Radon level in pCi/L: what the number means

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Short answer

A radon result is read against EPA’s action level of 4 pCi/L, which EPA prints as 150 Bq/m3: at that level EPA says to remedy the problem as soon as possible. Between 2 and 4 pCi/L (75 to 150 Bq/m3) EPA says consider fixing, and the Citizen’s Guide puts the average indoor level at about 1.3 pCi/L and the outdoor average at about 0.4 pCi/L. WHO’s reference level is 100 Bq/m3, about 2.7 pCi/L, with 300 Bq/m3 as the ceiling where 100 cannot be reached. To convert, multiply pCi/L by 37, from EPA’s 1 curie = 3.7 x 10^10 becquerel. One short-term test does not decide anything: EPA’s rule is that a result of 4 pCi/L or higher calls for a follow-up test, and the fix is a sub-slab depressurization system, which EPA’s Consumer’s Guide says typically cuts radon by 50 to 99 percent, installed by a state-certified or NRPP or NRSB-listed contractor.

The picks

Needs EPA's first short-term test, or the second one, with a lab result
PRO-LAB Radon Test Kit (short-term, lab analysed)
PRO-LAB RA100 page: two desiccated short-term liquid scintillation detectors, exposure at least 48 and no more than 96 hours, pre-paid return envelope, standard results within one week of receipt; the maker lists a separate lab fee
Check price on Amazon Why this one
Wants to watch the number across seasons, not decide a sale this week
Airthings Corentium Home 223 (continuous monitor, pCi/L)
Airthings manual: 3 AAA batteries, 2 years battery life, first reading after 6 to 24 hours, 1-day and 7-day averages plus a long-term average up to one year, typical accuracy 10% at 7 days and 5% at 2 months at 5.40 pCi/L, place 50 cm above the floor
Check price on Amazon Why this one

If you are about to call a contractor off one 4-day kit result, stop: EPA’s Citizen’s Guide says a two-test average of 4.1 pCi/L has about a 50 percent chance of a year-round average somewhat below 4, which is why EPA asks for a second test before anyone drills the slab.

What does your number mean?

Radon is measured in picocuries per litre of air (pCi/L) in the United States and in becquerels per cubic metre (Bq/m3) almost everywhere else. The bands below are EPA’s, in EPA’s words, with the Bq/m3 figures EPA prints next to them.

Your result	EPA’s Bq/m3 figure	What EPA says, as printed
4 pCi/L or higher	150 Bq/m3 and above	“Remedy the problem as soon as possible” (Health Risk of Radon page); the action level
2 to 4 pCi/L	75 to 150 Bq/m3	“Consider fixing” (Health Risk of Radon page); most homes can be reduced to 2 pCi/L or below (Citizen’s Guide)
About 1.3 pCi/L	50 Bq/m3	EPA’s estimate of the average indoor level in the US (Citizen’s Guide)
About 0.4 pCi/L	15 Bq/m3	What EPA says is normally found in outdoor air (Citizen’s Guide)

EPA has no “below 2 is fine” line. The Citizen’s Guide says that any radon exposure carries some risk and that reducing a home below 2 pCi/L is difficult; Congress set a long-term goal of indoor levels no higher than outdoor levels. On the health question itself, EPA’s Health Risk of Radon page states that radon is the number one cause of lung cancer among non-smokers by EPA’s estimates, and this page cites that statement and stops there.

How do pCi/L and Bq/m3 convert?

Multiply pCi/L by 37 to get Bq/m3. EPA’s Radiation Terms and Units page gives 1 curie as 3.7 x 10^10 becquerel, so a picocurie is 0.037 becquerel, and a litre is one thousandth of a cubic metre.

Figure	pCi/L	Bq/m3	Who prints it
EPA action level	4	150 (EPA’s rounding; 37 x 4 = 148)	EPA, Health Risk of Radon
EPA consider-fixing floor	2	75 (37 x 2 = 74)	EPA, Health Risk of Radon
WHO reference level	2.7 (derived)	100	WHO, Radon and health
WHO ceiling where 100 cannot be met	8.1 (derived)	300	WHO, Radon and health

WHO’s fact sheet uses Bq/m3 only and recommends a national annual-average reference level of 100 Bq/m3, with 300 Bq/m3 as the level a country should not exceed where 100 is not reachable. The pCi/L equivalents in that table are OwnerSpec’s arithmetic with the 37 factor, not WHO’s print.

Which test did your number come from?

A short-term test stays in the home from two days to 90 days depending on the device; a long-term test stays more than 90 days and, EPA says, is more likely to give the year-round average. Radon varies by season and by day, which is why EPA’s rule turns on a second measurement.

