 trousses de dépistage du radon" (100 Test Kits Challenge), a community program run through Take Action on Radon and the Association pulmonaire du Québec, in which residents place a long-term detector for the heating season and send it to a lab. In the winter of 2022, 108 Beloeil homes completed 91-day alpha-track tests. The results:

• 44% measured below 100 Bq/m³
• 33% measured between 100 and 200 Bq/m³
• 23% measured above the 200 Bq/m³ guideline

That 23% over-guideline rate is higher than the roughly 17% reported for Québec homes overall — a meaningful local signal. The City of Beloeil reports that across the winters of 2022–2023 and 2024–2025, 194 residents in total took part in the challenge, installing detectors in their homes between roughly December and March for lab analysis.

Beloeil has gone further than awareness. The city:

• Reimburses 50% of the cost of a dosimeter, up to $30, for property owners who buy a kit and actually send it in for analysis;
• Has required, since 2024, that all new residential construction include a complete passive radon-reduction system (a "level 2" rough-in) installed by a certified expert;
• Has formally asked the Québec government to tighten the construction code and create a financial-assistance program for radon mitigation.

If you live in Beloeil or a neighbouring Richelieu-valley community, the takeaway is simple: roughly one in four tested homes in your area came back above the guideline, so testing your own home is a sensible step — and your municipality may help pay for it.

How do you test a home in the Montérégie for radon?

Testing is straightforward, inexpensive, and the only way to learn your actual level. The key rules come straight from Health Canada:

1. Use a long-term test of at least 91 days. Radon swings hour to hour and day to day, so a long-term measurement is the only way to get a reliable estimate of your annual average. The 200 Bq/m³ guideline applies to that annual average, not to a quick snapshot. Short-term and digital monitors can be useful for a first look, but a 91-day-plus alpha-track test is what Health Canada bases corrective-action decisions on.
2. Test during the heating season where possible. Late fall through early spring (roughly October to April) is ideal, because closed windows and reduced ventilation produce the highest, most representative readings. The Association pulmonaire du Québec specifically recommends November to April. If you test outside that window, leave the detector in place for a full 12 months.
3. Place the detector in the lowest lived-in level. Put it in a room where someone spends more than four hours a day — a basement family room, a ground-floor living room, or a basement bedroom — away from drafts, heat and humidity.
4. Don't rely on a neighbour's result or a regional map. Health Canada and Québec public-health authorities are explicit on this point: a high reading nearby raises the importance of testing, but it does not predict your number. Every home is different.

RadonTest's radon test kit is a managed, long-term (91-day-plus) alpha-track test analyzed by a C-NRPP-certified laboratory, with prepaid return shipping included and the option to have your kit analyzed entirely in Canada — it meets every one of the criteria above.

What should you do if your Montérégie home is above 200 Bq/m³?

A result above 200 Bq/m³ is not an emergency, but it is a clear signal to act. Here is the current Health Canada guidance — and note that it replaces the older tiered timelines you may still see on some printed materials:

• Plan to reduce the level within one year. Health Canada recommends taking corrective action if your average annual radon level exceeds 200 Bq/m³, and the higher your result, the sooner you should act. (Older brochures sometimes quote a two-year window for 200–600 Bq/m³ and one year above 600 Bq/m³; the current guidance is simply to fix it within a year, sooner if the level is high.)
• Hire a C-NRPP-certified mitigation professional. A certified contractor will design a system suited to your home. The standard fix is active soil depressurization — a fan-and-pipe system that draws radon from beneath the foundation and vents it safely above the roofline. Done correctly, professional mitigation routinely cuts radon by more than 90%. For a breakdown of what it costs, see radon mitigation cost in Canada.
• Confirm the fix with a follow-up test. After mitigation, re-test to verify your level has dropped below the guideline and to confirm the system is working.

For a full, step-by-step walkthrough of next steps, read what to do if your radon level is above 200 Bq/m³.

There is no level of radon that is considered risk-free, so even a result below 200 Bq/m³ can be worth reducing if it can be done practicably — but above the guideline, action is clearly recommended.

