Radon in Your Income Property: Testing, the 200 Bq/m³ Guideline, Mitigation and Disclosure When Selling Your Plex

ImmoMulti — direct buyer of multi-unit properties on the North Shore — regularly sees radon surface during pre-purchase inspections. This naturally occurring radioactive gas, invisible and odourless, is the second leading cause of lung cancer in Canada after smoking, according to Health Canada. For a plex or income property owner, radon is not an abstraction: it is a measurable factor affecting your tenants' health, your liability as a seller and, indirectly, your building's value. The good news: testing is simple and inexpensive, and mitigation, when needed, is well understood. Here is what every multi-unit owner should know before selling.

What is radon — and why does it concern your plex?

Radon is a naturally occurring radioactive gas produced by the breakdown of uranium in the soil. Invisible and odourless, it seeps in through foundations and accumulates in basements and ground floors. According to Health Canada, it causes about 16% of lung cancers in Canada — over 3,000 deaths a year — and is the second leading cause of the disease after smoking.

Radon comes from uranium naturally present in the Earth's crust. As it decays, it releases a gas that migrates to the surface and enters buildings through foundation cracks, drains, slab joints and crawl spaces. In a plex, it is therefore the occupied basement and the lower units that are most exposed — often units rented to families.

The danger is real and well documented. According to Health Canada, radon exposure is linked to about 16% of lung cancers in Canada, or more than 3,000 deaths per year. The Government of Québec adds that radon is also the leading cause of lung cancer among non-smokers and is responsible for more than 1,000 deaths a year in the province.

Sources: Health Canada — Radon gas causes lung cancer; Government of Québec — Residential radon.

What is Health Canada's radon intervention guideline?

The Canadian guideline is 200 becquerels per cubic metre (Bq/m³). Above it, Health Canada recommends lowering the level within one year. In Québec, mitigation work must be done within the year following a result above 200 Bq/m³. No level of radon is deemed completely risk-free.

The Canadian radon guideline sets an intervention threshold of 200 Bq/m³. In practical terms, if your building's test shows a concentration above this level, Health Canada recommends taking corrective action within one year. The higher the concentration, the sooner you should act.

The Government of Québec follows the same guideline: when concentrations exceed 200 Bq/m³, mitigation work must be carried out within the year following the result. One key point: Health Canada stresses that no level of radon is considered completely risk-free — the goal is to reduce exposure as low as reasonably achievable, even below the threshold.

Sources: Health Canada — Government of Canada radon guideline; Government of Québec — Residential radon.

How do you test for radon in your income property?

Place a passive detector in the basement about one metre from the floor (or on the ground floor if the basement is unoccupied). Health Canada recommends a test of at least 91 days, ideally during the heating season. In Québec, measure at least one month, ideally three months between October and April — when radon is most concentrated.

Good news for owners: measuring radon is simple and inexpensive. You use a small passive detector left in place for several weeks. Health Canada recommends placing the device in the basement about one metre from the floor, or on the ground floor if the basement is unoccupied — which, in a plex, often corresponds to the lower unit.

Duration and season: why winter?

Duration matters. Health Canada recommends a test of at least 91 days for a valid estimate of the annual average, ensuring the measurement covers the heating season. The Government of Québec specifies measuring at least one month, ideally three months in winter (October to April).

Why winter? Because radon concentrations are generally higher in the cold season: windows stay closed, ventilation drops and the stack effect draws more soil gas indoors. A summer test reading below 200 Bq/m³ should in fact be repeated in winter to be reliable.

Sources: Health Canada — Testing your home for radon; Government of Québec — Residential radon.

How to reduce radon: active soil depressurization

The standard method is active soil depressurization (ASD): a pipe runs through the concrete slab and a fan draws radon-laden gas from under the building and vents it outside. According to Health Canada, ASD can reduce radon by roughly 90%. The system should be installed by a certified professional.

If your plex exceeds the threshold, mitigation is effective and proven. The most common technique is active soil depressurization (ASD). The principle is simple: a pipe is installed through the concrete slab and connected to a fan that continuously draws radon-laden soil gas from under the building and vents it outside, above the roof.

The effectiveness is remarkable: according to Health Canada, active soil depressurization can reduce radon concentrations by roughly 90%. For a multi-unit owner, it is a targeted intervention that durably resolves the problem without transforming the building. It should, however, be entrusted to a certified radon mitigation professional to ensure a properly sized system and a compliant follow-up result.

Source: Health Canada — Radon Reduction Guide for Canadians.

Do you have to disclose radon when selling your plex?

In Québec, the seller must disclose known factors affecting the value or use of the building. A high test result, or completed mitigation work, should be declared in writing. An undisclosed latent defect can expose you to liability after the sale. Conversely, a tested and corrected building, with documentation, is a reassuring selling point.

This is where the topic becomes strategic for a selling owner. In Québec, the seller has a good-faith obligation to disclose known factors that may influence the buyer's decision, the value or the use of the property. A high radon test result, or conversely mitigation work already completed, are elements best recorded in writing in the seller's declaration.

The risk of omitting is concrete: a known and undisclosed problem can be treated as a latent defect and expose you to liability even after closing. For an income property, where the buyer almost always conducts an inspection, hiding a problem is a poor strategy — it often surfaces at the worst moment of the negotiation.

The better strategy is the opposite: test early, correct if needed, document. A plex accompanied by a compliant test result (or a mitigation system installed with a follow-up test) is a reassuring building for the buyer — a differentiator, not a burden. For your specific situation, consult a notary, who will properly frame the seller's declaration.

Radon: the winning strategy for a multi-unit selling owner

A high radon test is not a reason to undersell your income property. It is a known, measurable and correctable problem — three qualities that, in real estate, are worth far more than uncertainty. Here is how to approach it if you are considering selling your plex on the North Shore:

• Test in winter, before going to market: you know your situation before the buyer and keep control of the timeline.
• Correct if needed: active soil depressurization installed by a certified professional durably resolves the problem, with a follow-up test to back it up.
• Document and disclose: a complete file turns a potential objection into a point of trust.
• Sell directly: ImmoMulti buys multi-unit properties across the North Shore — no broker, no commission, offer within 48 hours. A tested and corrected building trades more smoothly.

To prepare your building for a transaction under the best conditions, see also our guide to the pre-purchase inspection of an income property and our article on latent defects in an income property in Québec. If you would rather explore a direct sale, learn how to sell your income property fast on the North Shore.