What radon gas is, how it gets into your home, and which Canadian homes have more of it by building type, basement and community, per the 2024 survey.
Photo: Weirdmeister, CC BY-SA 4.0, cropped, via Wikimedia Commons.
20.4%, 11.1%, 7.9%
Share of single-detached, semi-detached and row properties at or above 200 Bq/m³ in the 2024 Cross-Canada Radon Survey, with geometric means of 93.4, 61.5 and 51.8 Bq/m³. Multifamily results are preliminary (1,089 tests, 9.3% at or above 200) and under-represent buildings of five or more storeys.
Radon is a radioactive gas you cannot see, smell or taste. It forms underground when uranium in rock and soil breaks down. Health Canada names radon the number one cause of lung cancer in people who do not smoke. Read how much radon raises the risk of lung cancer.
Radon builds up indoors, and some homes collect more of it than others. Health Canada’s guideline is 200 Bq/m³, the average annual level at which it recommends corrective action. Find out what 200 Bq/m³ means for your home and how soon to act.
Uranium in rock and soil breaks down over time through radioactive decay. One of the things it turns into is radium, a solid radioactive metal. Radium-226 then breaks down into radon-222, which is a gas. The rock and the radium stay where they are, while the gas mixes with other soil gases and moves toward the surface.
3.8
daysHalf-life of radon-222: about how long it takes for half of any amount to decay into solid radon decay products. Radon that enters a building is short-lived, and its decay products are what stay in the air.
Radon is a noble gas, so it does not readily react with other substances, including minerals or water. That lets it separate from the rock that made it and move quickly through soil. Being unreactive does not make it harmless, because it stays radioactive.
Within days, radon breaks down into a chain of solid radioactive products, beginning with polonium and ending in stable lead. Look up becquerel, decay products and soil gas in the glossary.
Soil gas carrying radon reaches the surface everywhere. Outdoors, it is diluted quickly in the atmosphere. A building in direct contact with the ground gives the gas a way in. Inside, dilution is limited, so high levels can build up. Health Canada lists these entry points:
Levels are generally highest in basements and crawl spaces, which are nearest the source and usually poorly ventilated. The 2024 Cross-Canada Radon Survey says: “even though radon is naturally made, the high levels we see indoors are a human made problem”. Health Canada says sealing the entry points is not an effective stand-alone way to reduce radon. See how a mitigation system deals with these entry points.
Warm air rises through a house and leaves through the upper floors. As it goes, the lower levels end up at lower air pressure than the ground and the air outside. That pressure difference pulls air in from the soil, and radon comes with it. The effect is generally stronger during the heating season, which is why a radon test runs through it. Read how to test, and why the heating season matters.
The 2024 survey compared radon by the floor where the test was placed. Lower floors average more.
89.9, 68.8, 54.4
Bq/m³Geometric mean radon levels below ground, at ground level and above ground, by the floor where the test was placed, in the 2024 Cross-Canada Radon Survey: 23.5% higher below ground than at ground level, and 20.9% higher at ground level than above. A population average, not a rule for any one home.
Below ground reads highest on average, then ground level, then the floors above. Upper floors are lower on average, and they are not radon free. The survey’s authors suggest upward air movement in the house, and dilution from windows and doors, as the most likely reason.
Rural communities on groundwater wells have more radon in their properties than nearby urban communities. In the survey, rural means a community of 1 to 29,999 people. The authors suggest gaps around well casings may let radon move more easily toward the soil under the house. A standard mitigation system still works in these homes. The survey suggests no action on the well itself at this time.
Usually not a problem
Health Canada's position on radon in drinking water: the larger risk is radon in indoor air. Water adds a little when the gas escapes during showers and household use, and very high levels in private well water can occasionally matter. Testing the air captures radon from every source.
Test the indoor air first, since an air test captures radon from every source, water included. Find out which level and room to test in your home.
Three factors set a building’s radon level: region, community and building design. Radon rises through the ground at a rate set by local geology, and every Canadian region has some radon-generating source material. Design, construction and ventilation matter too, and the survey reports a trend toward higher radon in newer Canadian homes. Read the geology behind the regional differences.
In the survey, a single-detached house is more likely than a semi-detached house to be at or above the guideline. A semi-detached house is more likely than a row house. The multifamily figure is preliminary.
Rural 23.8%, urban 17.4%
Share of properties at or above 200 Bq/m³ by community type in the 2024 Cross-Canada Radon Survey, with geometric means of 99.9 Bq/m³ (rural) and 86.3 Bq/m³ (urban). The survey counts a community of 30,000 people or more as urban and one of 1 to 29,999 as rural.
Rural homes, in the survey’s sense, also average more radon than urban ones, and a larger share are at or above the guideline.
9
%Share of households outside apartment buildings that had heard of radon and had ever tested for it, in 2021; 56% of households had heard of radon. A 2021 figure, not a current rate.
In 2021, most households that had heard of radon had never tested. See how common high levels are by province and place.