Radon is the leading cause of lung cancer in nonsmoking Canadians.


Radon is found in public buildings, daycares, schools, hospitals, and new and older homes. Levels are highest in indoor air and underground.


Radon gas enters buildings when the air pressure inside is lower than in the soil surrounding the foundation. This difference in pressure draws air and other gases, including radon, into buildings through cracks and openings in foundations, construction joints, gaps around pipes, sump pumps and drains, windows, or cavities inside walls. Radon can also be found in groundwater from private or small community wells. When this water is agitated through showering, clothes washing, and cooking, radon may be released into the home.

Environmental radon concentrations vary depending on a number of factors including building characteristics (e.g. ventilation and sealing), occupant lifestyle (e.g. using windows or fireplaces), and local geology and soil characteristics. The highest concentrations are found in areas with uranium and thorium ore deposits and granite formations (which have naturally high concentrations of uranium).  Radon levels in buildings also vary across seasons, and can change significantly in 24 hours (by a factor of two or three). The highest levels usually occur in winter because windows and doors are kept closed, sealing buildings and therefore decreasing ventilation. When buildings are sealed to conserve energy, higher levels of radon can accumulate as well.

Most Canadian homes contain some level of radon gas. In a cross-country survey of radon concentrations in homes conducted by Health Canada from 2009 to 2011, 6.9% of homes tested had indoor radon concentrations above the current Canadian guideline of 200 Bq/m3. [Source: the CAREX Canada Radon Profile.]

Learn more about this known carcinogen on the CAREX Canada website.

Radon Limits

Health agencies around the world have set guidelines for radon levels indoor. If radon levels exceed the guidelines, mitigation to reduce radon levels to as low as possible is the next step.

  • Health Canada’s radon guideline is 200 Bq/m3 (5.4 pCi/L).

  • The US EPA radon action level is 150 Bq/m3 (4 pCi/L).

  • WHO recommends action above 100 Bq/m3 (2.7 pCi/L).


Radon is a radioactive gas that enters buildings through the ground, air and water. Without dedicated exits, it concentrates.

Design factor

Building design will inhibit or permit radon entry.

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