Radon · stack · 6 quick cards
Radon Levels: What Your Result Means
7 cards · By the Air in the House editors, from EPA · how stacks are made · Updated
What does your radon number mean?
Radon levels of 4 pCi/L or higher mean EPA says fix the house; between 2 and 4 pCi/L, consider fixing; a single high short-term result needs a follow-up test first.
- EPA's action level is 4 pCi/L: at that level or above, fix the home.
- Between 2 and 4 pCi/L, EPA says to consider fixing; no level is risk-free.
- The average US home is about 1.3 pCi/L indoors; outdoor air is about 0.4 pCi/L (EPA).
- A first short-term result above 8 pCi/L (twice the action level) calls for a second short-term test right away.
- Radon causes about 21,000 lung cancer deaths a year in the US, about 2,900 of them among people who never smoked (EPA).
- Most homes can be brought down to 2 pCi/L or below, EPA says.
AI-made picture, not a photo of a real house What radon levels should worry you?
Radon levels of 4 picocuries per liter (pCi/L) or higher are the point where EPA says to fix your home. That number is the agency's action level. EPA lists the same threshold in metric as 150 Bq/m³ on its radon zones page.
For context, EPA's Citizen's Guide to Radon estimates the average indoor level at about 1.3 pCi/L and normal outdoor air at about 0.4 pCi/L. So a reading of 4 is roughly three times the national indoor average and ten times what you breathe outside. EPA also stresses that any exposure carries some risk; 4 pCi/L is a practical trigger for action, not a line between safe and unsafe.
Turn over for the detail
If you haven't tested yet, start with how to test for radon, since a result is only as good as the placement and closed-house conditions behind it.
What if the result is between 2 and 4 pCi/L?
A result between 2 and 4 pCi/L is EPA's "consider fixing" band: the home doesn't hit the action level, but lowering it still cuts risk. EPA's radon health page puts the band at 75–150 Bq/m³ and notes that getting below 2 pCi/L is difficult.
One sensible step is a long-term test, which shows where the year-round average really sits before you decide. How much time the family spends on the tested floor, such as a basement bedroom or playroom, can also weigh in the choice. Either way, the choice is yours; EPA frames the band as an option, not a requirement.
When do you need a follow-up test?
You need a follow-up test whenever a first short-term result comes back at 4 pCi/L or more. Radon swings from day to day and season to season, so EPA wants a second measurement before anyone spends money on a fix.
EPA gives two follow-up routes:
Lifetime lung cancer risk by radon level (EPA estimates per 1,000 people) Radon level People who smoked People who never smoked EPA advice 20 pCi/L About 260 About 36 Fix your home 8 pCi/L About 120 About 15 Fix your home 4 pCi/L About 62 About 7 Fix your home 2 pCi/L About 32 About 4 Consider fixing 1.3 pCi/L (average indoor) About 20 About 2 Reducing below 2 is difficult Turn over for the detail
- Long-term test (more than 90 days). Gives a better picture of the year-round average. Fix the home if it reads 4 pCi/L or more.
- Second short-term test. Gives a faster answer. Consider fixing if the average of the first and second results is 4 pCi/L or more.
The higher the first number, the more sense the quick route makes. If the first short-term result is more than twice the action level, above 8 pCi/L, EPA says to take the second short-term test immediately. Results close to the line are the least certain: EPA's guide notes that when two short-term tests average 4.1 pCi/L, there is about a 50% chance the year-round average is somewhat below 4.
Source the documents listed on this stack's sources card
Next Why is radon a health risk?↓Why is radon a health risk?
Radon is a health risk because breathing it over years can cause lung cancer. EPA estimates radon is responsible for about 21,000 lung cancer deaths in the US each year, with a range of 8,000 to 45,000, and about 2,900 of those deaths occur among people who have never smoked. That makes radon the leading cause of lung cancer among non-smokers and the second cause overall, after smoking.
