A high radon test result — what to do

Q: A high radon test result — what to do

In Canada, Health Canada sets the radon action level at 200 Bq/m3: above it, your home should be fixed. Start by making sure the number is real — if it came from a short-term test, confirm it with a long-term (91-day-plus) test, because radon swings a lot day to day. Sealing obvious entry points like foundation cracks and the sump pit helps a little but rarely enough on its own. The proven fix for a confirmed high reading is a sub-slab depressurization system installed by a C-NRPP-certified professional, who re-tests afterward to prove it worked. The higher the reading, the sooner to act — radon is Canada's second-leading cause of lung cancer.

A high radon number is unsettling, but radon is a long-term risk you have time to address the right way — not an emergency to fix tonight. The path from here is clear and well established in Canada.

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In Canada, the proven fix for a confirmed reading above 200 Bq/m3 is a sub-slab depressurization system installed by a C-NRPP-certified professional. It draws radon from under the slab with a continuously running fan and vents it above the roofline, and the installer re-tests afterward to prove the level has dropped. Noticing that your home has no such system is an observation, not a repair — the design and installation are a certified pro's job.

Ranked from real-world failure data. The order updates as you select what you’re noticing above.

Likely causes, in the order to check them

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Beginner-friendly Doable, with care Safety-critical Call a pro

verifiedWhy this one goes to a pro

Sub-slab depressurization looks like a pipe and a fan, but sizing the suction, sealing the slab, routing the discharge safely above the roofline, and proving the result all take training. A poorly designed system can fail to lower radon — or worse, pull soil gas or combustion exhaust into the house. In Canada the standard is a C-NRPP-certified radon mitigation professional, who re-tests afterward to confirm the level is below 200 Bq/m3. Use the C-NRPP directory to find one; this is not a task to improvise.

gavel Check your province first AB: homeowner may do this · BC: homeowner may do this · ON: homeowner may do this · PE: homeowner may do this · QC: homeowner may do this
Radon mitigation is not a legally licensed trade in most Canadian provinces, so a homeowner may legally install a system; however, the recognized national standard is a C-NRPP-certified professional who re-tests to confirm the level drops below 200 Bq/m3. Confirm any local requirement (e.g. discharge/venting under provincial building code) before publishing.

Your notes travel with the request — what you saw, what you ruled out, when it started — so the pro arrives with a head start instead of a mystery.

visibilityHow to confirm the reading

warning A short test is a screen, not a verdictDo not commit to costly work — or dismiss the risk — on a two-to-seven-day short-term number alone. Radon can read very differently across a few days versus a full season, so a long-term test is what tells you your real exposure. If a short-term result was extremely high, though, do not wait months to react: contact a C-NRPP-certified professional now and let them advise on interim steps while a longer test runs.

Buy a long-term alpha-track radon kit (sold through Health Canada-recognized suppliers and the Canadian hardware chains). A long-term test running 91 days or more is what Health Canada recommends for a decision against the 200 Bq/m3 action level.
Place the detector in the lowest level you regularly use — a finished basement, or the ground floor if the basement is unoccupied — about 1 to 2 metres up, away from drafts, exterior walls, and heat.
Leave it undisturbed for the full period, then mail it to the lab as instructed. Compare the reported annual-average result to 200 Bq/m3: at or above it, plan mitigation; the higher the number, the sooner.

buildlong-term-radon-test-kit schedule15 min

handymanHow to fix it

As an interim measure, seal accessible foundation cracks, slab penetrations, and the gap around a sump pit with a suitable sealant, and fit a sealed, gasketed sump cover — then re-test. Treat this as a stopgap while you arrange a proper mitigation system, not the final fix.

Clean out visible cracks in the foundation floor and walls and the gaps where pipes pass through the slab; remove loose debris so sealant can bond.
Fill cracks and penetrations with a suitable flexible sealant (polyurethane or a purpose-made concrete/radon sealant), tooling it into the gap so it fully bridges the opening.
If there's a sump pit, fit a sealed, gasketed cover — a purpose-made airtight lid — leaving a grommeted port for the pump's power and discharge lines.
Re-test with a long-term kit afterward. If the reading is still at or above 200 Bq/m3 — which is common — arrange a sub-slab depressurization system with a C-NRPP-certified professional.

schedule90 min savings$25–80

Concrete/radon crack sealantA flexible polyurethane or purpose-made radon-rated sealant for concrete cracks and slab penetrations Sealed sump pit coverAn airtight, gasketed sump lid with grommeted ports for the pump cord and discharge pipe

gavel Check your province first AB: homeowner may do this · BC: homeowner may do this · ON: homeowner may do this · PE: homeowner may do this · QC: homeowner may do this
Homeowner sealing of cracks and sump covers is permitted across Canada; this is an interim measure only and does not replace certified mitigation.

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Why the list is in this order

We don’t sort by likelihood alone. We sort the way a good, honest tradesperson thinks: rule out the free, safe, common causes first. That’s why the cheap checks outrank the expensive repairs — and why anything dangerous gets a hard stop instead of a pep talk. The order exists to save you money, not to sell you anything.