How do I test for radon in my Toronto basement and what level requires mitigation?

Testing for radon in your Toronto basement is simple and inexpensive — you can buy a long-term radon test kit for $30–$50 at most GTA hardware stores, place it in the lowest lived-in level of your home for 3 to 12 months, and send it to a lab for analysis. Health Canada's action level is 200 becquerels per cubic metre (200 Bq/m³) — if your results exceed this, you should install a radon mitigation system before finishing your basement.

Radon is a naturally occurring radioactive gas that seeps up from uranium in the soil and enters basements through cracks in the foundation, gaps around pipes, floor drains, sump pits, and the porous concrete itself. It's colourless, odourless, and the second leading cause of lung cancer after smoking, according to Health Canada. Radon levels in the GTA vary significantly by neighbourhood and even by individual property — two houses on the same street can have dramatically different radon levels depending on soil composition, foundation condition, and air pressure dynamics. Parts of the GTA built on glacial till and clay deposits — common across Scarborough, Markham, Vaughan, and north Mississauga — can have elevated radon levels, though high readings have been found throughout the entire region.

Long-term testing (3–12 months) is the most accurate method because radon levels fluctuate significantly with weather, season, soil moisture, and home ventilation patterns. A long-term alpha track detector kit is the standard choice — you place the small device in the basement (at breathing height, away from windows, doors, and exterior walls) and leave it undisturbed for the testing period. After the exposure period, you seal the kit and mail it to the lab for analysis. Results arrive within 2–3 weeks. The fall and winter heating season is the ideal testing period because homes are closed up, air exchange with the outdoors is minimal, and radon levels tend to be at their highest — giving you a realistic picture of the exposure you'd experience in a finished, occupied basement.

If you need faster results — for example, if your renovation is already scheduled — short-term test kits provide preliminary readings in 48 to 96 hours. These are less accurate than long-term tests because they capture only a brief snapshot, but a short-term result above 200 Bq/m³ is a strong indicator that mitigation is warranted. Professional radon testing using continuous electronic monitors provides the most detailed data (hourly readings over several days) and costs $150–$400 for a certified radon measurement professional to conduct.

If your test results exceed 200 Bq/m³, Health Canada recommends reducing radon levels within two years. If levels exceed 600 Bq/m³, action within one year is recommended. The most effective mitigation method is sub-slab depressurization, where a contractor installs a small PVC pipe through the basement floor slab, connects it to a radon fan in the attic or exterior, and creates a slight vacuum beneath the slab that draws radon gas out before it enters the basement. The gas is vented above the roofline where it disperses harmlessly. This system typically reduces radon levels by 80–99% and costs $2,000–$4,000 installed by a certified radon mitigation professional (look for C-NRPP certification — Canadian National Radon Proficiency Program).

The ideal time to address radon is during your basement renovation, before the floor and walls are finished. If mitigation is needed, installing the sub-slab pipe and preparing the rough-in during construction is far less expensive and disruptive than retrofitting after the basement is complete. Your contractor can also seal the most common radon entry points — foundation cracks, the floor-wall joint, sump pit (with a sealed, airtight cover), and pipe penetrations — as part of the finishing process. Even if your initial test comes back below 200 Bq/m³, consider having your contractor install a passive radon rough-in (the pipe through the slab capped for future use) during construction for $300–$500. If radon levels increase in the future — which can happen as foundations age and soil conditions change — activating the system with a fan is a simple, inexpensive addition.