Introduction: 

While many homeowners in Ottawa are aware of common household dangers like mold, lead paint, or carbon monoxide, few realize that the very ground beneath their homes may be releasing a radioactive gas that poses a major health threat — radon.

Radon is a naturally occurring, colorless, odorless gas formed from the decay of uranium in soil, rock, and groundwater. It seeps into homes through cracks and openings in the foundation and, when trapped inside, can accumulate to hazardous levels. Ottawa has become a focal point in Canada’s radon discussion due to its unique geological composition, particularly the types of soil and bedrock that dominate the region.

This article explores the science behind radon gas, explains how Ottawa’s soil and subsurface geology significantly contribute to elevated radon levels, and outlines what homeowners need to do to protect themselves.

What Is Radon and Why Is It Dangerous?

The Science Behind Radon

Radon is a radioactive gas formed during the natural breakdown of uranium in soil and rock. It moves upward through the soil and enters buildings through:

• Foundation cracks
• Sump pump holes
• Crawl spaces
• Utility penetrations
• Gaps around basement windows

Once inside, radon becomes trapped, particularly in lower levels like basements, and can accumulate to levels dangerous to human health.

Health Effects of Radon Exposure

Radon is the second leading cause of lung cancer in Canada, according to Health Canada, accounting for about 3,000 deaths per year. Long-term exposure, especially at high concentrations, significantly increases cancer risk — even among non-smokers.

Ottawa: A High-Risk Radon Region

What Makes Ottawa Unique?

Ottawa’s elevated radon levels are due in large part to its:

• Uranium-rich bedrock, including granite and shale
• Glacial till deposits from the last Ice Age
• Soil permeability and moisture retention
• Cold winters that keep homes sealed, reducing ventilation

More than 20% of homes in Ottawa exceed Health Canada’s radon guideline of 200 Bq/m³ (becquerels per cubic metre). This is significantly higher than the national average.

But why is radon so common here? The answer lies in Ottawa’s soil and geology.

The Role of Soil in Radon Gas Formation

Soil Composition: The Primary Source of Radon

Uranium is found in small amounts in virtually all rocks and soils, but the type of rock and its breakdown products determine how much radon is released. When soil and rock formations with high uranium content begin to degrade, radon is produced as part of the radioactive decay chain.

In Ottawa, the predominant soil types include:

• Clay-rich glacial till
• Loamy glacial outwash
• Sandy deposits and organic silts
• Granite and shale bedrock outcrops

Each of these contributes to radon levels in unique ways.

Clay-Rich Glacial Till

What Is It?

Glacial till is an unsorted mix of clay, sand, silt, gravel, and rock debris deposited by glaciers. It forms much of Ottawa’s subsurface layers.

How It Contributes to Radon

• High uranium content from weathered granite and shale fragments.
• The fine-grained nature of clay allows for radon accumulation in microscopic pore spaces.
• Despite being dense, glacial till contains fractures and fissures that create pathways for radon migration.

Sandy Soils

Common Locations

Found in pockets along the Ottawa River and certain southern suburbs, sandy soils are porous and well-drained.

Why They Matter

• Sandy soils allow rapid movement of radon gas toward building foundations.
• Though less radioactive than till or shale, their high permeability increases the transport efficiency of radon from deeper layers.

Shale and Granite Bedrock

Geological Background

Ottawa lies near the edge of the Canadian Shield, an ancient geological formation rich in granite, shale, and other metamorphic rocks — many of which contain uranium.

How It Impacts Radon Levels

• Shale and granite naturally contain more uranium than sedimentary rocks.
• Over time, uranium decays to radon, which rises to the surface through cracks and fissures.
• When these rocks lie near the surface or under thin soil layers, radon emissions are significantly higher.

Soil Moisture and Seasonal Changes

Moisture content affects how radon travels through soil. In Ottawa:

• Frozen ground in winter acts like a lid, trapping radon in homes.
• Wet clay soils can restrict radon movement temporarily but force it sideways and up through foundation openings.
• Dry sandy soils allow easy radon flow into basements, especially during dry fall and winter months.

How Radon Enters Ottawa Homes

Un

derstanding soil types explains the source of radon, but how does it actually get indoors?

