A radioactive gas that comes out of the ground, collects indoors, and causes lung cancer over long exposure. It is invisible, it is common in Canada, and it is fixable.
Uranium exists in small amounts in soil and rock nearly everywhere. As it decays it produces radium, and radium decays into radon, a gas. Outdoors that gas disperses into the atmosphere and does no harm. Indoors it can build up, because a heated building is at slightly lower pressure than the ground beneath it and quietly draws soil gas in through the foundation.
You cannot see it, smell it or taste it. The only way to know how much is in a building is to measure it.
Radon is the second leading cause of lung cancer after smoking, and the leading cause among people who have never smoked. The mechanism is that radon decays into solid radioactive particles which attach to dust, get breathed in, and lodge in lung tissue.
The risk is cumulative. It comes from years of exposure, not from any single day, which is why every guideline in the world is written as a long-term average and why a high reading is a reason to act rather than a reason to panic.
The 2024 Cross-Canada Survey of Radon found about 17.8% of people in Canada live in buildings at or above the 200 Bq/m³ guideline, with a further 24.2% between 100 and 199. The previous national survey put the figure at 6.9%. Part of that rise is better measurement, and part is that newer houses are built tighter and leak less.
There is no radon-free part of Canada. There are places where it is much more likely, and our national map shows which, but every province has homes above the guideline.
On Health Canada's own measurements, Regina and Winnipeg stand far above every other city, with 56% and 47% of tested homes at or above the guideline. Thunder Bay, Kingston, Halifax and Saskatoon follow. Vancouver, Victoria and Toronto are among the lowest in the country.
Two patterns hold nationally: rural communities generally show more radon than urban ones, and single-detached houses more than semi-detached, which show more than row housing.
Sealing cracks helps a little. What actually works is sub-slab depressurization, a pipe and a fan that vent soil gas above the roofline before it can enter the house. It is a day's work in most homes and routinely cuts levels by most of the way.
What does not work: air purifiers, houseplants, and opening windows as a permanent strategy.
Through the ground floor. Cracks in a slab, gaps around service pipes, sump pits, crawlspaces and unfinished floors. Warm air rising inside creates slightly lower pressure at the bottom of the house, which pulls soil gas in. This is why radon is generally higher in winter and often higher in newer, better-sealed homes.
It is the second leading cause of lung cancer in Canada after smoking, and Health Canada, the WHO and the US EPA all treat it as a genuine risk. It is also slow, cumulative and entirely fixable, which is a very different kind of danger from an emergency. The right response is to measure it and deal with it if it is high, not to worry about it.
A high reading is a negotiating point in a sale, but mitigation is a days-long job at a known cost, so it is usually handled as a line item rather than a deal-breaker. A house with a documented mitigation system and a confirming re-test is arguably easier to sell than one that has never been tested at all.
No. It reflects what is in the ground underneath, not the quality of the building. Two identical houses on the same street can read very differently, and a well-built modern house can read higher than an old draughty one precisely because it is better sealed.
Sources. Health Canada radon guidance and published measurement data; the 2024 Cross-Canada Survey of Radon; World Health Organization Handbook on Indoor Radon; United States Environmental Protection Agency; Canadian National Radon Proficiency Program. Costs described are general ranges, not quotes.