Turn over for the detail
EPA describes smoking and radon as working together, so smokers face a much higher risk. The table above, from EPA's risk charts in the Citizen's Guide, shows that at 4 pCi/L about 62 of every 1,000 people who smoked could get lung cancer from a lifetime of exposure, against about 7 of every 1,000 who never smoked. Those are lifetime estimates, not a forecast for any one person, and EPA notes the risk falls when the level falls, even after years in the same house.
Source the documents listed on this stack's sources card
Next What should you do next?↓What should you do next?
What you do next depends on the confirmed number. At 4 pCi/L or above after a follow-up, plan a fix with a certified radon mitigator; in the 2–4 band, decide whether to act or run a long-term test; below 2, keep the result on file and test again if you start living on a lower floor or remodel.
For a fix, look for a mitigator credentialed by the National Radon Proficiency Program or the National Radon Safety Board, or licensed by your state where it licenses radon work. EPA's page on finding a radon professional explains both routes, and your state radon office, listed on EPA's state radon contacts page, can point you to names. EPA says most homes can be fixed for about the cost of other common home repairs; ask more than one certified mitigator for an estimate. How the systems work, and how to check one afterward, is on the radon mitigation page.
Video from the Virginia Department of Health channel on YouTube: How To Test for Radon in Your Home. That channel has no tie to Air in the House; nothing loads from YouTube except the still picture until you press play. Turn over for the detail
Keep the paperwork. A dated lab report helps if you sell, and EPA's guide says buyers increasingly ask about radon before they buy or rent. If you're unsure, call a licensed pro.
Radon levels by the numbers
- 4 pCi/LEPA action level: fix the homeEPA Citizen's Guide to Radon
- 2–4 pCi/LEPA band where fixing is worth consideringEPA, Health Risk of Radon
- 1.3 pCi/Lestimated average indoor levelEPA Citizen's Guide to Radon
- 21,000US lung cancer deaths a year linked to radonEPA, Health Risk of Radon
- 2,900of those deaths among people who never smokedEPA, Health Risk of Radon
Chart drawn by Air in the House from figures cited in this stack. Source EPA
Next Quick cards↓
Quick cards
Short questions people ask next, one answer each.
Is 3 pCi/L a dangerous radon level?
It is below EPA's 4 pCi/L action level but inside the 2–4 band where EPA suggests considering a fix. A long-term test shows whether 3 is typical for your house across the year.
Why did two radon tests give different numbers?
Radon changes with weather, season and how the house is used, so two short tests a few weeks apart rarely match. EPA uses the average of the two short-term results, or a long-term test, to decide.
What does Bq/m³ mean on a radon report?
It is the metric unit, becquerels per cubic meter. EPA treats 4 pCi/L as about 150 Bq/m³ and 2 pCi/L as about 75 Bq/m³.
Is radon only a problem for smokers?
No. EPA estimates about 2,900 radon lung cancer deaths a year among people who never smoked, though the risk is far higher for smokers.
Can radon get down to zero?
Not in practice. Outdoor air itself carries about 0.4 pCi/L, and EPA says levels below 2 pCi/L are hard to reach, though most homes can get to 2 or below.
Does a low result in one season mean the house is fine?
It is a good sign, but a short test only covers a few days. EPA says you may want to test again in the future, and to retest if you begin living on a lower floor.
Where these cards come from
This stack draws on EPA's A Citizen's Guide to Radon (including its lifetime risk charts) and EPA's Health Risk of Radon page, read in October 2026. It leaves out mitigation prices, since EPA no longer publishes a dollar range, and does not rate any test kit or monitor.
- EPA EPA's action level
- EPA EPA says to consider fixing
- EPA radon zones page
- EPA EPA's page on finding a radon professional
- EPA EPA's state radon contacts page
Cards are written by the Air in the House editors from the documents above and dated when they were last checked. How stacks are made