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

Frequently Asked Questions

Is the Montérégie a radon hotspot? Parts of it show a clear signal. In Beloeil, 23% of tested homes measured above the 200 Bq/m³ guideline, and the wider Montréal metropolitan area (which includes the South Shore) averages 82.4 Bq/m³ with about 1 in 5 homes above the guideline — far higher than Toronto or Vancouver. The area around the Monteregian Hills, especially Mont-Saint-Hilaire, has documented elevated radon. That said, no town is uniformly "high," and the only way to know your own home is to test it.

Why is Mont-Saint-Hilaire associated with higher radon? Mont-Saint-Hilaire is an alkaline igneous intrusion — a body of nepheline syenite enriched in unusual elements — that can carry uranium and thorium, the parent elements that produce radon as they decay. A study of local well water measured radon as high as 310 Bq/L in wells tied to the intrusion and described the area as known for high indoor radon. The geology raises the odds of elevated radon in the area, but a long-term test is the only way to confirm any individual home's level.

Does living near a Monteregian Hill mean my home has high radon? No. Geology shifts the probability, not the result. Radon levels can vary widely between neighbouring houses on identical bedrock because the actual indoor level depends on foundation condition, sump pits, cracks, ventilation and how the home is built and used. Treat proximity to the hills as a strong reason to test — not as an answer.

What did Beloeil's radon kit program find? In the winter 2022 round of the "Défi 100 trousses" program, 108 Beloeil homes ran 91-day tests; 23% came back above 200 Bq/m³, 33% between 100 and 200, and 44% below 100. Across the 2022–2023 and 2024–2025 winters combined, 194 Beloeil residents took part. Beloeil also reimburses 50% of a dosimeter's cost (up to $30) and now requires a passive radon system in new construction.

Does Beloeil still help pay for a radon test? Beloeil's published program reimburses property owners 50% of the cost of a dosimeter, up to $30, provided they buy a kit and send it in for lab analysis. Program terms can change, so confirm current details directly with the Ville de Beloeil before relying on the rebate. The rebate is paid on proof of purchase and proof that the sample was mailed for analysis.

When is the best time to test for radon in Québec? Test during the heating season — roughly October to April, and ideally November to April per Québec public-health advice — when windows are closed and ventilation is lowest, producing the most representative readings. Use a detector for at least 91 days. If you test outside the heating season, leave it in place for a full 12 months.

How long does a proper radon test take? At least 91 days. Health Canada bases corrective-action decisions on tests of 91 days or longer because radon fluctuates constantly, and only a long-term measurement reliably estimates your annual average. Shorter digital readings can flag a potential problem but should be followed up with a 91-day-plus test.

What radon level requires action? Health Canada's guideline is 200 Bq/m³ as an annual average. Above that, plan to reduce the level within one year — and the higher the reading, the sooner you should act. Because there is no risk-free level, reducing radon below 200 is also worthwhile where it can be done practicably.

Sources

• Health Canada — Radon guideline — Canada's 200 Bq/m³ guideline; no risk-free level; corrective action within one year, sooner the higher the level (updated September 2025).
• Take Action on Radon — Beloeil, QC community report (2023) — 108 Beloeil homes, 91-day alpha-track tests, winter 2022: 23% above 200 Bq/m³, 33% between 100–200, 44% below 100.
• Ville de Beloeil — Radon — municipal program: 194 participants over winters 2022–2023 and 2024–2025; 50% dosimeter rebate up to $30; mandatory passive radon system for new construction since 2024.
• Evict Radon — Radon levels in Canada's six largest cities (2024) — Montréal metro area average 82.4 Bq/m³; roughly 1 in 5 homes above 200 Bq/m³.
• Pinti et al. — 222Rn activity in groundwater of the St. Lawrence Lowlands (ScienceDirect) — radon up to 310 Bq/L in wells associated with the Mont-Saint-Hilaire intrusion; area "known for its high level of indoor radon."
• Monteregian Hills (geological overview) — Cretaceous alkaline intrusive igneous geology of Mont-Saint-Hilaire and the other hills; Mont-Saint-Hilaire's nepheline-syenite, rare-element-enriched composition.
• Association pulmonaire du Québec — Radon — Québec testing advice (heating-season measurement, November–April) and the provincial finding that ~17% of Québec homes exceed 200 Bq/m³.