Radon Testing in Ontario Homes

Radon is a naturally occurring radioactive gas that seeps into homes from the ground. It is colourless, odourless, and tasteless. You cannot detect it without a test. Yet radon is the second leading cause of lung cancer in Canada after smoking, responsible for an estimated 3,200 lung cancer deaths per year nationwide. In Ontario, radon levels vary significantly by region, but elevated concentrations have been found in every part of the province. Testing is the only way to know whether your home has a radon problem.

Despite these facts, radon testing rates in Ontario remain low. Health Canada estimates that fewer than 10 percent of Canadian homes have been tested. This represents a significant gap between what we know about the risk and what we are doing about it. A radon test is inexpensive, straightforward, and could be one of the most important health decisions you make for your household.

Where Radon Comes From

Radon is produced by the natural decay of uranium, which is present in soil and rock everywhere on earth. As uranium breaks down through a chain of radioactive decay, it produces radon gas. This gas migrates upward through the soil and, in outdoor air, disperses harmlessly. The problem arises when radon enters a building and accumulates indoors.

Radon enters homes through any pathway that connects the interior to the soil. Cracks in the basement floor, gaps around pipes and service entries, floor drains, sump pits, and the porous concrete of foundation walls all provide entry points. The slight negative pressure inside a heated home, created by the stack effect and exhaust fans, actively draws soil gas inward. Essentially, your house acts like a low-grade vacuum cleaner, pulling radon from the ground.

The concentration of radon in a home depends on two factors: how much radon the underlying soil produces and how easily it enters and accumulates in the building. Homes built on granite bedrock or uranium-rich soils tend to have higher radon levels, but surprises are common. Two neighbouring homes built on the same soil can have very different radon levels because of differences in construction, foundation condition, and ventilation.

Radon Levels Across Ontario

Health Canada's cross-country radon survey found that approximately 7 percent of Ontario homes tested had radon levels above the Canadian guideline of 200 becquerels per cubic metre (Bq/m3). Certain regions have significantly higher rates. Parts of eastern Ontario, the Ottawa Valley, and areas underlain by Precambrian Shield rock have shown higher average concentrations.

However, radon is highly localized. High-radon and low-radon homes can exist on the same street. Regional averages are useful for understanding general risk, but they do not tell you what is in your specific home. The only way to know your home's radon level is to test it.

New construction in Ontario is required under the Building Code to include a radon rough-in: a sealed pipe through the basement floor that can be connected to a fan if radon levels are found to be elevated. This is a sensible precaution, but the rough-in alone does not reduce radon. It simply makes mitigation easier and less expensive if testing reveals a problem. Even new homes should be tested.

How to Test

Radon testing is simple and can be done by any homeowner. The recommended approach is a long-term test using an alpha track detector placed in the lowest lived-in level of your home for at least 91 days during the heating season, typically between October and April. Long-term tests give the most accurate picture of your average radon exposure because radon levels fluctuate daily and seasonally.

Test kits are available from several Canadian suppliers, including the Take Action on Radon campaign, which provides kits with analysis for approximately $50. You place the detector in your basement or lowest living area, leave it undisturbed for the test period, then mail it to the laboratory. Results are returned within a few weeks.

Short-term tests, which run for two to seven days, are available and useful for screening purposes or real estate transactions, but they are less reliable. Radon levels can vary by a factor of two or more from day to day, so a short-term test may not represent your typical exposure. If a short-term test shows elevated levels, follow up with a long-term test to confirm.

Place the test device in the lowest level of your home that is regularly occupied. For most homes, this is the basement. If you use your basement as a living space, recreation room, or bedroom, that is where the test should go. Place it away from exterior walls, windows, and doors, and away from drafts, humidity sources, and direct sunlight. Follow the instructions that come with the kit carefully.

Understanding the Results

Canada's guideline level for radon in indoor air is 200 Bq/m3. If your test result is below 200, Health Canada considers the risk acceptable, although there is no completely safe level of radon exposure. Risk increases with both concentration and duration of exposure.

If your result is between 200 and 600 Bq/m3, Health Canada recommends taking action to reduce radon levels within two years. If the result is above 600 Bq/m3, remediation within one year is recommended. The higher the level, the more urgently you should act.

For context, the World Health Organization recommends a reference level of 100 Bq/m3. Some countries, including the United States, use a lower action level than Canada. If your result is between 100 and 200, you may still want to consider mitigation, especially if you have young children or spend a lot of time in the affected area.

Reducing Radon Levels

The most effective radon mitigation technique is active soil depressurization, often called sub-slab depressurization. A pipe is installed through the basement floor into the gravel layer beneath the slab. A fan connected to the pipe creates a slight negative pressure under the floor, drawing radon from beneath the house and exhausting it above the roofline where it disperses harmlessly. This system is quiet, energy-efficient, and typically reduces radon levels by 80 to 95 percent.

If your home has a radon rough-in from new construction, activating it involves connecting a fan to the existing pipe. This is a relatively straightforward project. In older homes without a rough-in, a mitigation contractor will need to core a hole through the basement floor, install the pipe, and mount the fan. The total cost for professional installation typically ranges from $2,000 to $3,500.

Sealing cracks and gaps in the foundation is a useful supplementary measure but is rarely sufficient on its own. The number of potential entry points in a typical foundation is too large to seal completely, and new cracks develop over time. Sealing works best in combination with active depressurization.

Increasing ventilation in the basement can reduce radon levels, but it comes with trade-offs: higher energy costs, comfort issues in winter, and the possibility that increased air movement draws more radon from the soil. Active depressurization is the preferred approach because it addresses the source rather than diluting the result.

Radon and Real Estate

If you are buying a home in Ontario, asking about radon testing should be part of your due diligence. Sellers are not currently required to disclose radon levels in Ontario, but you can make your offer conditional on a radon test. A short-term test can be completed within the typical inspection period, though a long-term test would be more accurate.

If you are selling a home and have tested for radon with a good result, sharing that information with potential buyers is a positive signal. It demonstrates responsible homeownership and removes uncertainty. If levels are elevated and you have installed a mitigation system, documentation of the mitigation and post-mitigation test results can reassure buyers.

Radon is one of several environmental health factors worth considering when evaluating a home or neighbourhood. For a broader perspective on environmental factors in home selection, the guide to evaluating environmental quality when moving covers the full range of considerations. And for families considering what makes a neighbourhood liveable and safe, radon testing belongs alongside assessments of water quality, air quality, and community infrastructure in building a complete picture of community health.

Testing your home for radon takes minimal effort and costs less than a family dinner out. The peace of mind, or the early warning, it provides is disproportionately valuable. Every Ontario home should be tested. If yours has not been, the heating season is the ideal time to start.