Radon in Ontario – The Invisible Risk in Many Homes

Radon is a naturally occurring radioactive gas that forms from the decay of uranium in soil and rock. It is colourless, odourless, and tasteless — there is no way to detect its presence in a home without testing. It is found throughout Canada, including Ontario, and Health Canada considers it a significant indoor air quality concern worth addressing through testing in every home.

How Radon Enters a Home

Radon moves from the soil below a building into the indoor environment through gaps, cracks, sump openings, floor-wall joints, and other penetrations in the foundation. The pressure difference between the soil and the interior of a building — created by the warmth of the home, wind, and the stack effect — can pull soil gas upward into the living space.

This process is not limited to older homes or specific foundation types. Newer, tighter construction can actually concentrate radon more effectively because the building retains what enters rather than exchanging air through unintended gaps. Homes with basements, crawl spaces, slab-on-grade foundations, and any foundation with openings into the soil can all be pathways for radon entry.

What the Numbers Mean

Radon concentration is measured in Becquerels per cubic metre (Bq/m³). Health Canada's guideline is 200 Bq/m³. The World Health Organization reference level is 100 Bq/m³. When long-term testing reveals levels above these thresholds, mitigation is recommended.

The threshold numbers are guidelines, not cliff edges. The goal is to reduce radon to the lowest achievable level. Modern sub-slab depressurisation systems are effective at reducing radon substantially in most homes.

Why Long-Term Testing Matters

Health Canada recommends long-term testing — a minimum of three months in the lowest livable area of the home — because radon levels fluctuate with season, temperature, ventilation, and other variables. A short-term test can indicate whether radon may be present at elevated levels, but a long-term test provides a more accurate picture of the annual average exposure, which is the figure most relevant to understanding risk over time.

RBM Group follows CNRPP guidelines for monitor placement and test duration. Every test includes a written summary of results and, where levels indicate action is warranted, clear options for next steps.

What to Do

Test first. Radon levels can vary considerably between homes on the same street, between floors in the same home, and between homes with superficially similar foundations. There is no reliable way to estimate what is present without measuring it. If elevated levels are confirmed, mitigation is practical, effective, and available. If levels are within acceptable range, that is the outcome the test provides — and it is equally useful information.