What Causes Radon Gas in Homes? The Full Picture
You can stand in your kitchen and never know it’s happening. Radon gas is leaking up from the soil under your slab. It slips through hairline cracks. It pools in the basement.
About 1 in 6 Ontario homes test above the Health Canada action guideline of 200 Bq/m³.
This post walks through where radon actually comes from, why it ends up in homes, and which houses get more of it. Plain English. Real numbers.
The short version:
- Radon forms when uranium in soil and rock breaks down over millions of years
- It rises out of the ground as a gas, then gets pulled into your home by negative pressure
- Common entry points are slab cracks, sump pits, gaps around pipes, and well water in some homes
- Winter levels run higher because of the stack effect (warm air rises, cold air gets sucked in from below)
- Newer airtight homes can trap more radon, not less
- The only way to know your home’s level is to test
Where radon comes from
Radon is a noble gas. That means it doesn’t bond with anything else, so once it forms, it drifts.
It starts as uranium. Uranium sits in trace amounts in almost every Canadian rock and soil sample.
Uranium decays slowly. Over millions of years, it changes into radium. Radium then decays into radon. Each step shoots out a tiny bit of radiation.
Here’s the chain in plain English:
- Uranium-238 sits in the rock for billions of years
- It breaks down into radium-226
- Radium decays into radon-222
- Radon decays into a string of solid radioactive particles called “radon daughters”
The radon daughters are the part that hurts you. They stick to dust and the inside of your lungs. The damage builds up over years.
That’s the chain. Same in Ontario, same in Alberta, same in Norway. The only thing that changes is how much uranium is in the ground.
Why bedrock and soil composition matter
Some rocks hold more uranium than others. Granite, shale, and certain sedimentary rocks carry more. Limestone and sandstone carry less.
Soil is the next layer of the story. Loose, dry, gravelly soil lets radon flow freely up to the surface. Wet, packed clay slows it down.
So two homes a block apart can sit on top of very different gas paths. One basement reads 50 Bq/m³, the next reads 600, even with the same builder and the same year of construction.
That’s why “my neighbour tested low” is not a safe answer. It’s just a number from one house.
Why Ontario homes get more than their share
Ontario sits on a mix of two rock systems. The Canadian Shield covers the north and east. Glacial deposits and limestone cover the south.
Both have radon. The Shield sits on granite, which carries more uranium. Glacial till in the south got dumped here by ice sheets that scraped up uranium-rich rock from far away and dropped it on top of the bedrock.
Real numbers from Health Canada’s nationwide survey show Ontario averages around 92 Bq/m³ across tested homes.
Some pockets run much higher than the provincial average:
- Wellington County (Guelph area): average around 100 Bq/m³, with about 1 in 10 homes above guideline
- Ottawa Valley: Canada’s highest big-city average. Champlain Sea sediments plus Shield bedrock.
- Kingston: average above 130 Bq/m³, with 25 to 50 percent of homes above guideline
The Greater Toronto Area itself averages low, around 43 Bq/m³ city-wide. But individual homes in Oshawa, Oakville, Vaughan, or anywhere else can test far above that. See our city pages for local averages where you live, or the radon map of Ontario for the regional picture.
How radon actually gets inside
The science under the soil is one half. The other half is your house.
Your home is slightly negatively pressurized. That means the air pressure inside is a bit lower than the air pressure in the soil under your foundation. The house is constantly sucking air upward from the ground.
Anywhere the soil meets your living space is an entry point. The most common ones:
- Cracks in the basement slab. Even hairline cracks let gas through.
- Cold joints. The seam where the slab meets the foundation wall.
- Sump pits. Especially open ones with no airtight cover.
- Floor drains. A dry trap is an open pipe to the soil.
- Gaps around utility penetrations. Water lines, gas lines, sewer pipes, electrical conduits.
- Crawl spaces. Especially dirt-floor crawl spaces with no vapour barrier.
- Block walls. Hollow concrete blocks can act like a chimney for soil gas.
- Well water. In a small share of homes (usually rural ones on private wells), radon comes up dissolved in the water and gets released when you shower or run a tap.
Once radon is inside, it doesn’t escape on its own. It collects in the lowest, most enclosed parts of the house. That’s why basements are the focus.
The stack effect and why winter readings spike
You may have heard that radon is worse in winter. There’s a real reason for that.
It’s called the stack effect. Warm air rises inside your house. As it rises and leaks out through the upper floors, it pulls cold air in from below to replace it.
In summer, the temperature inside and outside is close. The stack effect is mild.
In winter, your furnace runs all the time. Indoor air is 20°C. Outdoor air is well below freezing. The pressure difference is huge.