• Pressure differentials between the soil and the inside of a home (stack effect) draw radon in.
• Homes act like vacuums — warm air rising inside creates negative pressure at the base.
• Radon-rich gas from the soil is pulled through even the tiniest gaps.
• Sump pits, floor drains, and expansion joints are common radon entry points.

Ottawa Neighborhoods with Elevated Radon Levels

While radon can occur anywhere, certain Ottawa neighborhoods have recorded consistently higher levels, often due to the underlying soil and geology.

High-Risk Areas Include:

• Barrhaven – Newer homes, tight seals, and granite-based soil.
• Kanata – Proximity to Canadian Shield rock and shallow groundwater.
• Orleans – Clay-rich soil and rapid residential development.
• Greely and Manotick – Large properties on glacial till and sediment.
• Westboro and Old Ottawa South – Older homes with foundation wear and potential for soil gas infiltration.

These regions have seen homes with radon readings over 1,000 Bq/m³, five times the Health Canada guideline.

Real-World Data: Radon Testing in Ottawa

According to Health Canada and community testing programs:

• The average radon level in Ottawa homes is approximately 180 Bq/m³.
• Over 20% of tested homes exceed the 200 Bq/m³ threshold.
• In some communities, more than 40% of homes exceeded safe levels.
• Newer energy-efficient homes tend to have higher average levels due to less ventilation.

Soil, Construction, and Climate: The Triple Threat

Ottawa’s soil type, combined with modern construction practices and climate, amplifies radon risk.

Airtight Construction

New homes are built to be energy-efficient with sealed envelopes, which can trap radon gas inside. Less natural air exchange means more opportunity for radon to build up.

Long, Cold Winters

Ottawa’s climate leads to closed windows and sealed homes for months, preventing dilution of radon-laden air.

Basements

Most Ottawa homes have basements built into or on top of radon-emitting soil. If unsealed or unfinished, these spaces are ideal entry points.

How to Protect Yourself: Testing and Mitigation

Step 1: Test Your Home

You cannot know your radon level without testing. Options include:

• DIY Test Kits: Available for $40–$80. Long-term (91+ days) testing recommended.
• Professional Testing: Uses digital monitors; costs $150–$400. Suitable for short-term or time-sensitive needs.

Place detectors in the lowest lived-in level, away from vents and windows.

Step 2: Interpret Results

Radon Level (Bq/m³)Action Needed

0–100 No action needed

100–199 Monitor and retest

200–600 Mitigate within 2 years

600+ Mitigate within 12 months

Step 3: Install a Mitigation System

If levels are high, a sub-slab depressurization (SSD) system is the gold standard. It:

• Vents radon gas from beneath your foundation
• Can reduce radon by up to 99%
• Costs $2,000 to $3,500 on average in Ottawa

The Role of Government and Community

Health Canada

• Sets the 200 Bq/m³ guideline
• Offers resources and maps of high-risk areas
• Recommends long-term testing and mitigation

Take Action on Radon

• Offers free radon test kits
• Hosts webinars and outreach programs
• Encourages “100 Test Kit Challenge” in communities

Ottawa Public Health

• Provides guidance and educational materials
• Encourages testing in schools and municipal buildings

FAQs: Ottawa’s Soil and Radon

1. Is my home safe if my neighbor has low radon?

No. Radon levels vary greatly — even from house to house. Test your own home.

2. Do homes on sandy soil have lower radon?

Not always. Sandy soils allow easier gas flow, which may increase radon infiltration.

3. Does gardening or digging affect radon?

Not significantly, but major landscaping or foundation work can open new pathways for radon.

4. Can air purifiers remove radon?

No. HEPA or carbon filters do not remove radon gas. Only specialized mitigation systems work.

Conclusion: Know What Lies Beneath

Ottawa’s soil, shaped by ancient glaciers and rich in uranium-bearing rock, plays a direct role in its elevated radon levels. Combined with tightly sealed homes and harsh winters, this geological reality makes Ottawa radon testing​ a non-negotiable step for every homeowner in the capital region.

Radon is invisible, but it’s also detectable, preventable, and fixable. By understanding the connection between soil and radon, you’re one step closer to safeguarding your home and health.