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.

How radon forms and enters a building

Uranium-238 in soil and rock decays through a chain of radionuclides that includes radium-226, which produces radon-222 gas. Radon's half-life is about 3.8 days, long enough for the gas to move through soil pores and into buildings, yet short enough that its radioactive progeny (polonium, lead, and bismuth isotopes) deliver most of the lung dose when inhaled. Health guidance focuses on indoor air concentration of radon gas as a practical control point for that chain.

Entry paths include cracks in floor slabs, gaps at wall-floor joints, unfinished dirt floors, sump pits, floor drains, and utility penetrations. Homes with basements or crawl spaces are common measurement sites because soil gas often collects at the lowest level. Airtight construction and balanced ventilation can reduce entry; depressurization of the home relative to the soil (the winter stack effect) can increase it.

Cross-Canada Survey methods (how to read aggregates)

Health Canada's Cross-Canada Survey of Radon Concentrations in Homes collected long-term alpha-track detector results from homes across the country and published geographic summaries through the Open Government portal. Those summaries support regional comparison: mean or geometric-mean concentrations and the share of results at or above 200 Bq/m³ for available geographies. They are survey aggregates of tested homes, with participation and geography grain that vary by release.

Important reading rules follow from that design. A municipal or CMA figure is a summary of participating tests in that geography, not a census of every dwelling. Year labels on survey products refer to the survey vintage, not an annual retest of the same homes. OpenStats radon rankings that span multiple City Score years may carry a survey vintage forward when Health Canada has not published a new geography table; those carried years are labeled as static survey context, never as fresh annual measurements invented for each calendar year.

Mitigation in practice

Active soil depressurization (often called sub-slab depressurization) is the most common permanent fix: a fan draws soil gas from beneath the slab or from a sump and vents it above the roofline. Sealing major entry points supports that system. Heat-recovery ventilators and other ventilation upgrades can help in some buildings but are usually secondary to soil-gas control when concentrations are well above the guideline.

Canadian National Radon Proficiency Program (C-NRPP) credentials identify professionals trained for measurement and mitigation. After mitigation, a follow-up long-term test confirms whether concentrations fell below the guideline. Building and renovating with radon-resistant practices (sealed membranes, rough-in pipes for future fans) is increasingly common in higher potential regions.

Workplaces and public buildings

The same 200 Bq/m³ guideline informs workplace and public-building programs in many jurisdictions, with occupational health rules adding exposure-time context. Schools, care homes, and ground-contact commercial space can accumulate radon for the same geological reasons as houses. Testing protocols still favour long-term detectors in regularly occupied ground-contact rooms.

Smoking and combined lung-cancer risk

Health Canada and international radiation-protection bodies describe a strong interaction between tobacco smoking and radon exposure: the combined risk of lung cancer is higher than adding the two risks as if they were independent. That is why guidance materials urge both radon testing and smoking cessation. A non-smoker in a moderately elevated home and a smoker in a lower-radon home can face very different absolute risks even when the Bq/m³ figures look close.

Community survey averages on OpenStats pages do not incorporate smoking prevalence. They describe indoor radon measurements from participating homes. Personal risk framing still belongs with a physician or public-health materials that address smoking status alongside radon results.

Buying, selling, and when short-term tests appear

Real-estate timelines often push buyers toward short-term radon tests of a few days. Provincial disclosure rules and standard forms differ, and practice varies by brokerage and region. A short-term result can support a conversation or a holdback for further testing; it remains a weaker estimate of the annual average than a three-month detector on the lowest lived-in level, especially outside the heating season.

If a purchase depends on radon, plan for a long-term test after closing when the short-term screen is inconclusive or elevated. Mitigation clauses and retest requirements are contract terms; the science recommendation from Health Canada stays the same: long-term measurement in the normal occupancy area.

OpenStats rankings and the radon atlas

The highest indoor radon ranking orders communities by Cross-Canada Survey mean concentration where geography allows (rank 1 is the highest mean). The radon atlas city pages show average concentration, the share of tested homes at or above 200 Bq/m³ when published, and the data tier (direct geography vs regional proxy). Those pages are YMYL context: they help prioritize testing, and they never replace a detector in your own home.

Decade City Score years without a new Health Canada geography table carry the survey vintage forward and flag it as static. OpenStats does not invent annual municipal retests. See the radon series notes and methodology for how carried years appear in rankings history. Practical steps also live on the atlas how to test and mitigation pages.

C-NRPP professionals and Canadian standards

Health Canada's Radon Reduction Guide for Canadians directs homeowners to certified measurement and mitigation contractors under the Canadian National Radon Proficiency Program (C-NRPP). C-NRPP training aligns professionals with Health Canada measurement protocols and Canadian General Standards Board (CGSB) practice for radon control in new construction and mitigation in existing buildings. Use the C-NRPP professional directory when a long-term result sits at or above 200 Bq/m³, and retest after any mitigation system is installed.

Why Canadian communities differ on survey averages

Uranium-bearing bedrock, glacial till, and soil permeability set regional radon potential. Parts of the Prairies, the Kootenays, and pockets of the Canadian Shield often show elevated survey means. House-to-house variation still routinely exceeds city-to-city average gaps because foundation type, sump design, sealing, and ventilation differ on the same street. Winter heating and tight envelopes raise indoor concentrations relative to summer. Survey participation is voluntary; treat municipal or CMA figures as regional potential indicators, not a census of every dwelling.

Caveats when using OpenStats radon figures

• A city average never replaces a detector in your own home.
• Survey vintages may be carried static across City Score years; they are not annual retests.
• Short-term real-estate screens need long-term confirmation.
• Smoking multiplies lung-cancer risk at a given Bq/m³; averages omit smoking status.
• Regional proxy tiers borrow broader geography when a municipality lacks its own breakout.

Primary sources

• Health Canada radon overview and guideline: canada.ca/health-canada radon
• Radon Reduction Guide for Canadians: canada.ca reduction guide
• Cross-Canada Survey of Radon Concentrations in Homes (Open Government): open.canada.ca dataset
• Canadian National Radon Proficiency Program (C-NRPP): c-nrpp.ca
• OpenStats: radon ranking, radon atlas, radon series
• Related hub pages: methodology, data sources