Huron-Perth
Radon testing and mitigation in St. Marys, Ontario
St. Marys is known as the Stonetown, with a downtown and many homes built of the local limestone, on the Thames River between London and Stratford. Here’s what’s known about radon in St. Marys, and how to find out about your home.
- Region
- Huron-Perth
- Population
- about 7,400
- Public health
- Huron Perth Public Health
- Local factors
- 5 reasons to test
Is radon a problem in St. Marys?
Radon can exceed Health Canada’s 200 Bq/m³ guideline in any St. Marys home. Cancer Care Ontario’s table of Health Canada survey data put Perth at 12.0% of homes at or above the guideline. Across Ontario, about 1 in 8 homes tested at or above the guideline in the 2024 national survey. Locally, shallow or fractured bedrock, Shield-derived glacial soils and sandy or gravelly soils are reasons to test. A 91-day test is the only way to know your home’s level.
Radon in St. Marys
St. Marys is known as the Stonetown, with a downtown and many homes built of the local limestone, on the Thames River between London and Stratford. It sits in Perth County, where the Health Canada survey data that Cancer Care Ontario tabulated by public health unit put 12.0% of the homes tested at or above Health Canada’s 200 Bq/m³ guideline. That is a county figure and a limited sample, so it is a reason to test and not a verdict on any one house.
The ground under St. Marys
Limestone bedrock comes close to the surface around St. Marys, which is why the town has a long quarrying history and why so many of its older buildings are stone. Where rock is near the surface, joints and fractures can carry soil gas toward foundations. Around it, glacial till and sand and gravel along the Thames valley complete a mix that varies from lot to lot.
St. Marys’s homes
Limestone homes with stone foundations and cellars stand next to post-war bungalows and newer subdivisions. Stone and rubble walls have more gaps than poured concrete, and the older houses often have a mix of basement, cellar and crawlspace under different parts of the building, each of which needs its own plan.
What the data says
Of homes tested in the Perth health unit area were at or above 200 Bq/m³ in the Health Canada survey data tabulated by Cancer Care Ontario (2009 to 2013).Source: Cancer Care Ontario — Risk of Residential Radon Exposure Varies Geographically (results by public health unit, Health Canada survey data 2009–2013)
Ontario homes at or above Health Canada’s 200 Bq/m³ guideline in the 2024 Cross-Canada Survey, more than double the share found a decade earlier.Source: Cross-Canada Survey of Radon Exposure in the Residential Buildings of Urban and Rural Communities (2024)
Higher average radon exposure for rural residents than urban residents in a 2024 national study of 42,051 homes, mostly linked to drilled wells.Source: Goodarzi et al., Scientific Reports (2024) — rural communities and drilled-well annuli
Local results
Building a real radon picture for St. Marys
Public radon data for Southern Ontario towns is thin. The national surveys tested only a handful of homes per area. So we’re building our own: anonymized long-term results from homes we test, published by area once there are at least 30, so no single home can be identified and small samples can’t mislead.
St. Marys: collecting results. Already tested? Check your number and share it anonymously when you book.
Local factors
What can push radon up in St. Marys
Not a prediction for any single home, just the reasons a test is worth it here.
Shallow or fractured bedrock
Where rock sits close to the surface, as on the Niagara Escarpment or the limestone plains near Lake Erie, joints and fractures can act as pipelines that carry soil gas toward foundations.
Glacial soils with Shield debris
The till and gravel left by glaciers here includes ground-up rock carried south from the Canadian Shield. Uranium travels with it, so radon can form close to your footings even where the bedrock itself is low in uranium.
Sandy or gravelly soils
Sand and gravel let soil gas move freely and over longer distances. More of the radon formed in the ground can reach the air around your foundation instead of decaying in place.
Century homes
Fieldstone, rubble and early concrete foundations, earth-floor cellars and open sumps give radon many ways in. Older doesn’t mean safer: these homes still need a test.
Rural homes on drilled wells
A 2024 national study of over 42,000 homes found rural residents were exposed to about 30% more radon than urban residents, with most of the gap linked to drilled wells. Researchers suspect gas travels up the gap around the well casing.
Where we work
St. Marys neighbourhoods and nearby communities
- Downtown St. Marys
- Stonetown heritage district
- Wildwood area
- Perth South
- Southern Perth County
Nearby: Radon in Stratford · Radon in London · Radon in Ingersoll
How it works
From test to fix
- Book
Tell us about your St. Marys home. We reply within one business day.
- Test for 91+ days
A certified technician places long-term detectors on your lowest lived-in floor. You get an early reading at 2 weeks.
- Get your result
Lab-verified average, explained against Health Canada’s guideline.
- Fix it if it’s high
A sub-slab system designed for your foundation, at a fixed price, with results guaranteed.
Questions
Radon in St. Marys: FAQ
Is radon a problem in St. Marys?
Radon can exceed Health Canada’s 200 Bq/m³ guideline in any St. Marys home. Cancer Care Ontario’s table of Health Canada survey data put Perth at 12.0% of homes at or above the guideline. Across Ontario, about 1 in 8 homes tested at or above the guideline in the 2024 national survey. Locally, shallow or fractured bedrock, Shield-derived glacial soils and sandy or gravelly soils are reasons to test. A 91-day test is the only way to know your home’s level.
Does limestone close to the surface mean more radon?
Limestone itself is usually low in uranium, but joints and fractures in shallow bedrock can carry soil gas from the surrounding ground toward foundations. That is why rock close to the surface is a reason to test, not a reason to relax.
Can you install a radon system in a stone house?
Usually, yes. Stone and rubble foundations have more openings, so we spend more time sealing. We may add a second suction point or treat a cellar or crawlspace separately, and we show you the design before we drill.
Do you offer radon testing and mitigation in St. Marys?
Yes. We test and mitigate homes throughout St. Marys, including Downtown St. Marys, Stonetown heritage district, Wildwood area, Perth South, Southern Perth County. Call (226) 271-4667 or book online.
Radon is fixable. First, know your number.
Serving St. Marys, Stratford, London and Ingersoll.