What is radon? Definition, levels and health risks

Where does radon come from?

Definition

Radon is a naturally occurring radioactive gas. It has no smell, no color and no taste. It forms in the ground, from the natural decay of uranium in rocks and soils. None of your senses can detect it. Only a measurement can tell you whether it is present in a building — and at what concentration. (WHO) (Health Canada)

How it enters buildings

Radon seeps into buildings from the ground. The entry routes: cracks in concrete floor slabs, joints between the slab and the walls, service-pipe penetrations, drains and sumps. Concentration is typically highest in basements and in floors in contact with the soil. It decreases on upper floors. (WHO)

What is a Bq/m³?

Bq/m³ = becquerels per cubic meter. It is the unit used to measure radon concentration in air. The higher the number, the higher the concentration. 1 Bq/m³ means one radioactive decay per second, on average, in each cubic meter of air. On the map, the reading grid follows the tiers published in the legend: under 100 Bq/m³ (low), 100 to 200 Bq/m³ (worth monitoring), above 200 Bq/m³ (elevated). (WHO)

Radon is a naturally occurring radioactive gas: it forms in the ground as the uranium present in certain rocks decays. Odourless, colourless and tasteless, it escapes from the soil continuously and seeps into buildings through foundation cracks, joints and pipe penetrations. Its source is the soil beneath the building — not materials, not furniture, not the occupants’ activities. Its concentration is measured in becquerels per cubic meter of air: 1 Bq/m³ means one radioactive decay per second, on average, in each cubic meter. No sense can detect it — only a long-term measurement can reveal its presence in a given home. (WHO) (Health Canada)

Is radon dangerous to your health?

Lung cancer

Yes: radon is an important cause of lung cancer. Among people who have never smoked, it is the leading cause. The risk comes from prolonged exposure, built up over years. The gas is inhaled without being noticed. No symptom announces its presence — only a measurement can reveal it. (EPA) (Take Action on Radon) (WHO)

The order of magnitude in Canada

In Canada, radon causes more than 3,000 lung cancer deaths every year. (Take Action on Radon) That is an order of magnitude, not an exact count. But it gives the scale: radon exposure in homes is one of the most significant known causes of lung cancer in the country. It can be measured, then corrected.

The dose-response relationship

The higher the long-term average concentration, the higher the risk. The relative risk of lung cancer increases by about 16% per 100 Bq/m³ of long-term average concentration. The relationship is assumed linear: every reduction in indoor concentration translates into a reduction in risk. (WHO)

The share of lung cancers attributable to radon

An estimated 3% to 14% of all lung cancers are attributable to radon, depending on the country. (WHO) That is a significant share, for a risk that can be measured and reduced.

Smoking and synergy

Radon and tobacco compound each other. At equal radon exposure, smokers are about 25 times more at risk of lung cancer than non-smokers. (WHO) The two levers of prevention remain distinct: not smoking, and knowing your indoor concentration by measuring it.

Other health effects

To date, no other health effect has been established for radon exposure in the home — no other cancer, no other disease. (WHO) Prevention therefore focuses on lung cancer, and on it alone.

Are you a parent? Children spend more time at home than most adults, often in the lowest rooms — and the risk builds up over years of exposure. Two reflexes: know the concentration in the home your family lives in, and re-measure every time you move, because two neighbouring houses can have very different levels. A long-term measurement requires neither work nor particular skills: a dosimeter exposed for 91 days or more is enough. No symptom will announce the gas’s presence in the meantime — only the measurement can. The full protocol is described further down this page, in the “Measuring radon in your home” section.

What are the thresholds in Canada?

The thresholds that apply in Canada

The map’s reading grid follows the tiers published in the legend: under 100 Bq/m³, acceptable; from 100 to 200 Bq/m³, worth monitoring; above 200 Bq/m³, Health Canada recommends mitigation. The 200 Bq/m³ guideline is the one that applies in Canada. (Health Canada) (WHO)

International context only: WHO recommends that national programs aim for a reference level of 100 Bq/m³ where achievable, never exceeding 300 Bq/m³. That reference steers national programs — it does not replace the Canadian guideline applied on this site. (WHO)

Canada’s radon guideline is clear: above 200 Bq/m³ as an annual average in an occupied space, Health Canada recommends reducing the concentration — most often through a mitigation installation. Under 100 Bq/m³ the concentration is considered acceptable; between 100 and 200 Bq/m³ it deserves monitoring: re-measure, ideally during the heating season, before deciding. This 100/200 Bq/m³ grid is the one the map uses to classify sectors. It is specific to Canada: internationally, WHO recommends aiming for 100 Bq/m³ where achievable, never exceeding 300 Bq/m³. Whatever the measured value, the decision starts with a reliable long-term measurement: a few-day snapshot is not enough to place a home on this grid. (Health Canada) (WHO)

To find out where you stand, the “Measuring radon in your home” page explains the recognized protocol.

No measurement at home yet? See where to get a detector.

How do you measure radon at home?

The Health Canada protocol

The measurement that counts is long. Health Canada recommends 91 days or more — ideally 3 to 12 months — preferably during the heating season. That duration spans the seasonal variations: it averages them out. A few-day reading reflects the moment’s weather. It says nothing about the air you breathe through the year. (Health Canada) (WHO)

The limits of short-term tests

A short-term test is a snapshot, not an average. Concentration varies from hour to hour and from day to day: ventilation, weather, how the house is lived in. Below 91 days, the result freezes a single moment. Hence the map’s rule: short-term tests are labeled “indicative” and excluded from sector averages. They inform; they do not feed the statistics. (WHO)

Where to place the detector

As a general rule: the lowest room you actually use regularly. Often the basement, if it is genuinely lived in. Place the detector at breathing height, away from drafts, windows, exterior walls and ventilation equipment. Health Canada’s guide details placement for your house’s configuration. Also follow the instructions that come with your detector. (Health Canada)

Three families of devices coexist on the market. The passive long-term detector (roughly $30 to $60) is exposed for 91 days or more, then mailed back to a laboratory for analysis: the recommended default choice, the least expensive route to the measurement that counts. The continuous electronic monitor (roughly $150 to $250) counts as a long-term measurement too — Health Canada recognizes both device families — under two conditions: C-NRPP-approved, and left in place for 91 days or more. A few-day reading, or an unapproved model, is no substitute for the long-term measurement. And the detector with C-NRPP-accredited laboratory analysis is the full-service option: the device, the accredited analysis and the report all come from the same accredited Canadian service. (C-NRPP) (C-NRPP)

See where to get a detector to choose yours.

Where to get a detector?

Two families of devices, two routes. The passive long-term dosimeter: the least expensive route, laboratory analysis included. The continuous electronic monitor: C-NRPP-approved and left in place for 91 days or more, a long-term measurement in its own right. The commented selection — monitors and the Quebec Lung Association’s all-inclusive dosimeter — now lives on its own page: the detector guide. The criterion is the same everywhere there: C-NRPP approval and duration — 91 days or more.

What to do after the measurement?

After the measurement

The result is read with the map’s grid. Under 100 Bq/m³, acceptable. From 100 to 200 Bq/m³, worth monitoring: re-measure, or consider a long-term test. Above 200 Bq/m³, Health Canada recommends mitigation, and the “Reducing radon” page explains the options.

And whatever the value, your measurement matters to the map: the more measurements there are, the more sectors stand out. Add my measurement to the map.

Last verified: 2026-08-28

Next step

You now know what radon is and how to measure it. Two ways to continue: Reducing radon · Submit my measurement