Radon levels and the Canadian guideline

What is the radon guideline in Canada?

Health Canada sets the guideline for radon in indoor air at 200 becquerels per cubic metre (Bq/m³) and recommends corrective action when a home's annual average is above itsource. A becquerel is one radioactive decay per second, so the unit counts how much radon is decaying in each cubic metre of the air you breathe.

Two details in that sentence matter more than the number. The first is "annual average". Radon rises and falls by the hour and by the season, so the guideline is compared with a long measurement, not with one reading. Health Canada asks for a test of three to twelve months, with 91 days as the minimumsource. The second is where the measurement is taken: on the lowest level of the home that someone occupies for four hours a day or moresource. A result from an unused cold room or a crawl space is not the number the guideline refers to.

Is there a safe radon level?

No level of radon is considered risk-free, and the guideline does not claim otherwise. It marks the point where Health Canada says action is needed, which is a different thing from the point where risk begins. Health Canada lists a higher chance of lung cancer as the only known health effect of radon, describes that risk as long-term rather than immediate, and ties it to the radon level, the years of exposure and smokingsource.

That is why the same guidance that names 200 Bq/m³ also asks homeowners to bring radon down as far as they reasonably can. A home at 180 Bq/m³ is under the guideline. It is also carrying nearly the same exposure as a home at 210 Bq/m³, and its owner is free to decide that lowering it is worth doing, especially when someone sleeps or works in the basement every day.

What is a dangerous radon level?

In Canada, any long-term average above 200 Bq/m³ is the level at which Health Canada recommends fixing the home, and urgency grows with the number. There is no official threshold labelled "dangerous". What changes as the result climbs is how quickly the guidance expects you to act and how much exposure builds up for each year you wait.

Radon gives no warning at any concentration. A basement at 1,500 Bq/m³ looks, smells and feels the same as one at 50 Bq/m³, so the only way to place a home on the scale is to measure it. Health Canada also notes that people who smoke and are exposed to radon carry a much higher combined risk than non-smokerssource, which makes a high result more pressing in a household with a current or former smoker.

How quickly should I act at each level?

Above 200 Bq/m³, Health Canada's guidance is to reduce radon within one year, and faster as the level risessource. Below the guideline that recommendation does not apply and the choice belongs to the homeowner; lowering the level remains a reasonable one, since lower is better. The table groups results into four ranges to make that easier to read. The ranges are our own reading aid for this site; the only official line in Canada is the one at 200 Bq/m³.

Each range has its own page with typical next steps: below 100 Bq/m³, 100 to 200 Bq/m³, 200 to 600 Bq/m³ and above 600 Bq/m³. If your report is in American units, start with a result of 2 pCi/L or a result of 4 pCi/L.

The timeline assumes the number came from a long-term test. A short test of a few days is a snapshot. If a snapshot reads several times the guideline, it makes sense to talk to a professional straight away. If it reads near 200 Bq/m³, a long-term test is the way to find out which side of the line the home is really on. Our page on radon testing in Calgary explains how both kinds of test are run.

How does Canada compare with the US and the WHO?

Canada's number is the highest of the three, which surprises people who read American websites. The US Environmental Protection Agency recommends fixing a home at 4 pCi/L, which is about 148 Bq/m³source. The EPA also suggests that Americans consider fixing a home that tests between 2 and 4 pCi/L, roughly 74 to 148 Bq/m³source. The World Health Organization recommends a national reference level of 100 Bq/m³, and no more than 300 Bq/m³ where local conditions make 100 unachievablesource.

For a home in Calgary, the reference that applies is the Canadian one: 200 Bq/m³. The other two are still useful. Many test kits and digital monitors sold here were designed for the American market and flag a result at 4 pCi/L, so knowing that this equals 148 Bq/m³ stops an alarm on a screen from being read as a Canadian exceedance. And a homeowner who wants a stricter personal target than the guideline can borrow the WHO figure.

How do Bq/m³ and pCi/L relate?

One picocurie per litre equals 37 becquerels per cubic metresource, so you multiply pCi/L by 37 to get Bq/m³ and divide Bq/m³ by 37 to go the other way. The Canadian guideline of 200 Bq/m³ works out to about 5.4 pCi/Lsource. Canadian laboratories report in Bq/m³. American devices usually default to pCi/L, and many let you switch in the settings.

Before comparing two results, check that they are in the same unit. A report that says "4.0" with no unit is almost always in pCi/L. Our radon unit converter does the arithmetic, and the test result checker places a number on the scale above.

Why do radon levels change through the year?

Indoor radon is usually highest when the house is closed up and heated. Health Canada asks that at least 91 days of a test fall inside the heating season, when windows are shut and indoor radon tends to peaksource. A heated house draws air in at the bottom as warm air leaves at the top, and part of that replacement air is soil gas pulled through cracks, joints, drains and sump pitssource.

In Calgary the furnace runs for much of the year, so the season that drives radon up is a long one here. A reading taken in July with the windows open can sit well below the same home's winter average. This is the reason a summer result under the guideline should not close the question, and the reason we suggest starting a long-term test in the fall. Day to day, wind, air pressure and frozen or wet ground move the number as well. None of that is a fault in the detector. It is the behaviour a long test is designed to average out.

Are levels different in the basement and on upper floors?

Yes. Radon comes from the ground, so the level is normally highest on the floor that touches the soil and lower on each storey above it. The guideline follows people, though, not floors: it applies to the lowest level that is regularly occupied.

That has practical consequences in Calgary homes. In a bungalow with a developed basement, a basement bedroom or home office is where the detector belongs. In a two-storey house whose basement holds only the furnace and storage, the main floor is the lowest lived-in level, and the result will usually be lower than a basement reading would have been. If the basement is finished later, or becomes a suite, the earlier result no longer describes how the house is used and the test should be repeated downstairs.

What level should I aim for after mitigation?

Aim for as low as reasonably achievable, not simply for 199 Bq/m³. Health Canada reports that a radon reduction system lowers the level by more than 80% in most homessource. Applied to a home that starts at 500 Bq/m³, a reduction of that size would leave it under 100 Bq/m³. That is an illustration of the arithmetic, not a promise: the result in a particular house depends on the soil, the slab and how well openings can be sealed.

The follow-up test is what tells you where you ended up. A short test soon after installation shows the system is working, and a long-term test in the next heating season gives the new annual average to compare with the guideline. Our page on radon mitigation in Calgary describes the systems and how results are confirmed.

What should I do with my own number?

Start by checking three things on the report: the unit, the length of the test and where the detector sat. A long-term result in Bq/m³ from the lowest lived-in level can be compared with the guideline directly. Anything else needs converting, repeating or both.

• Under 200 Bq/m³ on a long-term winter test: whether to act is your decision, because Health Canada's recommendation starts above 200 and no level is considered risk-free. Keep the report, and test again after renovations.
• Above 200 Bq/m³ on a long-term test: plan mitigation inside the one-year window, sooner for a higher result.
• Any result from a test of only a few days: treat it as a first look and confirm it with at least 91 days, unless it is far above the guideline.
• No test yet: a long-term test this heating season is the first step, whatever the age or location of the home. Our page on radon test kits in Calgary shows the do-it-yourself route.

If you send us your result with the form on this page, include the unit, the test dates and the room. Those three details let us tell you plainly whether the number calls for action, for another test or for nothing at all.