Radon in London
London is our home base and the city we test in most.
London radon guide
Radon in London & Middlesex
Radon testing and mitigation in London & Middlesex
Our home base. Century homes in the core, fast-growing subdivisions on the edges, and sand-and-gravel valleys along the Thames.
Radon in London & Middlesex
London and Middlesex County hold roughly half a million people and almost every kind of house Southern Ontario builds: brick century homes in Old East Village and Wortley Village, post-war bungalows, student rentals, and large two-storey homes on the north and south edges of the city. Radon doesn’t care which one you live in. Levels are set by the ground under the footings and the way the house breathes, and both vary block by block.
What’s under the ground
Glaciers left London sitting on layers of till, moraine ridges and old meltwater channels, with sand and gravel concentrated along the Thames valley and its former spillways around Byron, Komoka and Delaware. Those glacial deposits contain rock carried south from the Canadian Shield, a source of uranium.
West of the city, toward Strathroy and Lambton County, the bedrock includes the Kettle Point Formation, a black shale that Ontario Geological Survey core samples put at over 30 ppm uranium on average. Black shales are the most common uranium-radium-radon source rock in southern Ontario.
The homes
Two groups stand out. Older homes in the core often have rubble or early poured foundations, floor drains and open sumps. Newer homes in Hyde Park, Fox Hollow, Lambeth and Talbot Village are larger, taller and tighter, and Canadian research has found newer homes tend to test higher. London’s push for additional residential units also means more people sleeping in basement suites, where radon is usually highest.
Public health: Middlesex-London Health Unit.
What the data says
1 in 8
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)
~100
Roughly how many homes per health region the first national survey (2009–2011) could test. Too few to clear any neighbourhood, which is why public maps can’t tell you about your house.Source: Health Canada — Cross-Canada Survey of Radon Concentrations in Homes, Final Report (2012)
+30%
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 factors
Why London homes should test
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.
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.
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.
Communities
Where we work in London & Middlesex
Radon in Strathroy
Strathroy-Caradoc sits 30 minutes west of London, where two radon factors overlap: a uranium-bearing shale band underground and sandy, permeable soils at the surface.
Strathroy radon guide
Radon in Komoka & Middlesex Centre
Middlesex Centre wraps around London’s west and north sides: Komoka and Kilworth on the Thames, Ilderton and Arva to the north, Delaware to the southwest.
Komoka radon guide
Radon in Dorchester & Thames Centre
Thames Centre covers the country just east of London: Dorchester, Thorndale, Mossley, Putnam and the farmland between them.
Dorchester radon guide
Questions
Radon in London & Middlesex: common questions
Is radon a problem in London, Ontario?
Radon has been measured above Health Canada’s 200 Bq/m³ guideline in homes across Ontario, and no public data rules out any London neighbourhood. Province-wide, about 1 in 8 homes tested at or above the guideline in the 2024 Cross-Canada Survey. A 91-day test is the only way to know your own level.
Should landlords test basement apartments in London?
Yes. Health Canada’s guideline applies to any part of a home where someone spends more than four hours a day, and basement units are usually the highest-radon space in a house. Testing the suite itself gives tenants and owners a clear answer.
Book a radon test in London & Middlesex
C-NRPP certified testing and mitigation across London, Strathroy, Komoka, Dorchester.