Long-Term Radon Monitoring in Ontario: Why 91 Days Beats 3 Days
A 3-day radon test taken in July can read half of what the same house shows in January. Same house. Same basement. Same gas.
That gap is why Health Canada’s measurement guide calls for 91 days or more before you make a big decision about radon.
This guide covers what long-term monitoring is, what it costs in Ontario, who genuinely needs it (and who can skip it), and how to do it right.
The short version:
- Long-term = 91+ days of continuous measurement, ideally spanning a heating season
- It’s Health Canada’s recommended basis for the mitigate-or-not decision
- Radon swings 2 to 3x between seasons; short tests can miss the real average
- DIY long-term kits run $40 to $80. Our monitored 91-day service is $299.
- You need it most if a short test landed between 100 and 200 Bq/m³, or before a Tarion claim
- You can skip straight to mitigation if a short test came back very high (over 600)
What counts as long-term monitoring
Any radon measurement that runs 91 days or longer counts as long-term under Health Canada’s protocol. Most run 3 to 12 months.
Two ways to do it:
- Alpha-track detector. A small puck that sits in your basement and records radon damage on a film inside. After 91+ days you mail it to a lab. One number comes back: your true average for the period.
- Continuous radon monitor (CRM). An electronic device that logs readings hour by hour for the whole window. You see the average AND the swings: winter spikes, ventilation dips, what happens when the furnace kicks in.
The alpha-track answers “what’s my average?” The CRM answers “what’s my average, and why?”
Why short tests get it wrong (sometimes)
Radon isn’t steady. It breathes with your house.
Cold weather seals your windows and strengthens the stack effect, the pressure difference that pulls soil gas up through your foundation. The ground freezes and pushes more gas toward your basement instead of letting it vent through soil. The result: winter readings often run 2 to 3 times higher than summer ones.
A 3-day snapshot inside that swing can mislead in both directions:
| Short test says | Reality might be |
|---|---|
| 150 Bq/m³ in July | 300+ averaged over the year |
| 250 Bq/m³ during a cold snap | 180 averaged over the year |
That’s why the decision rule matters: Health Canada’s 200 Bq/m³ action guideline is meant to be compared against a long-term average, not a one-week reading.
The honest framing: short-term tests are screening tools. Long-term monitoring is the diagnosis. Both have a job. Most homeowners should start short and confirm long.
Who actually needs long-term monitoring
Not everyone. Here’s the honest split.
You need it if:
- A short-term test landed in the 100 to 200 Bq/m³ grey zone. The annual average decides whether you mitigate, and only a long test gives you that.
- You’re filing a Tarion warranty claim. The claim trigger is a C-NRPP certified result at or above 200 Bq/m³, and a long-term result is the strongest version of that evidence.
- You mitigated and want proof the system holds through a heating season, not just the week after install.
- You live in a higher-radon region (Guelph, Kitchener, Cambridge, Hamilton’s escarpment neighbourhoods) and want the real annual number before finishing a basement.
You can skip it if:
- A short test came back very high (600+). The seasonal swing won’t save you. Move to mitigation; verify after.
- A short test came back very low (under 50) in winter. Winter is the worst case. If your worst case is clean, your average is cleaner.
DIY vs professional: the real cost breakdown
| Option | Cost | What you get | Best for |
|---|---|---|---|
| DIY alpha-track kit | $40 to $80 | One mailed-in average after 91+ days | Anyone, anywhere in Ontario, on a budget |
| Our monitored 91-day service | $299 | Calibrated CRM, hour-by-hour data, written report, placement done right, we handle everything | Grey-zone confirmations, Tarion claims, post-mitigation proof |
The DIY route is genuinely fine for most homes. Buy the kit, put it in the lowest lived-in level (50 cm off the floor, away from walls, drafts, and heat sources), leave it through a heating season, mail it in.
The $299 service earns its keep when the result has to hold up. A Tarion claim, a real estate negotiation, a mitigation verification. You get the hour-by-hour chart, a placement that follows protocol, and a report you can hand to a builder, an agent, or an inspector. We serve the Greater Toronto Area plus surrounding Southern Ontario.
In the grey zone, or starting a Tarion claim? Book a 91-day monitored test or call (416) 605-3093. We place the monitor, you get the defensible number.
How to run a long-term test right
Five rules, whichever route you pick:
- Lowest lived-in level. Basement if anyone sleeps, works, or plays there. Main floor if not.
- 91 days minimum, heating season included. October to January beats April to July. A full year is even better.
- Placement matters. 50 cm off the floor, a metre from exterior walls, away from drafts, sun, and humidity sources like bathrooms.
- Live normally. Don’t seal the house up or air it out more than usual. The point is your real average.
- Write down the dates. Lab results without start and end dates are hard to defend later.
What your result means
- Under 100 Bq/m³: good news. Re-test every 5 years or after major renovations. Our radon levels guide covers the full scale.
- 100 to 200: below the guideline. With kids in a basement bedroom or a smoker in the house, some families mitigate anyway. Risk scales with exposure; there’s no magic safe line.
- 200 or higher: Health Canada recommends mitigation within 2 years.
The fix is a one-day install that cuts levels by 80 to 99 percent. Costs and process in our mitigation cost guide. New-build owners should check the Tarion path first, because the builder may pay for all of it.
Frequently asked questions
Is a long-term test more accurate than a short-term test?
For the annual average, yes. A 91+ day window absorbs the seasonal swings that make short tests noisy. Health Canada bases its action guideline on the long-term average, so the long test answers the question the guideline actually asks.
Can I just buy a consumer radon monitor instead?
A $200 to $260 consumer monitor (we use and like the Airthings Corentium line) gives you ongoing readings and is a great way to watch trends. For decisions with paperwork attached, like Tarion claims, you’ll still want a protocol-compliant test. Our radon detector guide compares the options.
When’s the best time to start a long-term test in Ontario?
Early fall. An October start captures the full heating season by January, which is the highest-radon stretch of the year. A spring start works too; it just takes longer to include the months that matter most.
Do you offer long-term monitoring outside the GTA?
Our monitored $299 service covers the Greater Toronto Area plus surrounding Southern Ontario, including Guelph, Kitchener, Cambridge, and Hamilton. Outside that area, a DIY alpha-track kit from a certified lab gives you the same protocol-compliant average for $40 to $80, and we’re happy to point you to reputable options.
My short-term test was 450. Should I run a long-term test before mitigating?
No. At that level the seasonal swing won’t change the answer. Mitigate, then use a follow-up test to verify the system works. Save the 91 days for proof, not for delay.
Radon decisions are annual-average decisions. Get the number that actually represents your home, and the rest of the path (mitigate, claim, or relax) sorts itself out.
Book a $299 long-term monitored test
Start with a $99 short-term screen instead
Authored by the Breathe Radon Free team. Every claim in this guide is backed by a primary source linked above. Full bibliography at /sources.
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