What do radon levels mean? How to read your test result

The number on a report is a radon concentration in air, written in whichever unit the lab's country uses. A result of 4 in pCi/L and a result of 200 in Bq/m³ are not the same concentration. Reading one against the other starts with the conversion factor, then with the threshold the relevant agency actually published.

Health Canada's guide for radon measurements in homes states the factor directly: “Note: 37 Bq/m³ is equal to 1 pCi/L.” Multiply a pCi/L result by 37 to get Bq/m³. Divide a Bq/m³ result by 37 to get pCi/L. That is the only gas-concentration conversion this page uses.

The chart applies that factor and nothing else. The Bq/m³ column is labeled calculated because each cell is the pCi/L value times 37, shown as the whole number in that column. It is not a second set of measurements, and it is not the rounded Bq/m³ figure EPA prints when it restates its own levels.

Two rows line up with reference levels you will meet again below. The 2.7 pCi/L row calculates to 100 Bq/m³, which is the WHO national reference level. The 5.4 pCi/L row calculates to 200 Bq/m³, which is Health Canada's guideline. The 4 pCi/L row calculates to 148 Bq/m³. EPA does not use 148 when it states that same level in becquerels.

EPA's action-level pagerounds instead. EPA rounds 4 pCi/L to 150 Bq/m³, 2 pCi/L to 75 Bq/m³, 1.3 pCi/L to 50 Bq/m³, and 0.4 pCi/L to 15 Bq/m³. Where this page quotes EPA, it keeps those rounded figures. The chart keeps the straight multiplication: 4 pCi/L is 148, 2 pCi/L is 74, 1.3 pCi/L is 48, and 0.4 pCi/L is 15. The outdoor average matches in both places. The other three do not, and the gap is rounding, not a different measurement. A sentence that says “4 pCi/L (150 Bq/m³)” is EPA's wording. A sentence that says 148 Bq/m³ is this chart.

Results can also be reported in working levels (WL). EPA's working-level page sets the action level at 0.016 WL: fix at or above 0.016 WL when the report is in WL. Health Canada's measurement guidewarns that converting WL to radon gas “depends on a number of factors.” Because of that warning, this page does not publish a fixed WL-to-pCi/L factor. A result reported in WL is read against 0.016 WL.

Each row is one published figure and the action the publishing body attaches to it. EPA's becquerel equivalents in this table are EPA's rounded ones (150 and 75 to 150), not the calculated 148 and 74 from the chart above. Health Canada's row is an annual average in the normal occupancy area. WHO's row is a national annual-average reference level, with a ceiling where that reference level cannot be reached.

EPA's action-level pagealso gives two averages that sit on the chart. The U.S. average indoor level is about 1.3 pCi/L, which EPA states as 50 Bq/m³. The average outdoor level is about 0.4 pCi/L, which EPA states as 15 Bq/m³. On the calculated chart those same pCi/L values are 48 Bq/m³ and 15 Bq/m³. EPA's page says the action level to follow is the one set by your country, state, or local jurisdiction, so a program can name its own figure even when EPA's 4 pCi/L is the number people quote first.

One U.S. program threshold is HUD's. HUD radon requirements use 4.0 pCi/L as the mitigation threshold for the multifamily rules that page explains. For an existing large building, radon mitigation under SGM-MFLB-2023 is written against that same 4.0 pCi/L action level. In new construction, the choice between a passive stack and a fan is settled by a post-construction test compared with 4 pCi/L, which is the decision on passive versus active radon control.

In Canada the comparison figure is Health Canada's 200 Bq/m³, not EPA's 4 pCi/L. British Columbia states that an active fan is triggered only if post-occupancy testing exceeds 200 Bq/m³. Ontario and Alberta set out the new-construction rough-in for those provinces. The radon code index lists the jurisdictions covered here. Those pages are building-code context for the 200 Bq/m³ guideline. They are not a second Canadian action level.

The steps below are the agencies' own decision sequences. They are not a blended checklist. A result in pCi/L is read on EPA's steps or on CDC's restatement of them. A result in Bq/m³ that you are comparing with the Canadian guideline is read on Health Canada's steps. WHO's 100 Bq/m³ and 300 Bq/m³ figures are reference levels for national policy, which is why they appear in the table above and not as a home-by-home test sequence here.

A Citizen's Guide to Radon (pages 5 to 6) gives three steps:

1. Take a short-term test. If the result is 4 pCi/L or higher, follow up.
2. For the follow-up, take a long-term test to learn the year-round average, or take a second short-term test if you need results quickly. If the first result is more than twice 4 pCi/L, take the second short-term test immediately.
3. Fix the home if the long-term result is 4 pCi/L or more. If the follow-up was a second short-term test, consider fixing if the average of the two short-term tests is 4 pCi/L or more.

How a short-term test and a long-term test differ is covered on short-term versus long-term radon tests. The steps above are only the sequence EPA attaches to the 4 pCi/L line.

CDC's radon testing page, updated August 26, 2026, restates the same levels:

• At or above 4 pCi/L, take a second test. Have a licensed professional install a reduction system if the average of two short-term tests is at or above 4 pCi/L, or if any long-term test is at or above 4 pCi/L.
• From 2 pCi/L through 4 pCi/L, consider a reduction system.
• Below 2 pCi/L, CDC's words are “No action needed.”

Health Canada's homes measurement guide, under interpretation of results, says to compare only a long-term average of 91 days or more with 200 Bq/m³. Above 200 Bq/m³, mitigate within 1 year, and sooner if the level is higher. The guideline page says to take corrective action, and that the higher the level, the sooner. The figure the comparison uses is the annual average in the normal occupancy area, which is the same 200 Bq/m³ row as the table above.

A result that prints just over a threshold is not, by itself, a precise statement of the year-round average. Both EPA and Health Canada publish a caution about results that sit on the line. They describe that caution differently, and the two descriptions are not interchangeable.

EPA's Citizen's Guide (page 6) ties a probability to one specific case. When the average of two short-term tests is 4.1 pCi/L, EPA says there is about a 50% chance the year-round average is somewhat below 4 pCi/L. The 4.1 figure and the 50% chance belong to that pair of short-term tests. They are not a general error bar you can paste onto every result.

Health Canada's measurement guidecites a tolerance in the standards instead of a probability. CSA and ANSI standards accept repeated results that vary by up to ±25%. A result printed just under 200 Bq/m³ may truly be just over 200 Bq/m³, and a result printed just over 200 Bq/m³ may truly be just under it. The guide's point is the width of that accepted spread, not a chance assigned to one reading the way EPA assigns a chance to 4.1 pCi/L.

EPA's Citizen's Guide(page 4) states: “Nearly 1 out of every 15 homes in the U.S. is estimated to have elevated radon levels.” That is the estimate EPA publishes, and it is quoted here with that source.

EPA estimates that radon causes about 21,000 lung cancer deaths per year in the United States. EPA states that figure on the action-level page and in A Citizen's Guide to Radon.

The Citizen's Guide (page 11) prints two lifetime risk tables, one headed for people who smoke and one for people who have never smoked. Quoted from the never-smoked table at 4 pCi/L, one cell reads “About 7 people” per 1,000 over a lifetime. The full tables are in the Guide. Credit for that cell and for the 21,000 estimate is EPA's.

The next reading is the test itself, not a product. Short-term versus long-term radon tests is the comparison of the two test lengths EPA's steps mention. To find a test kit or a measurement and mitigation professional, use EPA's finder. For what lowering a level in a Canadian home involves, Health Canada publishes Reducing radon levels in your home.