Radon by area
Radon in Southern Ontario, town by town
From Windsor and the Detroit River to Sarnia, Goderich and the Lake Huron shore, through London and the Thames valley to Kitchener-Waterloo and Brantford: what’s under the ground, what kinds of homes sit on it, what the data actually says, and what to do about it.
Base map: Natural Earth (public domain). Thames and Detroit rivers traced approximately. Kitchener and Waterloo are drawn slightly apart.
Windsor-Essex
Windsor’s riverfront neighbourhoods and the Essex County towns along Lake Erie, on flat clay plain between the Detroit River and the lake.
Chatham-Kent
One large municipality of flat farmland, river towns and shoreline on Lake Erie and Lake St. Clair, centred on Chatham on the Thames.
Lambton
Sarnia and the St. Clair River, the Lake Huron shore from Point Edward toward Grand Bend, and farm country over the Kettle Point shale.
London & Middlesex
Our home base. Century homes in the core, fast-growing subdivisions on the edges, and sand-and-gravel valleys along the Thames.
London · Strathroy · Komoka · Dorchester
Elgin & St. Thomas
Railway-city heritage homes, rapid new growth around St. Thomas, and Lake Erie shoreline cottages turned year-round.
Huron-Perth
Stratford’s Avon River heritage core, St. Marys and its limestone, and Goderich on the Lake Huron bluffs, with farm country in between.
Oxford County
Woodstock, Ingersoll and Tillsonburg: manufacturing towns with heritage cores, new subdivisions, and rural homes on wells.
Waterloo Region
Kitchener, Waterloo and Cambridge on the Grand River, over the sand and gravel of the Waterloo Moraine, with farm country in the townships.
Norfolk
Sandy Norfolk farm country and the Lake Erie ports of Simcoe and Port Dover.
Brant & Brantford
Grand River valley city and towns, from Brantford’s older wards to Paris’s cobblestone heritage and new growth in St. George.
Also serving: Hamilton · Ancaster · Haldimand (Caledonia) · Niagara Region: Grimsby, Beamsville, St. Catharines, Niagara-on-the-Lake, Thorold, Niagara Falls, Welland, Fonthill, Fort Erie, Port Colborne.
How to read these pages
No made-up risk scores. Just what’s known.
Most radon maps colour whole regions red or green from a few dozen tests. That can falsely reassure people in “green” areas, where plenty of homes still test high.
Each of our area pages answers four questions instead:
- What’s under the ground? Bedrock, glacial soils and how easily soil gas moves through them.
- What’s on top? The local housing: century foundations, new airtight builds, cottages, basement suites.
- What does the data say? Only published, sourced figures. When there’s no local data, we say so.
- What should you do? The same answer everywhere: a 91-day test (with an early reading at 2 weeks), then act on your own number.
Ontario homes at or above the 200 Bq/m³ guideline in the 2024 Cross-Canada Survey.Source: Cross-Canada Survey of Radon Exposure in the Residential Buildings of Urban and Rural Communities (2024)
Of homes tested in the Windsor-Essex County Health Unit’s 2015 to 2018 study were above the guideline: 18% in Essex County and 6% in the City of Windsor.Source: Windsor-Essex County Health Unit — Radon: Know Your Level, 2015–2018 report
Higher average exposure in rural communities than urban ones, linked mostly to drilled wells.Source: Goodarzi et al., Scientific Reports (2024) — rural communities and drilled-well annuli
Homes have tended to test higher than older ones in Canadian research: bigger, taller and tighter.Source: Simms et al., Scientific Reports (2021) — radon in newer North American residences
Local factors
What pushes radon up in Southern Ontario
These show up again and again across our service area. None of them predicts a single home, but each is a reason to test.
Uranium-bearing shale nearby
Black shale is the most common uranium source rock in southern Ontario. The Kettle Point shale that runs under Lambton and western Middlesex and Elgin averages over 30 ppm uranium in Ontario Geological Survey drill core, roughly ten times a typical rock.
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.
Clay plains
Dense clay slows soil gas, but it cracks in dry spells. Gas then follows the easiest route: the gravel backfill, weeping tile and utility trenches that lead straight to a basement.
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.
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.
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.
New, airtight construction
Canadian research shows newer homes have tended to test higher than older ones. Bigger, taller and tighter homes create a stronger stack effect that draws soil gas in through the slab.
Lived-in basements
Radon is usually highest on the lowest floor. Basement bedrooms, home offices and rental suites mean people spend hours a day where exposure is greatest.
Cottages and crawlspaces
Lakeshore cottages converted to year-round homes often sit on crawlspaces, piers or patched slabs. Once winterized and heated, they can start pulling in soil gas they never did as summer places.
Don’t see your town?
We serve all of Windsor-Essex, Chatham-Kent, Lambton, Huron-Perth, Waterloo Region, London, Middlesex, Elgin, Oxford, Norfolk and Brant, and also Hamilton, Ancaster, Haldimand and the Niagara Region. If you’re nearby, ask: we’ll tell you honestly whether we can help.