That difference pulls more soil gas into your basement. Studies show winter readings can run 2 to 3 times higher than summer readings in the same home. That’s why Health Canada’s measurement guide pushes for long-term tests of 91 days or more.
If you tested in July and got a low number, your true winter exposure may be much higher.
Which home types are most affected
There’s no home that’s immune. But some patterns hold.
| Home type | Why it can run higher |
|---|---|
| Older homes with cracked foundations | More entry points, settling cracks, and block walls |
| Newer airtight builds | Less ventilation, so trapped gas stays in longer |
| Finished basements | More time spent below grade means more exposure |
| Homes with sump pumps | Open pits are a direct path for soil gas |
| Crawl spaces with dirt floors | No real barrier between the soil and the home |
| Homes on hilltops over fractured bedrock | Gas paths up through the rock are wider |
| Rural homes on private wells | Radon can come up dissolved in the water itself |
A common myth: only old homes have a radon problem. Not true. The Evict Radon study, run by Canadian researchers, found that homes built after 1980 actually averaged higher radon than older ones.
Another myth: only basement-level rooms matter. Mostly true, but not fully. Slab-on-grade ground floors and crawl-space-under-slab homes can read high on the main floor too.
DIY vs hire a pro
You can do a lot of the front end yourself. Some things need a trained set of eyes.
| Task | DIY is fine | Hire a pro |
|---|---|---|
| Buying a long-term test kit | Yes, $35 to $60 at Canadian Tire or direct from a lab | Only for a real estate timeline or a Tarion claim |
| Placing the kit correctly | Yes, with the Health Canada placement rules | If your home is unusual (multiple slabs, large footprint) |
| Reading the result | Yes, the lab gives you a clear number in Bq/m³ | If you’re between 100 and 200 Bq/m³ and unsure what to do |
| Sealing visible cracks | Yes for surface cracks, with the right caulk | If cracks are wide, structural, or inside a sump pit |
| Designing a full mitigation system | No | Yes, sub-slab depressurization needs proper sizing and venting |
| Replacing a worn-out fan on an existing system | Sometimes, if you’re handy | Usually yes, especially if the system is under warranty |
The honest line: if you’ve never tested your home, a $40 mail-in long-term kit is the right first move. Don’t pay for a pro test until you have a number. We’d rather you spend $40 with someone else than $99 with us if you don’t need us.
If your test comes back high, that’s when professional help pays off. We can confirm the number with a 3 to 7 day pro test for $99, then walk you through the next step.
What to do next
Three steps, in order.
1. Test your home. Long-term kit if you have time. Short-term pro test if you’re on a deadline.
2. Read the number. Under 100 Bq/m³, retest in five years. Between 100 and 200, consider a long-term retest. Above 200, plan to mitigate.
3. If you need to fix it, get a quote. Sub-slab depressurization works in 80 to 99 percent of homes. We’ve published our pricing on the services page.
Need more on the basics first? Start with What is radon? A complete guide for Ontario homeowners. It covers the health side in more depth.
If you have a quick question, our FAQ page has the most common ones answered.
Frequently asked questions
Where does radon gas in homes actually come from?
It comes from uranium in the soil and rock under your foundation. Uranium decays into radium, which decays into radon. The gas rises out of the ground and gets pulled into your home through cracks and gaps.
Is radon worse in older homes or newer homes?
Both can run high. Older homes have more cracks and entry points. Newer homes are tighter, so once radon gets in, it’s harder for it to escape. The Evict Radon study found post-1980 builds averaged slightly higher than older ones.
Why does radon spike in the winter?
Your furnace runs more. Indoor air rises and leaks out at the top of the house, which pulls cold air in from below. That extra suction pulls more soil gas into the basement. Winter readings can run 2 to 3 times higher than summer ones.
Can I tell I have a radon problem without testing?
No. Radon has no colour, no smell, and no taste. It causes no short-term symptoms. The Canadian Cancer Society lists it as a leading cause of lung cancer for non-smokers in Canada.
Does well water bring radon into homes?
In a small share of homes, yes. Rural homes on private wells in granite-heavy areas can get radon dissolved in the water. It releases into the air when you shower or run a tap. Most municipal water systems are not a meaningful source.
Will sealing my basement cracks fix my radon problem?
Sealing helps a little, but it almost never fixes a high reading on its own. The standard fix is a sub-slab depressurization system, which uses a fan and pipe to pull soil gas out before it can enter. Sealing on its own is a Band-Aid.
Authored by the Breathe Radon Free Team. Every claim is backed by a primary source. Full bibliography at /sources.
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