How to read your radon test results

What information is on a radon test report?

A handful of items: the result, its unit, the dates, the location and the device. A laboratory report or a professional's report should state all of them. With a mail-in kit, some of them live in your own notes instead, which is why those notes are worth keeping with the report.

Read the supporting lines before the headline number. A figure with no unit, no dates or no room attached is hard to use later, when an installer or a future buyer asks where it came from.

Which unit is the result in?

Canadian laboratories report in becquerels per cubic metre, written Bq/m³, and many devices built for the American market report in picocuries per litre, written pCi/L. The two are tied by a fixed factor: 1 pCi/L equals 37 Bq/m³source. Put on the American scale, Canada's 200 Bq/m³ guideline comes to about 5.4 pCi/Lsource.

The mistake to avoid is setting a number in one unit against a guideline in the other. A small figure with a decimal point is often in pCi/L. A consumer monitor may be set to either unit, so open its settings before trusting the screen. The conversion tool for Bq/m³ and pCi/L does the multiplication for you.

What does "annual average" mean, and why does the guideline use it?

It is the level a home holds across a year, expressed as one figure. Health Canada's guideline of 200 Bq/m³ is written against that figure: corrective action is recommended when the annual average is above itsource.

Nobody measures every day of a year, so a long test stands in for it. The recommended measurement runs 3 to 12 months and never less than 91 dayssource, and those 91 days should fall in the heating season, when windows are closed and indoor radon tends to be highestsource. A result that meets both conditions is the one to place beside the guideline.

This is why the dates on a report carry as much weight as the number. An identical figure means one thing from a winter-long test and something far weaker from a few days in midsummer. Our guide to winter radon testing in Calgary covers the timing in detail.

How does the number compare with the reference levels?

For a home in Canada, the comparison that counts is with 200 Bq/m³. Other figures turn up on packaging, in apps and on foreign websites, and it helps to know where each one comes from.

The United States uses an action level of 4 pCi/L, about 148 Bq/m³, which is a lower figure than Canada's once both are stated in Bq/m³source. The US EPA further recommends that Americans consider fixing a home that measures between 2 and 4 pCi/L, about 74 to 148 Bq/m³source; the Canadian reference stays at 200 Bq/m³. The World Health Organization recommends a reference level of 100 Bq/m³, and no higher than 300 Bq/m³ where 100 cannot be achievedsource.

A monitor that shows a warning at the American level is not reporting that the Canadian guideline has been passed. It is not wrong either. The three figures are different answers to one question: where to draw a line on a scale that has no risk-free point. Our page on the Canadian radon guideline and the ranges below and above it sets them out in full.

Is a short test read the same way as a long one?

No. A short result describes a few days and a long result describes a season, and only the second can be compared with the guideline directly. Short-term tests typically last between 2 and 7 dayssource, while Health Canada does not recommend any measurement period under 91 dayssource.

A short reading far above the guideline is still a strong signal, and a reason to speak with a professional soon. One that sits near the guideline or under it settles nothing in either direction.

Around a home sale the same limit holds. Health Canada does not recommend conducting a radon test during a real estate transaction, because a three-month test is the only way to find out a home's level accuratelysource. A short reading taken while a purchase is under way should be read as a snapshot and as nothing more than that.

How should you read a consumer radon monitor?

Read its long-term average, and treat that average as a guide. C-NRPP has tested consumer-grade electronic radon monitors and publishes which models it approves; it states that these devices cannot be professionally calibrated and are not approved for use by radon measurement professionals, and it advises leaving them in place for at least 3 monthssource.

Most of these devices show more than one figure: a recent reading and a longer average. The recent one moves about, and watching it rise and fall is the main source of confusion among monitor owners. The longer average is the figure worth writing down, once the device has sat in one suitable spot for the period C-NRPP advises. Before then, the screen is showing part of a test.

A monitor average over the guideline is a sound reason to act, or to confirm with a detector that a laboratory analyses. A monitor average under it is useful to know, and a laboratory report remains the stronger record to keep with the house papers.

What should you do with a result above 200 Bq/m³?

Plan to bring it down. For a result over 200 Bq/m³, Health Canada's advice is mitigation within one year, sooner when the level is higher, with the aim of getting radon as low as reasonably achievablesource.

A long-term average over the guideline does not have to be confirmed by another test first. The next step is an assessment of the house by a mitigation professional. Health Canada reports that a radon reduction system can usually be installed in less than a day and reduces radon by more than 80% in most homessource. Nobody can promise the final figure for one particular house ahead of time, which is why the work ends with a measurement. How radon mitigation is carried out in Calgary homes describes the process from assessment to follow-up.

What should you do with a result below 200 Bq/m³?

Decide for yourself, knowing what the number does and does not mean. Health Canada's recommendation to act applies above 200 Bq/m³, and below that line the decision belongs to the owner. No level is risk-free and lower is better, so a result under the guideline is not a statement that nothing could be gained.

Several things can inform the choice. One is how close the average sits to the guideline. Another is how many hours a day somebody spends on the lowest level. A third is whether you would prefer to hold your home to the World Health Organization's lower reference level. A fourth is whether changes to the building are coming.

• Keep the report and your notes with the documents for the house, whatever you decide.
• Consider a second long-term test in another heating season when the first average came in close to the guideline.
• Ask for an assessment if you would like to know what lowering the level would involve before choosing.

The reasons an owner might care about a moderate level are set out in our summary of the evidence on radon and health, which keeps to what Health Canada states.

How do you read a result taken after mitigation?

As two separate measurements with different jobs. The installer takes a short verification reading once the system is running, and that reading shows the fan is drawing the level down. The long-term confirmation uses an independent laboratory kit that the homeowner places and mails, so the record of the new average does not rest on the person who did the installation.

Health Canada recommends testing after a system is installed, retesting every 5 years while an active system is in service, and testing again after any renovation that affects the structure or ventilation of the homesource. Read each of those later reports in the order you read the first one: unit, dates, room, device, then the average. File them together, because a run of reports from one address is worth more than any single page.

When does a result need repeating before you rely on it?

When the supporting details do not hold up. The number may still be right, but nobody can show that it is, and a fresh long-term test is the honest way to settle it.

• The test was shorter than the 91-day minimum.
• Most of the test fell outside the heating season.
• The detector sat in a room nobody uses, or in a kind of room the placement rules exclude.
• The detector was moved partway through, or the dates were filled in from memory.
• The house has changed since: a renovation, or a lowest level that is now lived in.

On the third point, Health Canada's guide calls for a regularly occupied room on the lowest lived-in level, away from kitchens, bathrooms, laundry rooms, vents and heat sourcessource. A figure from a furnace room or a storage room describes that room and not the air the household breathes for most of the day.

If none of those apply, the result stands and you can act on it. The checker for a radon test result will place the figure against the reference levels in either unit. If a repeat is needed and you would like it documented from the first day, our long-term radon test service in Calgary handles the placement, the records and the written report.