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Radon Test After Mitigation: When and How to Check the Fix

Updated November 1, 2026 · General information, not medical advice

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The short answer: EPA says to test within 30 days of installation, but no sooner than 24 hours after the system is running with the fan on. Use a two- to seven-day test with windows and doors closed. EPA also says to get an independent follow-up test, and to retest at least every two years.

Not medical advice. This page repeats EPA and CDC guidance in plain words. Follow your state radon program and a certified radon professional for decisions about your home, and talk to a doctor about health concerns.

EPA's steps for the first test

Wait at least 24 hours after the system is operating with the fan on, if it has a fan.
Test within 30 days of installation.
Close windows and doors 12 hours before the test and keep them closed during it, except for normal coming and going.
Use a two- to seven-day measurement. Our test kit guide covers placement.
Get an independent result. EPA says the contractor may run a test, but you should also have an independent tester run one, or do it yourself.

What result to expect

EPA says that in all cases, most homes today can be reduced to 2 pCi/L or below. EPA also says some systems can reduce radon by up to 99 percent. EPA lists a guarantee to reduce radon to 4 pCi/L or below as a question to ask a contractor, and says to ask how long the guarantee lasts. If your result is still 4 pCi/L or higher, call the contractor. Enter your number in our result checker.

Keep checking after the first test

Look at the warning device. EPA says an active system must have a warning device, such as a gauge, alarm or light, and that you should look at it on a regular basis. If it shows a problem, call a contractor.
Do not turn the fan off. EPA says it must run continuously.
Retest at least every two years.
Expect to replace the fan someday. EPA says fans may last five years or more, and manufacturer warranties tend not to exceed five years.
Retest after remodeling. EPA says to retest in the lowest lived-in area to make sure the construction did not reduce the system's effectiveness.

Need a kit for your follow-up test?

Testing is the only way to know your radon level. A mail-in kit is a low-cost way to start, and our guide shows how to place it so the result is reliable.

How to use a radon test kit Browse radon test kits

Common questions

When should I test after a radon system is installed?

EPA says a post-mitigation test should be done within 30 days of installation, but no sooner than 24 hours after the system is running with the fan on, if it has one.

Can the installer do the test?

EPA says the contractor may test to check the work and the system's first results, but recommends you also get an independent follow-up measurement. An independent tester, or a test you do yourself, removes any conflict of interest.

What kind of test should I use?

EPA recommends a two- to seven-day measurement to check how well the system works. Windows and doors must be closed 12 hours before and during the test, except for normal coming and going.

How often should I test after that?

EPA says it is a good idea to retest at least every two years to be sure radon levels stay low. EPA also says to retest in the lowest lived-in area after a remodel.

Can I turn the fan off to save power?

No. EPA says the fan should never be turned off. It must run continuously for the system to work correctly.

Related guides

Sources

Free printable: Radon Test Day Checklist

EPA's testing steps in one page, plus what your result means. Download the one-page PDF now, or get it by email along with a short series of five helpful emails.

Download the PDF