What radon is, and why it only becomes a problem indoors.

Where it comes from

Uranium is present in small quantities in soil and rock almost everywhere on earth. As it decays it passes through a chain of products, and one of them is radon — a gas. Being a gas, it moves: up through soil, through fractures in bedrock, through fill and gravel, and eventually to the surface.

Outdoors this is a non-event. The gas disperses into an effectively infinite volume of air. The problem is only ever a building, because a building is a container sitting directly on the source.

How it gets into a house

Houses are not sealed boxes. Warm air rises and leaves through the upper storeys, and the air that replaces it has to come from somewhere. Some comes in around windows and doors, and some is drawn up out of the ground. That slight negative pressure at the lowest level of the house is the engine: it actively pulls soil gas through whatever openings exist.

• Cracks and joints in a concrete slab, including where the slab meets the wall
• An open or loosely covered sump pit, which connects straight to the drainage around the footings
• Gaps around plumbing, electrical and drainage penetrations
• Floor drains, crawlspaces, and exposed earth or rock
• Porous or unsealed foundation walls

This is why radon levels rise in winter. A house that is closed up and heated for months has the pressure difference running hard, with nothing diluting what comes in. It is also why two identical houses next door to each other can test completely differently.

Why the health risk is real

Radon itself is inhaled and largely exhaled again. What does the damage are its decay products — solid, electrically charged particles that attach to dust, get breathed in, and lodge in lung tissue, where they continue to emit radiation. Over years, that steadily raises the chance of a cell mutation that becomes cancer.

Health Canada identifies radon as the number one cause of lung cancer in people who have never smoked, estimates that about 16% of lung cancers — more than 3,000 deaths a year in Canada — are linked to radon exposure, and notes that radon and smoking multiply each other’s risk: the World Health Organization puts a smoker’s risk from radon at roughly 25 times a non-smoker’s.

The exposure is slow and entirely symptomless. There is no smell, no irritation and no early warning of any kind — which is the whole reason measurement is the only sensible response.

What the numbers mean

Radon is measured in becquerels per cubic metre (Bq/m³). Health Canada’s guideline says to take action above 200 Bq/m³; the World Health Organization’s reference level is 100 Bq/m³. There is no level known to be perfectly safe — the risk falls as the level falls — so the guideline is a threshold for action, not a line between safe and dangerous.

What you can actually do about it

Two things, in order. Measure the house, because nothing about its age, price, neighbourhood or appearance predicts the result. Then, if the level warrants it, fix it — a well-understood piece of work with a verifiable outcome, not an open-ended renovation. Health Canada notes a properly installed system reduces radon by more than 80% in most homes.

Start with how testing works, or if you already have a number, how mitigation works.