Indoor Radon in Canada: Guideline, Testing, and Geology

Radon is a colourless, odourless radioactive gas produced by the natural decay of uranium in soil and rock. It is present everywhere in Canada at some concentration, and it moves from soil into buildings through foundation cracks, sump pits, and gaps around pipes and utility entries. Outdoors, radon disperses quickly and poses negligible risk. Indoors, especially in basements and lower levels with limited air exchange, it can accumulate to concentrations high enough to raise long-term health risk.

Health Canada considers indoor radon exposure a leading cause of lung cancer after smoking, and the risk compounds for smokers, who face a meaningfully higher combined risk from radon exposure than non-smokers at the same radon concentration.

The 200 Bq/m³ guideline

Health Canada's guideline for indoor radon is 200 becquerels per cubic metre (Bq/m³), averaged over at least a three-month period, for the normal occupancy area of a home (the lowest level of the home that is lived in regularly, which for most houses is the basement if it is finished and used, or the main floor if the basement is not). The guideline applies to residential and workplace exposure and reflects a balance between health risk and practical mitigation capability, not a hard line between safe and unsafe.

Risk from radon exposure is understood to increase with concentration and does not have a known threshold below which there is zero risk. The 200 Bq/m³ line is the level at which Health Canada recommends remedial action be taken. Below that level, exposure is still non-zero, but the guideline treats mitigation as most warranted at or above it.

Why testing takes months, not minutes

Radon concentration inside a building fluctuates by season, weather, and even time of day, driven by temperature differences, wind, barometric pressure, and how a home is heated and ventilated. Concentrations are typically higher in winter, when homes are sealed tightly against cold and heating creates a stack effect that draws soil gas upward through the foundation. A short-term test taken over a few days can land well above or below the true annual average purely by chance of timing.

Health Canada recommends long-term testing, a minimum of three months and ideally covering part of the heating season, using an alpha-track or similar passive detector placed in the lowest lived-in level of the home. Short-term tests (two to seven days) have a role in real estate transactions where a fast answer is needed, but they are less reliable and should be treated as a screening step until a long-term test confirms the result.

Why radon varies with geology

Radon potential depends primarily on the uranium content of the underlying soil and bedrock, soil permeability, and how easily gas can migrate upward, none of which respect municipal boundaries. Two houses on the same street can post very different readings depending on foundation type, sump pit design, and how well the building is sealed and ventilated. Regions with granite bedrock, certain shale formations, or glacial till derived from uranium-bearing source rock tend to show higher average readings, which is why some provinces and specific regions, including parts of the Prairies, the Kootenays in British Columbia, and pockets of the Canadian Shield, show consistently elevated averages in national radon surveys.

Because of this house-to-house variability, a city-level average is a useful starting point for understanding regional risk, but it cannot substitute for testing an individual home. A low city average does not guarantee a low reading in any specific house, and a high city average does not guarantee every house in that city is elevated.

How OpenStats radon pages are built

The radon topic site draws from Health Canada's cross-Canada radon survey data, which aggregates long-term test results by geography. Each city page reports the average concentration and the share of tested homes above the 200 Bq/m³ guideline for that geography, along with a risk category derived from those figures. Where a municipality is not separately broken out in the underlying survey, the page uses the nearest available regional or provincial figure as a proxy and states that clearly, since radon potential can shift meaningfully between a city core and its surrounding region depending on local geology.

The survey data reflects homes that were tested and voluntarily reported, not a random sample of every home in a region, so it should be read as an indicator of regional radon potential. It is an indicator of regional risk, and it is less precise than a population-wide average from a random sample.

What to do with an elevated result

• Confirm with a long-term test if the initial result was short-term.
• Above 200 Bq/m³, plan mitigation; a certified radon mitigation professional can install a sub-slab depressurization system, the most common and effective fix.
• Retest after any mitigation work and periodically afterward, since radon levels can shift after renovations, foundation repairs, or changes to home ventilation.
• Test again if you move into a home without a recent long-term result on file, regardless of the neighbourhood's general reputation.

Radon test kits are inexpensive relative to the information they provide, and testing is the only way to know the concentration in a specific home. Regional averages published on OpenStats city pages are context for that decision, not a substitute for it.