Radon is a colourless, odourless radioactive gas produced when uranium in soil and bedrock decays. It seeps into buildings through foundation cracks, sump pits, floor drains and service penetrations, and in an airtight house in a cold climate it accumulates. Health Canada identifies it as the leading cause of lung cancer among Canadians who have never smoked, and the second leading cause overall after smoking.
How much radon is too much?
Health Canada’s guideline for remedial action in a dwelling is 200 becquerels per cubic metre (Bq/m³), measured as an annual average in the normal occupancy area. The guidance is to remediate within two years above 200 Bq/m³, and within one year above 600 Bq/m³.
Two things worth understanding about that number. First, it is a practical action threshold, not a safety line — risk is continuous, and reducing a level from 180 to 60 still reduces risk. Second, the World Health Organization recommends a reference level of 100 Bq/m³, which is why some Canadian health professionals argue the national guideline is set too high.
Does Ottawa have a radon problem?
Health Canada’s 2024 Cross-Canada Survey found approximately 17.8% of Canadian homes — roughly one in five — at or above 200 Bq/m³. Ontario’s overall average sits well below the guideline, but averages hide everything that matters here: radon is a local geological phenomenon, and the Ottawa Valley’s bedrock and glacial deposits are the kind that produce it.
The critical practical point is that results vary enormously between neighbouring houses. Two homes built by the same builder in the same year on the same street can differ by a factor of five, because what governs the result is the specific soil, the specific foundation, the specific cracks and the specific pressure dynamics of that building. Your neighbour’s reading tells you nothing about yours.
Long-term versus short-term testing
The 91-day test is the one Health Canada recommends and the only one that should drive a mitigation decision. A detector sits in the lowest lived-in level for a minimum of three months during the heating season, and the result reflects what you are actually breathing across a year. Radon swings enormously with weather, wind, barometric pressure, whether windows were open, and how the furnace is running — a long test averages all of that out.
The 48-hour test uses a continuous radon monitor under closed-building conditions and exists because real estate transactions do not wait three months. It is genuinely useful: a high short-term result is strong evidence and a very low one is reassuring. But it is a screening tool, and a report that presents a 48-hour number as if it were a definitive annual average is overstating what it knows.
Either way, the person placing the device should be certified through the Canadian National Radon Proficiency Program (C-NRPP). Do-it-yourself long-term test kits are also available and are a completely legitimate option — Take Action on Radon and various public health units run periodic low-cost kit programs.
What mitigation costs and how well it works
Radon is one of the few serious home problems with a reliable, permanent, reasonably priced fix. The standard solution is active sub-slab depressurisation: a pipe sealed into the soil beneath the basement floor, connected to a continuously running fan that vents above the roofline. It reverses the pressure differential that draws gas into the house.
- Typical Ottawa cost: $2,500 to $4,000 installed for a standard home.
- Typical reduction: 80% to 90%, frequently more.
- Running cost: a continuously running fan, roughly comparable to a lightbulb, plus a modest heating penalty.
- Always re-test after installation to confirm the system worked.
Sealing cracks alone is not mitigation. It helps a properly designed system work better, but on its own it rarely produces a meaningful reduction.
If you are buying a home in Ottawa
Ask the seller whether the home has ever been tested and request the results. If it has been mitigated, ask for the installation documentation and the post-mitigation test. If it has not been tested, you have two sensible options: run a short-term screening test during your conditional period, or buy the home and run a proper 91-day test through your first heating season, budgeting for mitigation if it comes back high.
Neither approach is wrong. What is wrong is never testing at all, which is what happens in the overwhelming majority of Ottawa transactions.
Frequently asked
What is a safe radon level in Canada?
Health Canada sets a guideline of 200 Bq/m³ for remedial action in homes, recommending action within two years above that level and within one year above 600 Bq/m³. The World Health Organization recommends a lower reference level of 100 Bq/m³. No level is considered entirely risk-free.
How long does radon testing take?
Health Canada recommends a minimum 91-day test during the heating season for a decision-grade result. A 48-hour continuous-monitor screening test is available when a real estate deadline makes the long test impossible.
How much does radon mitigation cost in Ottawa?
A standard active sub-slab depressurisation system typically costs $2,500 to $4,000 installed and usually reduces levels by 80% or more. Always re-test afterwards to confirm.
Can a home inspector detect radon?
Not by inspection. Radon is invisible and odourless and must be measured with a detector over time. Look for a C-NRPP certified measurement professional, and be sceptical of anyone who offers an opinion on radon without a measurement.