Short answer
Use a long-term radon detector for at least 91 days, ideally between October and April. Place it on the lowest level where someone spends more than four hours a day, at breathing height, away from exterior walls, windows, drafts, heat sources and the sump. Then send it to the lab. A do-it-yourself kit works; a C-NRPP certified professional adds correct placement, a verified report, advice, and an early reading in as little as 2 weeks.
Key facts
- Test length: at least 91 days (3 months). Longer is fine.
- Best season: October to April, when windows are closed.
- Location: lowest lived-in level, at breathing height.
- Short tests (48 hours to a week) and digital monitors are screening tools. A single one can falsely reassure or falsely alarm.
- A professional early reading at 2 weeks can tell you sooner whether the answer is likely high.
Long-term vs. short-term tests
Radon levels rise and fall with the weather, the season and the way a house is used. A reading taken over two days can be several times higher or lower than the home’s yearly average. That’s why Health Canada bases its guideline on long-term measurements of at least three months.
| Test type | Duration | Best used for |
|---|---|---|
| Long-term alpha-track detector | 91 days to 12 months | Deciding whether to mitigate; post-mitigation confirmation; Tarion claims |
| Electret ion chamber (professional) | Short or long term | Professional measurements, including real estate screening |
| Professional continuous monitor | 2 days to several weeks | An early reading alongside a long-term test; real estate screening; checking a system is working |
| Consumer digital monitor | Continuous | Watching trends and screening |
| Short-term kit (48 hours to 7 days) | 2–7 days | Quick screening only. Not a basis for deciding to mitigate or not |
Why a 48-hour test kit can mislead you
Many inexpensive kits on the shelf run for 2 to 7 days. They aren’t useless, but they’re easy to over-trust. C-NRPP counts a test as short-term when it runs from 48 hours up to three months, says every short-term result should be followed by a long-term one, and says the decision to mitigate should rest on a long-term test.
The reason is the one in the first paragraph above: radon moves with the weather, the season and how the house is used, and a few days can land on either side of the yearly average. That cuts both ways:
- A low result can falsely reassure. A mild week, open windows or a summer test can show a number well below the year’s average, and the home looks fine when it isn’t.
- A high result can falsely alarm. A cold snap or a storm can push a few days above 200 in a home whose long-term average is below it.
- Results near 200 are the hardest to read. One review of 20 studies found one-week tests were reliable when levels were low and right only about half the time at moderate levels.
Short tests do have a place: real estate deals with a closing deadline, a quick check after a system is installed, and a first rapid indication. Treat the result as a screen, and use it to decide how soon to start a long-term test, not whether to skip one. What to do with a short-term result.
Where to place the detector
Health Canada’s protocol puts the detector on the lowest level of the home that is occupied for more than four hours a day. If you have a finished basement you use, that’s where it goes. If the basement is unfinished storage and nobody spends time there, test the main floor.
- Breathing height: roughly 0.8 to 2 metres off the floor.
- Away from exterior walls, windows and doors.
- Away from drafts, heat sources, direct sunlight, sumps and floor drains.
- Somewhere it won’t be bumped, moved or covered for three months.
- Not in kitchens, bathrooms or laundry rooms, where humidity and exhaust fans skew readings.
Large homes, split levels, additions on separate foundations and homes with a basement apartment may need more than one detector.
Radon swings hour to hour and season to season. Three months or more, through the heating season, gives a true annual average. An early reading at day 14 tells you sooner if it’s clearly high.
Can I get a result sooner?
Yes, an early one. A professional test can pair the long-term detector with a calibrated continuous monitor for the first two weeks. If that early reading is far above 200, your 91-day result is very likely high too, and you can start planning a fix. If it’s near 200, you wait for day 91. Either way the long-term test runs to the end. How the 2-week early reading works.
When to test
Any time is better than never, but the most representative results come from tests that run through the heating season, between October and April. Run the test under normal living conditions: there’s no need to keep windows shut beyond what you normally would.
DIY or professional?
A do-it-yourself long-term kit is inexpensive and reliable when used correctly. Choose one from a C-NRPP listed supplier, and follow the placement instructions closely. A professional test makes sense when:
- You need a report another party will rely on: a buyer, a seller, a landlord, a tenant or a Tarion warranty claim.
- Your home is complex: several foundations, a rental suite, a crawlspace, or a previous mitigation system.
- You want someone to explain the result and what to do next, with no conflict of interest in the advice.
- You’d like an early reading at two weeks rather than waiting three months for a first answer.
Some libraries in Ontario, including Oxford County Library, lend digital radon monitors free for a few weeks. They’re a great way to get familiar with radon in your home, but Health Canada recommends confirming any decision with a long-term test.
After a high result
If your long-term average is above 200 Bq/m³, the next step is a mitigation assessment by a C-NRPP certified mitigation professional. If it’s between 100 and 200, you’re below the guideline; consider re-testing in a few years or after renovations, or reduce it if you’d like to. How to read your results.
Common questions
Are 48-hour radon tests reliable?
They’re a screen, not a verdict. Radon levels swing with weather and how the house is used, so a few days can sit well above or below the yearly average. C-NRPP says every short-term result should be followed by a long-term test, and that the decision to mitigate should rest on the long-term result. A short test is most useful when it’s far from 200 Bq/m³ and taken in winter.
How long should a radon test be?
At least 91 days (three months). Health Canada’s guideline is based on long-term averages, and tests of a few days can differ greatly from the yearly level.
Where can I buy a radon test kit in Ontario?
Long-term kits are sold online and through some hardware stores, health units and lung health organizations. Look for a kit from a supplier listed with C-NRPP so the lab analysis meets Canadian standards.
Can I get radon results faster than 91 days?
A professional test can give you an early reading in as little as 2 weeks from a calibrated continuous monitor. It’s reliable when the reading is far above or below 200 Bq/m³, less so near it. The 91-day result is still the one to decide on.
Should I test in summer?
You can, but summer readings are often lower than the yearly average. If you test in summer, consider re-testing during the heating season.
Do I need to test every floor?
No. Test the lowest level where someone spends more than four hours a day. Add detectors for separate foundations, additions or rental units.
Sources
- Health Canada — Guide for Radon Measurements in Residential Dwellings (Homes)
- Health Canada — Government of Canada radon guideline
- Canadian National Radon Proficiency Program (C-NRPP)
- C-NRPP — Statement on Short Term Testing (2017)
- C-NRPP — Protocol for Conducting Short-Term Radon Measurements (2018)
- Mphaga et al., Sensors (2024) — Short-term vs. long-term: a critical review of indoor radon measurement
- Oxford Today — Oxford County Library offers radon monitors for loan
- Take Action on Radon