Continuous radon monitors: how they work and how to read them

C-NRPP sorts electronic radon monitors into consumer-grade and professional-grade, which separates devices made for households from those made for professional use. The old term, continuous radon monitor (CRM), was renamed professional-grade ERM to stress the quality and use of the professional kind.

The trade-off fits in one sentence: some technically advanced active devices offer anti-interference features, and although they may cost more, they may give a more reliable result. If a result has to stand up for someone else, a certified professional with a professional monitor is the safer route; see radon professionals.

Every monitor reports averages over set periods, and each one does it differently. Read the maker's user guide to see how your monitor computes its long-term average, because each model averages its data in its own way. This is how the six consumer monitors C-NRPP approved in 2024/25 report their readings:

1. 1

   Find the long-term average

   On the screen or in the app. Only the long-term average should be compared with a guideline. Some monitors do not report one, and you can calculate it from the interval readings.

2. 2

   Give it time

   At least 3 months, preferably in the heating season. With less than 91 days of data, follow up with a long-term test.

3. 3

   Treat hourly figures as weather

   A few hours or days can read much lower or much higher than your real average.

4. 4

   Check the units

   U.S. guidance is in pCi/L; some monitors default to Bq/m3. Our results interpreter reads either.

The value of a continuous record is the pattern. The report from an active device can reveal unusual swings in the radon level during the test, and in a home sale a device that records frequently is one way to spot swings that suggest interference. At home, the same record lets you see how the level moves with the seasons and how it responds to a change, such as a new mitigation system.

• Seasons. Indoor radon levels are typically highest in the heating season; a monitor run through the year shows it in your own home.
• A fix. A post-mitigation test is still needed within 30 days of installation, no sooner than 24 hours after the system starts; a monitor lets you watch the drop and keep watching afterwards.
• Lag. A 2024 field study found consumer monitors' response times ranged from 1 to 15 hours behind a calibrated reference, so a sudden change shows up late.

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1. Day of installation

   The system goes in

   Keep the fan running at all times: it must run continuously for the system to work correctly.

2. No sooner than 24 hours

   Earliest post-mitigation test

   Test no sooner than 24 hours after your system is in operation with the fan on, if it has one.

3. Within 30 days

   Post-mitigation test done

   A two- to seven-day measurement is recommended, and EPA recommends an independent follow-up radon measurement as well as any test the contractor runs.

4. At least every two years

   Retest

   Retest your home at least every two years to be sure radon levels remain low.

Test conditions: Windows and doors must be closed 12 hours before and during the test, except for normal entry and exit. Not drawn to scale.

After a remodel. After you remodel, retest in the lowest lived-in area to make sure the construction did not reduce the effectiveness of the radon reduction system.

The 2024 study's missing records are a practical point too: a monitor that drops readings, or runs out of storage, leaves gaps in the record. Some monitors store less than 3 months of data and need periodic downloads. For C-NRPP's chamber results and what its grades mean, see independent accuracy results explained.

1. 1

   Lowest lived-in level

   A normal occupancy area: a room someone spends at least 4 hours in each day.

2. 2

   Off the floor, away from walls and drafts

   Away from an exterior wall, sunlight, vents, doors, fans and windows, and away from electronic or network devices, which may affect measurements.

3. 3

   Keep power and storage going

   A plug-in monitor needs an outlet in the room; a battery monitor needs batteries for the whole test. Download or sync before storage fills.

4. 4

   Leave it in place

   Moving it mixes two rooms into one average. Run it for at least 3 months for a decision.

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Test	How long it runs	What it is for
Short-term test	Stays in the home from 2 to 90 days, depending on the device, and for a minimum of 48 hours.	A faster screening result. Because radon levels vary from day to day and season to season, it is less likely than a long-term test to tell you the year-round average.
Long-term test	Stays in the home for more than 90 days.	It is more likely than a short-term test to tell you the home's year-round average radon level.
Continuous monitor	Logs readings for the whole test, run as a short-term or a long-term test under the same durations in EPA guidance.	Logs a graph over time, so you can see trends and spot anomalies rather than a single number.

In a sale, EPA accepts testing the home with a continuous monitor for at least 48 hours as one of three short-term options, and recommends fixing the home if the average radon level is at or above 4 pCi/L. Any real estate test should include steps to prevent or detect interference, such as a device that frequently records radon levels, a motion detector, or records of barometric pressure and temperature.

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If you are testing in a real estate transaction and you need results quickly, any of the following three options for short-term tests are acceptable in determining whether the home should be fixed.

1. Option 1. Two tests at the same time

   Passive

   Take two short-term tests at the same time in the same location for at least 48 hours.

   What to do nextFix the home if the average of the two tests is 4 pCi/L or more.

   Two tests taken at the same time (simultaneous) would improve the precision of this radon test.

2. Option 2. One test, then a second

   Passive

   Take an initial short-term test for at least 48 hours. Immediately upon completing the first test, do a second test using an identical device in the same location as the first test.

   What to do nextFix the home if the average of the two tests is 4 pCi/L or more.

   One test followed by another test (sequential) would most likely give a better representation of the seasonal average.

3. Option 3. A continuous monitor

   Active

   Test the home with a continuous monitor for at least 48 hours.

   What to do nextFix the home if the average radon level is 4 pCi/L or more.

   Both active and passive devices may have features which help to prevent test interference.

Any real estate test for radon should include steps to prevent or detect interference with the test device. Your state radon office can help you decide which option is best.