What Is a Continuous Radon Monitor?
A continuous radon monitor, or CRM, is an electronic instrument that measures radon in the air and records a reading every hour. It’s the standard tool for professional radon testing, especially in real estate. Unlike a charcoal kit that you mail to a lab, a CRM gives a result right after the test, along with a full record of how radon levels moved during it.
Step by Step: The CRM Test
Step 1: Closed-house conditions
For 12 hours before the test and for the whole test, keep windows and outside doors closed except for normal coming and going. Don’t run whole-house fans or window units that bring in outside air. Your furnace and central air can run as usual. These rules keep the test from being skewed by fresh air.
Step 2: Placement
An Iowa HHS-credentialed technician places the monitor on the lowest livable level of the home. That’s the lowest floor that’s used, or could be used, as living space. In most Iowa homes, that’s the basement.
Good placement means:
- About 20 inches or more off the floor
- Away from exterior walls, windows and doors
- Away from drafts, vents and heat sources
- Not in a bathroom, kitchen, laundry room or crawlspace
- Not right next to a sump pit
Step 3: The 48-hour minimum
The test runs for at least 48 hours. During that time, the monitor samples the air and logs an hourly radon reading in picocuries per liter (pCi/L).
Step 4: Tamper and pressure tracking
Along with radon, the monitor records other data. Most professional units log:
- Motion or tilt, to flag if the monitor was moved
- Barometric pressure, since pressure changes can push radon levels up or down
- Temperature and humidity, which help spot an open window or door
If the data shows the monitor was moved or the house was opened up, the test may need to be repeated.
Step 5: Retrieval and the report
The technician picks up the monitor and downloads the data. Your report shows your average radon concentration over the test and a chart of the hourly readings.
How to Read Your CRM Report
| Report item | What it tells you |
|---|---|
| Average radon (pCi/L) | The main result; compare it to the EPA 4.0 action level |
| Hourly readings | How levels rose and fell over the test |
| Test start and end | Confirms the 48-hour minimum was met |
| Barometric pressure | Helps explain spikes or dips |
| Tamper flags | Whether the monitor was disturbed |
| Placement notes | Where in the home the test was taken |
It’s normal for hourly readings to swing up and down. Radon often rises overnight and early morning when the house is closed and temperatures drop, then dips in the afternoon. The average is what you act on.
Reading the number
At or above 4.0 pCi/L, the EPA recommends mitigation. Between 2.0 and 3.9, it suggests considering it. Our guide on what radon level is safe explains each range.
CRM vs. Other Test Types
A CRM test is a short-term test. It’s fast and detailed, which makes it the right choice for real estate deals and post-mitigation clearance. Long-term tests of 90 days or more better reflect your year-round average. Charcoal kits are a low-cost first check but are easy to mishandle. Compare them in test types compared and DIY vs. professional testing.
Why Use a Professional CRM Test?
- Speed. Results right after the test, not days later from a lab.
- Detail. Hourly data shows the pattern, not just one number.
- Trust. Tamper tracking and credentialed placement hold up in a home sale.
- Guidance. If your result is high, we explain it and offer a free virtual estimate for mitigation.
If you’re planning a test, let us know your deadline, whether it’s a real estate test, and the town you’re in. We’ll schedule placement and pickup around your timeline.