Short-term vs long-term radon tests

Radon tests fall into two groups based on how long the device stays in place. Each has a job.

Short-term tests (2 to 90 days)

Short-term tests are the usual first step. Common devices include activated charcoal canisters, alpha-track or electret detectors used for short periods, and electronic continuous radon monitors. Charcoal kits typically run 2 to 7 days and are mailed to a lab.

Strengths: fast and inexpensive; good for screening and for real estate deadlines. Limits: radon changes from day to day and season to season, so a short test is a snapshot. It may read higher or lower than your annual average.

Long-term tests (more than 90 days)

Long-term tests typically use alpha-track or electret detectors left in place for 3 months to a year.

Strengths: they average over changing weather and seasons, giving a result closer to your year-round exposure. EPA describes a long-term test as giving a better picture of your average level. Limits: you wait months for an answer.

Continuous radon monitors

Electronic monitors record radon hour by hour. Professional-grade monitors are used by credentialed testers, especially for real estate. Consumer digital monitors can be useful for tracking trends over time; check whether a model is listed or certified for accuracy before relying on it for a decision.

How EPA combines them

EPA’s Citizen’s Guide recommends:

1. Take a short-term test.
2. If the result is 4 pCi/L or higher, take a follow-up test: long-term for a better average, or a second short-term if you need results quickly.
3. Fix the home if the long-term result is 4 pCi/L or higher, or if the average of your first and second short-term tests is 4 pCi/L or higher.

If your first short-term result is below 4 pCi/L, EPA suggests considering a test again in the future, especially if your living patterns change. Our follow-up planner applies these steps.

Seasonal effects to keep in mind

Winter tests with the house closed up often read higher than summer tests. That is one reason a long-term test is valuable when a short-term result is near the decision point, such as 3 to 5 pCi/L.

Quality matters more than type

Whatever you choose, follow the placement and closed-house instructions. EPA notes that test devices meeting recognized standards are reliable when used correctly. Most bad results come from bad placement or open windows, not bad devices. See how to test your home.

Choosing a test by situation

Situation	Good choice	Why
First screening of your home	Short-term test (2 to 7 days)	Fast and inexpensive, and tells you whether to look further
First short-term result at or above 4 pCi/L	Second short-term test, or a long-term test	EPA recommends confirming before fixing
Real estate transaction	Two short-term tests at once, or a continuous radon monitor	Fast results within the deal timeline
Deciding about mitigation at 2 to 4 pCi/L	Long-term test (90+ days)	Captures seasonal variation, giving a better picture of average exposure
Checking a mitigation system	Short-term test 24 hours to 30 days after installation, then periodic retests	Confirms the system works
Ongoing peace of mind	Consumer digital monitor plus periodic lab tests	Shows trends day to day

How EPA uses follow-up results

EPA's guidance to homeowners (in A Citizen's Guide to Radon) works roughly like this:

• If your first short-term test is 4 pCi/L or higher, take a follow-up test.
• If you follow up with a long-term test, fix the home if the long-term result is 4 pCi/L or more.
• If you follow up with a second short-term test, fix the home if the average of the two tests is 4 pCi/L or more.
• If results are between 2 and 4 pCi/L, EPA suggests considering a fix, since there's no known safe level.

Are tests accurate?

EPA has examined device accuracy, including a contractor review of measurement proficiency data done in consultation with its Office of Inspector General. EPA says the results support its position that radon testing devices provide accurate and reliable results. Accuracy in practice depends on following the instructions. Most errors come from bad placement, opening windows, or sending the kit to the lab late.

Consumer digital monitors

Plug-in or battery digital radon monitors have become popular. They're useful for spotting trends: higher levels at night, in winter, or during storms. Things to know:

• Many need a day or more to give a stable reading, and short-term readings fluctuate.
• Consumer devices vary in calibration. For real estate and mitigation decisions, use a lab-analyzed test or a professional continuous monitor.
• A monitor is excellent for checking that a mitigation system keeps working between formal retests.

When results don't match

It's common for two tests to give different numbers. Radon varies with weather, season, HVAC use, and where in the house you test. If results differ:

1. Check whether both tests followed the same conditions and placement.
2. Look at the average rather than either number alone.
3. Consider a long-term test for a better picture.
4. If any result is well above 4 pCi/L, don't wait. Plan to fix.

How often to retest

EPA suggests retesting if your living patterns change, such as starting to use a lower level of the home, and testing again sometime in the future even if earlier results were below 4 pCi/L. Its home buyer's guidance notes that a buyer may want a new test if the last test wasn't recent (for example, within two years) or if the home was renovated after testing. Homes with mitigation systems should be retested periodically. See living with a radon mitigation system.

Whatever test you pick, the best one is the one you actually complete correctly. Start with a simple short-term kit if you haven't tested at all.