Short answer
Radon is a colourless, odourless radioactive gas that forms naturally when uranium in soil and rock breaks down. Outdoors it dilutes to very low levels, but it can seep into homes through foundations and build up, especially in basements. Long-term exposure is the leading cause of lung cancer in non-smokers. A long-term test is the only way to know your level.
Key facts
- Radon comes from the natural decay of uranium in soil and rock.
- You can’t see, smell or taste it. Only a test can detect it.
- Health Canada’s guideline is 200 Bq/m³ as a yearly average.
- Every home in Ontario has some radon; the question is how much.
Where radon comes from
Uranium is present in small amounts in almost all soil and rock. As it decays, it turns into a chain of other elements. One of them, radium, decays into radon, the only gas in the chain. Because it’s a gas, radon can move through cracks in rock and the spaces between soil particles until it reaches the surface.
Radon itself has a half-life of 3.8 days. It breaks down into short-lived radioactive particles called radon decay products (or progeny), mainly isotopes of polonium. These are what damage the lungs.
Why it matters in Southern Ontario
Many people assume radon is a problem only on the Canadian Shield or out west. Southern Ontario has its own sources. The glaciers that shaped the region ground up Shield rock and spread it across the south as till, sand and gravel. Uranium travelled with it. In parts of southwestern Ontario, black shales such as the Kettle Point Formation add more: Ontario Geological Survey drill core puts its uranium content at over 30 parts per million on average, roughly ten times a typical rock.
The result is that homes above Health Canada’s guideline turn up across the province. The 2024 Cross-Canada Survey estimated about 1 in 8 Ontario homes at or above 200 Bq/m³. In Hamilton, a public health survey found about 14% of the homes it tested above the guideline.
How radon gets into a house
Air pressure inside a heated home is usually slightly lower than in the soil around the foundation, especially in winter. That small difference acts like a gentle vacuum, drawing soil gas in through any opening: cracks in the slab, the joint where walls meet the floor, sump pits, floor drains, gaps around pipes, and earth or crawlspace floors. Once inside, radon can build up because indoor air is replaced much more slowly than outdoor air. More on how radon enters homes.
How radon is measured
In Canada, radon is measured in becquerels per cubic metre (Bq/m³). One becquerel is one radioactive decay per second, so 200 Bq/m³ means 200 decays every second in each cubic metre of air. U.S. test kits often report in picocuries per litre (pCi/L): 1 pCi/L equals 37 Bq/m³.
| Reference level | Bq/m³ | pCi/L |
|---|---|---|
| WHO reference level | 100 | 2.7 |
| U.S. EPA action level | 148 | 4.0 |
| Health Canada guideline | 200 | 5.4 |
| Health Canada: reduce within 1 year | 600+ | 16+ |
| Canada’s guideline before 2007 | 800 | 21.6 |
Is any level of radon safe?
There’s no known threshold below which radon carries zero risk. The risk rises steadily with concentration and with years of exposure. Health Canada’s 200 Bq/m³ guideline is the level at which it recommends action, and it encourages keeping levels as low as reasonably achievable. The World Health Organization suggests a reference level of 100 Bq/m³.
What to do next
Test. Radon levels vary widely between homes, even next door, so neighbours’ results, maps and averages can’t tell you about yours. A long-term test of at least 91 days, ideally between October and April, gives you an annual average you can act on. If it’s high, radon is very fixable: a sub-slab depressurization system typically reduces levels by 90% or more.
Common questions
Does radon have a smell?
No. Radon is colourless, odourless and tasteless. The only way to detect it is with a radon test.
Is radon only found in basements?
Radon enters from the ground, so concentrations are usually highest on the lowest floor. But it can move through a home on air currents, so upper floors aren’t automatically free of it.
Can radon come from granite countertops or building materials?
Building materials can release small amounts, but in Canadian homes the soil beneath the house is by far the main source. Testing your lowest lived-in level captures the radon that matters.
Sources
- Health Canada — Radon: what you need to know
- Health Canada — Government of Canada radon guideline
- Cross-Canada Survey of Radon Exposure in the Residential Buildings of Urban and Rural Communities (2024)
- Ontario Geological Survey — Soil Radon Gas Study of Southern Ontario (OFR 5847)
- Global News — Hamilton public health survey of 294 homes
- World Health Organization — WHO Handbook on Indoor Radon (2009)