First result (short-term)	EPA’s next step (Citizen’s Guide)	Then
Below 4 pCi/L	No follow-up required by EPA’s steps; consider fixing if between 2 and 4	Retest every two years, and after any renovation or when you start using a lower level
4 pCi/L to 8 pCi/L	Follow up with a long-term test for the year-round picture, or a second short-term test if you need results quickly	Fix if the long-term result is 4 or more, or the average of the two short-term tests is 4 or higher
Above 8 pCi/L (more than twice the action level)	Take a second short-term test immediately	Fix if the average of the two is 4 pCi/L or higher

The test conditions matter as much as the device. EPA’s guide says to keep windows and outside doors closed as much as possible during a short-term test, closed for at least 12 hours before a 2 to 3 day test, and not to test during unusually severe storms or high winds. Place the kit in the lowest lived-in level, in a regularly used room that is not a kitchen or bathroom, at least 20 inches above the floor, away from drafts, high heat, humidity and exterior walls.

A continuous monitor is a different instrument. Airthings’ manual for the Corentium Home 223 says the first reading appears after 6 to 24 hours, that first-day results are an indication only, and that typical accuracy at 5.40 pCi/L is 10 percent at 7 days and 5 percent at 2 months. It tracks a 1-day average, a 7-day average and a long-term average of up to one year. The radon detector review compares the monitors’ published specs.

Which fix does the number point to?

EPA’s Consumer’s Guide to Radon Reduction names active sub-slab suction, also called sub-slab depressurization, as the main technique: a vent pipe through the slab and a fan pull radon from beneath the house and vent it above the roof. The guide’s table gives a typical reduction of 50 to 99 percent for active sub-slab suction and 30 to 70 percent for a passive (fan-less) system, and says sealing cracks makes either more effective.

Question	What EPA’s guides say
Who installs it	A state-certified contractor, or one listed by a private proficiency program: EPA’s Consumer’s Guide names the National Radon Proficiency Program (NRPP) and the National Radon Safety Board (NRSB)
What standard	The Consumer’s Guide says mitigation should follow ASTM E2121, the practice for installing radon mitigation systems in existing low-rise homes
How you know it works	A fan-powered system carries a warning device, usually a manometer; retest after installation and at least every two years after that
How long the fan lasts	Five years or more; EPA notes maker warranties tend not to exceed five years
What it costs	Neither EPA guide prints a dollar range: both say about the same as other common home repairs, and less where a passive system was built in during construction

If the home is on a private well, radon can also enter through the water. The uranium and radon in well water guide covers that path, which an air system does not touch.

Before you sign a mitigation contract

Run EPA’s second test first. The PRO-LAB kit above is two lab-analysed short-term detectors, which PRO-LAB describes as for side-by-side testing, so check whether you need two sequential tests or a pair run together; the flaw is that PRO-LAB’s page lists the lab fee separately, so the shelf price is not the whole price. The Airthings Corentium Home 223 will show you the season-to-season swing a kit cannot, but Airthings itself calls the first day an indication only, so if you need a dated result for a sale this month, skip the monitor and mail the kit.

What should you do first?

Write down the unit next to the number. 150 is an action level in Bq/m3 and an emergency in pCi/L; they are not the same reading.
Place your result in EPA’s table above: 4 or higher, 2 to 4, or below 2.
At 4 pCi/L or higher, run EPA’s follow-up: a long-term test, or a second short-term test if you need an answer fast. Above 8, run the second short-term test immediately.
If the long-term result or the two-test average is 4 pCi/L or higher, get more than one estimate from a state-certified or NRPP or NRSB-listed contractor, and ask for ASTM E2121.
Retest after the system goes in, and at least every two years after that.

Frequently asked questions

Is a radon reading of 3.5 pCi/L acceptable?

It sits inside the band EPA labels consider fixing, which runs from 2 to 4 pCi/L (75 to 150 Bq/m3). EPA's Citizen's Guide says most homes today can be reduced to 2 pCi/L or below, and that the average indoor level is about 1.3 pCi/L. A 3.5 is below the 4 pCi/L action level, but EPA does not describe it as a level to ignore.

How do I convert pCi/L to Bq/m3?

Multiply by 37. EPA's Radiation Terms and Units page gives 1 curie as 3.7 x 10^10 becquerel, so 1 picocurie is 0.037 becquerel and 1 pCi/L is 37 Bq/m3. EPA's own pages round: it prints 4 pCi/L as 150 Bq/m3 and 2 pCi/L as 75 Bq/m3, where the factor gives 148 and 74. To go the other way, divide Bq/m3 by 37.

