How to Read Radon Test Results

Your radon test result is usually a number in picocuries per liter of air (pCi/L), and the key number to compare it with is 4 pCi/L (148 Bq/m³), the U.S. EPA action level. The EPA recommends fixing your home if one long-term test, or the average of two short-term tests, shows 4 pCi/L or higher. A single short-term result at that level means you should take a follow-up test first.

This guide follows U.S. EPA guidance. Other countries use different reference levels, so check your national or local guidance if you live outside the United States. It explains test results only. It isn’t medical advice, and it doesn’t replace a qualified radon professional.

What your radon number means

For comparison, the EPA estimates that the average indoor radon level is about 1.3 pCi/L (48 Bq/m³), and that about 0.4 pCi/L (15 Bq/m³) is normally found in outdoor air. With today’s technology, it says radon levels in most homes can be reduced to 2 pCi/L (74 Bq/m³) or below.

Check which kind of test you took

The same number can lead to a different next step depending on the test.

• Short-term test (2 to 90 days): this is a snapshot. Because radon levels tend to vary from day to day and season to season, it is less likely than a long-term test to tell you your year-round average.
• Long-term test (more than 90 days): this is more likely to reflect your home’s year-round average, so the EPA treats a result of 4 pCi/L or higher as a reason to fix your home.
• Two short-term tests: use the average of the two, not the higher or lower number.

For more on the differences, see Short-Term vs Long-Term Radon Testing.

Check how the test was done

Before you rely on a result, check that the test was set up the way the EPA describes. A test run under different conditions may not tell you what you need to know.

• Location: the kit should be in the lowest lived-in level of your home, in a room used regularly, but not the kitchen or bathroom.
• Placement: at least 20 inches above the floor, away from drafts, high heat, high humidity and exterior walls.
• House conditions: for a short-term test, windows and outside doors should stay closed as much as possible. For a 2- or 3-day test, close them at least 12 hours before it starts as well.
• Weather: tests of just 2 or 3 days shouldn’t be run during unusually severe storms or unusually high winds.
• Duration: the kit should stay in place for as long as the package says.

If something went wrong, the simplest fix is to test again. See Where to Put a Radon Detector and How to Test for Radon.

If a professional used a continuous monitor

Continuous monitors measure and record radon or its decay products over the test period. The EPA notes that many provide a report that can reveal unusual swings during the test, and that a qualified tester can explain the report to you. If you were given one and aren’t sure what it shows, ask the tester to walk you through it.

What to do next

If your first short-term result is 4 pCi/L or higher, take a follow-up test. The higher the first result, the more certain you can be that a short-term follow-up is the right choice. How Long Does a Radon Test Take? can help you plan for the timing.

If your long-term result or short-term average is 4 pCi/L or higher, look into fixing your home. Your state radon office can help you find qualified professionals. See How to Reduce Radon for an overview.

If your result is between 2 and 4 pCi/L, you may want to consider fixing your home. A long-term test can give you a better sense of your year-round average before you decide.

If your result is low, keep the report. You may still need to test again later, for example if you start using a lower level of your home or complete a major renovation. See How Often Should You Test for Radon?.