What Is Radon? A Complete Guide for Ontario Homeowners
This cornerstone guide was first published in April 2026 and substantially refreshed on September 22, 2026. We updated the health stats, corrected the fan lifespan, aligned pricing, and added the Lungs Matter grant and Tarion warranty paths.
Chances are you’ve heard the word “radon” somewhere. Maybe in a home inspection report. Maybe in a news story about lung cancer. Maybe from a friend who just bought a house and was told to test.
Here’s what most Ontario homeowners don’t know. Radon is the number one cause of lung cancer for people who’ve never smoked in Canada.
It kills more than 3,000 Canadians every year.
This guide tells you what radon is, why it matters, how it gets into your house, and what to do about it. No fluff. No sales pitch. Just the facts in plain English, each one backed by a source you can check.
The short version:
- Radon is a natural radioactive gas that seeps up from the soil under your home.
- You can’t see it, smell it, or taste it.
- About 1 in 8 Ontario homes tests above the Canadian guideline, and up to 1 in 5 in some regions.
- It causes about 16% of all lung cancer deaths in Canada.
- The only way to know your level is to test.
- If your level is high, it’s fixable for a few thousand dollars.
Want the longer version? Let’s go.
What is radon, in plain English?
Radon (its chemical symbol is Rn) is a radioactive gas. It forms in the ground as uranium in soil and rock breaks down over millions of years.
Uranium is everywhere. Not in scary amounts, but in trace levels across most Canadian soil. It’s higher in the granite-rich Canadian Shield.
As uranium decays, it turns into radium. Radium decays into radon. And because radon is a gas, it doesn’t stay put. It rises.
Radon has no color. No smell. No taste. You could live in a home with high levels for decades and never know.
Here’s the part that matters for your health. When radon breaks down further, it releases tiny radioactive bits. You breathe them in, and they stick to your lung tissue.
Year after year, they can damage the DNA in your lung cells.
It’s not like carbon monoxide. You won’t collapse from radon. It’s slow, quiet damage that builds over 10, 20, or 30 years.
Why Ontario homeowners should care
Let’s get specific. Here’s what the data shows.
- Radon causes about 16% of all lung cancer deaths in Canada.
- CAREX Canada lists radon as a Group 1 known human carcinogen, the same top-risk group as asbestos and tobacco smoke.
- One peer-reviewed study estimated radon was behind roughly 13% of Ontario lung cancer deaths, close to 850 deaths a year in the province.
- Children are more sensitive than adults because their cells divide faster.
For context, radon kills far more Canadians each year than house fires.
Your risk scales with three things:
- How much radon is in your home, since a higher level means higher risk.
- How long you breathe it in, since longer exposure means higher risk.
- Whether you smoke, since radon and smoking multiply each other’s risk instead of just adding to it.
Even at levels below the guideline, decades of exposure add up. That’s why Health Canada says every home should be tested.
How radon gets into your home
Here’s the mechanical reality. Your home sits at slightly lower pressure than the soil under it. That happens because:
- Warm air rises inside the house. This is the “stack effect.”
- Exhaust fans, dryers, and HVAC systems pull air out.
- Wind creates pressure changes around the house.
That lower pressure means your home is gently pulling air up out of the soil. If there’s radon in that soil, and in most of Ontario there is, your home draws it inward through any gap it can find.
Common entry points:
- Cracks in the concrete basement slab
- Gaps around water, gas, and sewer lines
- Sump pits, especially uncovered ones
- Crawl spaces with dirt floors
- The cold joint where the slab meets the foundation wall
- Floor drains
Once radon is inside, it pools in enclosed lower levels, mostly basements. If your basement is finished and you spend time there, your exposure is higher than someone who never goes down.
Older homes with cracked foundations have more entry points. But newer homes are often more airtight, so they trap radon once it’s inside. No home type is immune.
For a deeper look at the science, see our companion post on what causes radon gas in homes.
Who’s at risk in Ontario?
Short answer: every Ontario home could have high radon, but some areas run higher than others.
Here’s what the surveys show:
- Greater Toronto Area: lower on average, but far from zero. Individual homes still test high.
- Ottawa and the Ottawa Valley: among the highest big-city levels in Canada.
- Kingston, Thunder Bay, and parts of eastern and northern Ontario: high average levels, with large shares of homes above the guideline.
So some regions look calmer on average. But averages hide the tail. A single home in Oakville, Oshawa, or Toronto can test far above the guideline even when the house next door tests low. Soil, foundation, and HVAC all vary house to house.
