Radon infiltration in a building

Radon is a radioactive gas that comes from the natural breakdown of uranium present in soil and rock. It is an invisible, odourless and insipid gas.

When radon is released into the atmosphere, it gets diluted and poses no risk. However, in confined spaces, such as houses, it can sometimes reach very high concentrations that can pose a risk to the health of the occupants.

Radon is the second cause of lung cancer after smoking. It is also the leading cause of cancer in non-smokers. In Quebec, 10% to 16% of lung cancer deaths are associated with radon. This represents over 600 deaths per year. Radon is present in ALL buildings. Its levels vary depending on the region, the specific nature of the land, the construction of the building and can even vary over time in a given building based on its changing condition. Research carried out abroad has also shown that energy renovations (e.g. replacing windows and doors, adding insulation, increasing airtightness) can reduce air exchange and increase radon levels in indoor air.

The good news is that it is easy to measure and reduce the radon concentration in a building.

To find out the radon concentration in a building, it must be measured using a device called a dosimeter. It is recommended to take this measurement for a period of at least three months and to do it during the winter. The owner of an immovable can measure the radon concentration himself or hire the services of a professional.

Radon is measured in becquerels per cubic metre of air (Bq/m³).

Health Canada recommends corrective action when the average annual radon concentration exceeds 200 Bq/m³ in the occupied spaces in a dwelling.

Better results can be achieved by applying corrective measures, such as:

  • caulking foundation cracks
  • sealing openings in contact with the ground
  • ensuring that sumps are covered and ventilated to the outside
  • improving building ventilation, especially in the basement

If the radon concentration in a home is very high, these measures may not be sufficient, as they do not completely prevent radon infiltration. In this case, a qualified contractor should be called to install a system that will remove the radon present under the foundation before it enters the building’s living spaces.

Long and short tests

The main issue with radon in a real estate transaction is the time of year and the time required to perform the test. Health Canada recommends that radon levels be measured for three months during winter.

However, many real estate transactions are concluded in under three months, at any time of the year.

The broker must ask the seller questions to determine whether radon tests or work to correct a radon problem have been performed in the past, and disclose this information to any prospective buyer. By carrying out a test and, if necessary, mitigation measures before putting the property up for sale, the seller will benefit from an additional selling point.

If a buyer is concerned about radon, the broker may advise him to perform a radon test after the sale. Moreover, a mechanism is provided to make the Promise to Purchase conditional upon a sum being held in the trust account.

Standard clause 3.21 – Radon measurement test states the following:

The parties instruct the acting notary to withhold from the proceeds of the sale an amount of $________. This amount shall be remitted to the BUYER if the report produced by a certified radon measurement professional states, following a test to measure radon levels in the living space of the IMMOVABLE, that the average annual radon concentration is equal or exceeds 200 Bq/m3 and if a copy of the report is submitted to the SELLER within ___________ days following the signing of the deed of sale. Failing which, the amount shall be remitted to the SELLER, at the expiry of that time frame.

The amount mentioned above is intended to compensate the BUYER for the cost of work required to reduce the average radon levels in the living space of the IMMOVABLE. Upon receipt of this amount, the BUYER waives any claim and any other recourse in this regard.

All costs and fees relating to the management of the amount withheld by the notary shall be borne by the SELLER.

For more details: Standard clauses - Promise to Purchase - Radon Measurement Test


How to prevent radon from affecting your real estate transaction


Interactive map of the Quebec Lung Association on the presence of radon in Quebec

In November 2021, the Quebec Lung Association published on its website an interactive map on the presence of radon in Quebec, illustrating the data collected following the laboratory analysis of dosimeters sold online by the organization. By searching by postal code, users can view the results of tests conducted in different administrative regions of the province.

Although this awareness-raising tool is available to the public to illustrate the geographical distribution of the phenomenon, it does not allow anyone to decide on the potential presence of radon in a given building or specific area.

 It is impossible to determine the presence of radon in a building based on test results performed in another building, no matter how close they are, because of differences in land and construction. Furthermore, radon levels can vary over time in the same building based on its changing condition.

Therefore, brokers and agency executive officers must not extrapolate radon levels in a location from results available to them in such a way as to issue opinions similar to those of professionals in the field or otherwise mislead parties to a transaction.

Licensees have an ethical obligation to objectively advise and inform all parties of such an adverse factor without exaggeration, concealment or misrepresentation. Read this article to find out more about broker’s radon obligations.

Also remember that previous expert results, such as dosimeter analyses, should be made available to the buyer by answering the specific questions to that effect (clauses D12.1, D12.2 and D13.9) in the Declarations by the seller of the immovable form. For more details, read this article.



Gouvernment of Québec: Residential Radon

Health Canada: Radon: About

Association pulmonaire du Québec: Radon


Last updated on: October 31, 2023
Reference number: 208812