My first short-term test came back at 4.6 pCi/L. Do I need mitigation?

Not yet. EPA's Citizen's Guide says a short-term result of 4 pCi/L or higher calls for a follow-up: a long-term test for the year-round picture, or a second short-term test if you need an answer quickly. Fix the home if the long-term result is 4 pCi/L or more, or if the average of the two short-term tests is 4 pCi/L or higher.

Can a continuous radon monitor replace the lab test kit?

It can tell you what the number does across seasons, which a 4-day kit cannot. It is a poor fit for a deadline: Airthings' manual says the first reading of the Corentium Home 223 appears after 6 to 24 hours and that first-day results are an indication only, with typical accuracy of 10% at 7 days. For a sale, a lab-analysed kit gives a dated result on paper.

What does radon mitigation cost?

Neither of EPA's two guides prints a dollar figure. Both say most homes can be fixed for about the same cost as other common home repairs, and the Citizen's Guide adds that the cost is much lower where a passive system was built in during construction. Get more than one estimate from a state-certified or NRPP or NRSB-listed contractor, as EPA's Consumer's Guide advises.

Fact sheet and how to cite this page

Answer: EPA’s action level is 4 pCi/L (EPA prints 150 Bq/m3), where EPA says remedy the problem as soon as possible, and between 2 and 4 pCi/L EPA says consider fixing. WHO’s reference level is 100 Bq/m3, about 2.7 pCi/L, with 300 Bq/m3 as the ceiling. A first short-term result of 4 pCi/L or higher calls for a follow-up test before any mitigation contract.
Basis: EPA’s Health Risk of Radon page and A Citizen’s Guide to Radon (EPA 402/K-12/002) set the 4 pCi/L action level, the 2 to 4 pCi/L consider-fixing band and the follow-up test rule; WHO’s Radon and health fact sheet sets the 100 Bq/m3 reference level and 300 Bq/m3 ceiling; EPA’s Radiation Terms and Units page gives 1 curie = 3.7 x 10^10 becquerel, from which 1 pCi/L = 37 Bq/m3.
Assumptions: The reader’s number came from a home test kit, a continuous monitor or a home-inspection report, in pCi/L (US) or Bq/m3 (most other countries), and was taken on the lowest lived-in level.
EPA’s Bq/m3 figures (150, 75, 50, 15) are EPA’s rounded pairs; multiplying by 37 gives 148, 74, 48 and 15. The table shows EPA’s printed values.
WHO’s 300 Bq/m3 ceiling converts to about 8.1 pCi/L by the 37 factor; no source opened for this page prints that pCi/L figure, so it is labelled as derived.
Neither EPA guide opened for this page prints a dollar range for mitigation, so this page gives none.
Method: OwnerSpec read EPA’s Health Risk of Radon page, A Citizen’s Guide to Radon (EPA 402/K-12/002, PDF), the Consumer’s Guide to Radon Reduction (EPA 402/K-10/005, 2025 edition, PDF), EPA’s Radiation Terms and Units page, WHO’s Radon and health fact sheet, the Airthings Corentium Home 223 manual and PRO-LAB’s RA100 kit page on 3 October 2026, and compared only the figures those documents print. No radon measurement was made for this page; the two listed products are a spec comparison, not a test.
Primary source: EPA, A Citizen's Guide to Radon (EPA 402/K-12/002, PDF) (government or regulatory source)
Facts verified: 3 October 2026
Last updated: 3 October 2026
Canonical URL: https://ownerspec.com/air/guides/radon-level-pci-l-what-it-means/
Cite as: OwnerSpec, "Radon level explained: what your pCi/L or Bq/m3 number means against EPA's 4.0 pCi/L action level, and what to do next", https://ownerspec.com/air/guides/radon-level-pci-l-what-it-means/, facts verified 3 October 2026.

Every figure above also appears, with its source, in the article. All sources · How OwnerSpec verifies numbers

Sources

EPA, Health Risk of Radon (government or regulatory source)
EPA, A Citizen's Guide to Radon (EPA 402/K-12/002, PDF) (government or regulatory source)
EPA, Consumer's Guide to Radon Reduction (EPA 402/K-10/005, 2025 edition, PDF) (government or regulatory source)
WHO, Radon and health fact sheet (manufacturer or vendor document)
EPA, Radiation Terms and Units (government or regulatory source)
Airthings, Corentium Home (Model 223) user manual (PDF) (manufacturer or vendor document)
PRO-LAB, Radon Gas Test Kit RA100 (manufacturer or vendor document)