The only question that matters: is your home high? You find out by testing. Our radon map of Ontario breaks down the regional numbers if you want to see where your area lands.
What counts as “high”?
Radon is measured in Becquerels per cubic metre, or Bq/m3. Canada’s guideline is 200 Bq/m3.
Below 100 Bq/m3. You’re in good shape. Re-test every 5 years, or sooner if you renovate or change your HVAC.
100 to 200 Bq/m3. Below the guideline, but not zero risk. Some families choose to fix it anyway, especially with young kids or heavy basement use. Your call.
Above 200 Bq/m3. Health Canada says fix it within two years, or within one year if the level is very high.
How to test: DIY kit vs professional
There are two real ways for Ontario homeowners to test. Both work. The right one depends on your budget and your timeline.
| DIY test kit | Professional test | |
|---|---|---|
| Cost | About $35 | $99 to $399 |
| How long | 91+ days, then mail to a lab | 3 to 7 days, or 48 to 72 hours for a home sale |
| Accuracy | Good over a long window | High, with expert placement |
| Report | Basic lab number | Full report with next steps |
| Best for | Patient homeowners on a budget | Home sales, or a fast, clear answer |
You can buy a kit at Canadian Tire, Home Depot, or through Take Action on Radon.
Our honest recommendation:
If money is tight, do a DIY long-term kit. It beats not testing, every time.
If you want a fast, accurate answer, book a pro test. That’s the better path if you’re buying or selling, or if the result decides something real, like whether your kids sleep in the basement.
Either way, don’t skip testing. It’s the only way to know. Our residential test is $99 flat, and a faster real estate test is $399.
How radon mitigation works
A radon mitigation system is more boring than it sounds. It’s:
- A plastic pipe that runs from under your basement slab up through the roof
- A quiet electric fan mounted outside or in the attic
- Some sealant to close the obvious gaps
The fan creates suction under the slab. Instead of radon being pulled up into your home, it’s pulled out through the pipe and vented above the roofline.
This method is called sub-slab depressurization. It’s what Health Canada and the EPA both recommend.
Typical cost in Ontario runs $2,800 to $3,800, based on how complex your home is. Most installs take one day.
The system runs all the time on about $5 to $10 of electricity a month. The fan usually lasts 10 to 15 years, then gets swapped for a couple hundred dollars in parts.
That’s it. No magic. Just a pipe and a fan.
Want the deeper mechanics? See our post on sub-slab depressurization.
Help paying for it
If your test comes back high, you may not be on your own for the cost.
Tarion warranty. If your home was built in Ontario after 2021, radon mitigation may be covered by your new home warranty, up to a large builder-funded cap.
Lungs Matter grant. The Canadian Lung Association runs a program that can put money toward mitigation for eligible homeowners.
Take Action on Radon. Some regions run free or low-cost test kit programs through this national campaign.
What to do next
Three steps, in order.
1. If you haven’t tested, test. Pick a method. Pick a date. Do it. A $35 kit or a $99 pro test, the only wrong answer is not testing.
2. If you have tested, re-test on schedule. If your last test was more than 5 years ago, it’s stale. Levels shift with seasons, renovations, and soil.
3. If your test came back high, fix it. It’s fixable and not outrageous next to the risk. Every month you wait is another month of exposure.
If you’re in the Greater Toronto Area or surrounding Southern Ontario and want us to handle it, our prices are public at /services. Get a quote in about 60 seconds, book a call if you’d rather talk, or just read more and come back later. No pressure, no sales chase unless you ask.
Your lungs are playing the long game. Make sure you are too.
Frequently asked questions
Is radon really that common in Ontario?
Yes. About 1 in 8 Ontario homes tests above the Canadian guideline, and it’s up to 1 in 5 in some regions.
Can I smell or see radon?
No. Radon has no color, no smell, and no taste.
How much does it cost to fix high radon?
In Ontario, a mitigation system usually runs $2,800 to $3,800. If your home was built after 2021, a Tarion warranty claim may cover it.
Is a single high test an emergency?
No. Radon harms you slowly, over years. Health Canada says to fix a level above the guideline within two years, or within one year if it’s very high.
What’s the fastest way to find out my level?
A professional test. A technician drops off a calibrated monitor, it runs for a few days, and you get a full report. Our residential test is $99 flat, and a faster test for a home sale is $399.
Authored by the Breathe Radon Free team.
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