A radon level is a number, and what it means depends on the test behind it. EPA’s action level is 4 pCi/L (picocuries per liter of air, the usual US unit). EPA says to fix your home at 4 or higher and to consider fixing between 2 and 4. EPA’s Citizen’s Guide (2025 cover) also says “no level of radon is safe”, so 4 is a decision point, not a safety line.
This page takes a result in the order most people need: what to do with it, what 4 means, the units, and why the number moves. Each section points to the guide or tool that goes deeper.
Start with your number
EPA’s next step depends on the test as well as the level. A short-term test stays in the home for 2 to 90 days and a long-term test for more than 90. EPA says a long-term test is more likely to tell you your year-round average, which is why one short test at 4 or higher leads to another test, not a repair.
| Your test and level | EPA’s next step, in its words |
|---|---|
| Below 2 pCi/L | “Even if your test result is below 4 pCi/L, you may want to test again sometime in the future.” (Citizen’s Guide) |
| 2 to just under 4 pCi/L | “Consider fixing if your level is between 2 and 4 pCi/L” (EPA zone page) |
| First short-term test, 4 to 8 pCi/L | “Follow up with either a long-term test or a second short-term test” (Citizen’s Guide) |
| First short-term test, above 8 pCi/L | “you should take a second short-term test immediately” (same page) |
| Long-term test, 4 pCi/L or more | “Fix your home if your long-term test result is 4 pCi/L or more.” (same page) |
| Two short-term tests averaging 4 pCi/L or more, no sale | “Consider fixing your home if the average of your first and second test is 4 pCi/L or higher” (same page) |
| Two short-term tests averaging 4 pCi/L or more, in a home sale | “Fix the home if the average of the two tests is 4 pCi/L or more.” (Home Buyer’s and Seller’s Guide, March 2024) |
EPA’s guides really do differ on “fix” and “consider fixing”. One short-term test in a sale is a separate case: it isn’t yet one of EPA’s sale options, so another test comes first. Our radon test result reader takes your number and test type, picks the row and goes through what to check on your report.
Numbers close to a line deserve less weight than they seem to carry. EPA’s example: if two short-term tests average 4.1 pCi/L, there’s about a 50% chance the year-round average is somewhat below, or above, 4. EPA doesn’t treat 3.9 as a pass either. It says to consider fixing. If you have time, a long-term test is the better tie-breaker.
What 4 means, and what it doesn’t
An action level tells you when to act. It doesn’t mark where risk starts. CDC says “There is no known safe level of radon” and that reducing it will always help reduce your risk of lung cancer, even below 4 pCi/L.
For scale, EPA gives about 1.3 pCi/L as the average indoor level in US homes and about 0.4 for outdoor air. Treat 1.3 as a rough marker: it comes from a survey completed in 1991, and a 2025 analysis with EPA staff among its authors says no newer national data exist. Other bodies draw other lines. WHO suggests a national reference level of 100 Bq/m³, about 2.7 pCi/L, for countries to set, and Canada’s guideline is 200 Bq/m³, about 5.4 pCi/L. Each does a different job, and none is a pass mark.
EPA’s risk table shows how levels relate to lung cancer over a lifetime of exposure. At 4 pCi/L, about 62 of 1,000 people who smoked could get lung cancer, against about 7 of 1,000 who never smoked. These describe groups of people, not your own risk, and one test doesn’t put you on a row. Our guide to what EPA says about safe radon levels has the full level chart and risk table, the WHO and Canadian comparison, and how low a home can realistically go.
The units on your report
Some monitors and the WHO and Canadian lines use Bq/m³, becquerels per cubic meter. The conversion is exact: 1 pCi/L corresponds to 37 Bq/m³. That makes EPA’s 4 pCi/L exactly 148 Bq/m³, though EPA prints it as 150. It’s the same line, rounded. The pCi/L to Bq/m³ converter goes both ways and sets your number against the EPA and WHO lines.
Two results that look similar don’t convert this way. Working levels (WL) measure radon’s decay products, so no single factor links WL to pCi/L. EPA says to fix the home at 0.016 WL or higher, and the simplest course is to ask the lab for the pCi/L figure. Water results also come in pCi/L, but they aren’t comparable with air: a water result of 4 is nothing like an air result of 4. See radon in water for those numbers.
Why levels move
The same home can give different numbers. EPA says radon levels “tend to vary from day to day and season to season”. The federal toxicological profile for radon (May 2012) says radon enters mainly with soil gas, pulled in by small pressure differences caused mostly by indoor and outdoor temperature differences and wind. EPA’s Consumer’s Guide (2025 cover) says “your home acts like a vacuum, drawing radon in through foundation cracks and other openings.”
Levels also change with the floor. Lower floors tend to read higher because radon comes from the ground below them, not because it settles there. It is about 7.5 times heavier than air, but EPA says it “is easily influenced by air movements and pressure”. Our page on whether radon is heavier than air explains why upper floors aren’t free of it.
That makes location part of the result, so keep it with the number. We found no separate EPA level for basements. EPA says to test in the “lowest lived-in level of the home (for example, the basement if it is frequently used, otherwise the first floor)”, and to retest if you start using a lower level. Homes differ too: EPA says levels “can vary greatly from home to home”, so a neighbor’s result can’t stand in for yours.
Only a test gives you a level
Nothing you can notice in the house tells you the number. EPA says “You can’t see radon. And you can’t smell it or taste it.” A dry, clean-smelling basement can test high and a musty one can test low. If a smell is what brought you here, our page on whether radon has a smell covers what it may be instead, including what to do first if you smell gas.
Device limits matter as well. The professional testing standard, ANSI/AARST MAH-2023, says results below about 2 pCi/L are normally lower than devices can accurately measure, so don’t read much into the gap between 0.9 and 1.4. Home monitors can be well off near 4: in a 2024 UK Health Security Agency study at about 4.5 pCi/L, one of seven consumer monitor types had an error under 10% and the others ranged from 20% to 50%. See our guide to radon monitors.
What to do next
EPA’s next step, from the Citizen’s Guide:
Test your home for radon—it’s easy and inexpensive.
Fix your home if your radon level is 4 picocuries per liter (pCi/L) or higher.
Radon levels less than 4 pCi/L still pose a risk, and in many cases may be reduced.
- No test yet: start with how to test for radon.
- One short test at 4 or higher: test again first. Our testing guides compare the follow-up options.
- A result you can rely on at 4 or higher, or weighing a fix from 2: EPA says “You should use a contractor who is trained to fix radon problems.” Our guides to fixing radon cover systems and costs.
- After any fix: test again, then at least every two years, EPA’s Consumer’s Guide says.
- Worried about your health: talk to a doctor, and tell them if you smoke or have lived with high radon levels. Our radon and health guides set out what the health agencies say.
Two things worth asking. Your tester or lab: was this a short-term or long-term test, and on which floor? Your state radon office: does the state set its own level? EPA lists contacts by state, and we haven’t surveyed every state’s rules.
Radon Handbook sells no tests or systems and refers no one. This page is general information, not medical, engineering or legal advice.
Sources checked on 4 November 2026.
