Radon Levels Explained: What Your Test Result Means

A radon result arrives as a single number, often with little explanation. Whether it came from a free TDEC kit, a store-bought test or a professional monitor during a home sale, this page explains what the number means and what to do next. We're a free referral service, not a contractor; we connect Rutherford County homeowners with independent certified radon professionals when a result calls for one.

The units: pCi/L and Bq/m³

In the U.S., radon in air is reported in picocuries per liter (pCi/L), a measure of radioactive decay per liter of air. Much of the world, including the World Health Organization, uses becquerels per cubic meter (Bq/m³). One pCi/L equals 37 Bq/m³, so 4 pCi/L is about 148 Bq/m³ (the EPA rounds it to 150) and 2 pCi/L about 75. Some digital monitors let you switch units, so check which one your screen shows before you worry.

The benchmarks that matter

• 0.4 pCi/L: the average level in outdoor air, according to the EPA.
• 1.3 pCi/L: the average indoor level in U.S. homes.
• 2 pCi/L: the EPA says to consider fixing between 2 and 4. It also notes that levels below 2 are difficult to reduce further.
• About 2.7 pCi/L (100 Bq/m³): the WHO's recommended reference level. The WHO says countries shouldn't set it above 300 Bq/m³ (about 8 pCi/L).
• 4 pCi/L: the EPA action level, where it recommends fixing the home.
• 8 pCi/L and up: more than twice the action level. The EPA says to take a second short-term test immediately rather than waiting.

How Rutherford County results compare

Rutherford County is in EPA Radon Zone 1. The CDC's tracking data for 2008 to 2017 shows 977 buildings tested, with a median of 1.5 pCi/L and 15.3% of results at or above 4. TDEC reports that about one in four homes requesting its free state kit test above 4. Those numbers point the same way: most homes are moderate, but a meaningful share are high, and only your own test tells you which group you're in.

What the EPA says to do next

The EPA's Citizen's Guide lays out a simple sequence:

• First short-term test below 4: no immediate action, but consider testing again later, especially in a different season or after renovations.
• First short-term test at 4 or higher: follow up. For the best picture of your year-round average, use a long-term test (more than 90 days). If you need an answer quickly, use a second short-term test.
• Followed up with a long-term test: fix the home if the long-term result is 4 or more.
• Followed up with a second short-term test: consider fixing if the average of the two is 4 or higher. The higher both results are, the more certain you can be.

Short-term devices stay in place for 2 to 90 days; long-term devices for more than 90 days. See radon testing in Murfreesboro for which kit to use.

Why two tests in the same house disagree

Radon changes hour to hour and season to season. In Middle Tennessee, it tends to run higher in winter, when the house is closed and warm air rising inside draws soil gas through the foundation. Heavy rain, wind and how hard the HVAC runs all shift the number. Test placement matters too: the EPA's guidance is the lowest lived-in level, about 20 inches above the floor, away from drafts, with windows and doors closed for 12 hours before and during a short-term test. A kit left on a basement floor near a sump, or in a kitchen with the windows open, won't give a fair reading.

Continuous monitors and home-sale tests

Professional continuous monitors log hourly readings for at least 48 hours and report the average. Hourly spikes above 4 are normal even in homes with a low average; the average is what you act on. During a sale, the EPA's guidance on real estate testing applies, and the clock is short. Our home sale radon page covers the timeline.

After a system is installed

A working system often brings levels well below 4. The EPA says sub-slab suction typically reduces radon 50% to 99%. Retest within 30 days, but at least 24 hours after the system starts, and again every two years. Most certified installers put their reduction guarantee in writing. Costs typically run about $805 to $1,326 nationally (Angi 2026); see the Murfreesboro cost guide and system maintenance.

Frequently asked questions

Is 3 pCi/L of radon bad?

It's below the EPA's 4 pCi/L action level but above the WHO reference level of about 2.7 pCi/L. The EPA says to consider fixing between 2 and 4. A long-term test gives a better picture of your year-round average before you decide.

What does 148 Bq/m3 mean in pCi/L?

About 4 pCi/L. One pCi/L equals 37 Bq/m3, so divide becquerels by 37 to get picocuries.

My test was 9 pCi/L. What should I do?

The EPA says that when a first short-term result is more than twice the 4 pCi/L action level, you should take a second short-term test immediately. If the average of the two is 4 or higher, plan to fix the home.

Why is my radon higher in winter?

A closed, heated house draws soil gas through the foundation, a process called the stack effect. Rain-saturated soil and less ventilation can also push levels up in cooler months.

What level should radon be after mitigation?

Below 4 pCi/L at a minimum. Many systems get well under that; the EPA says sub-slab suction typically cuts radon 50% to 99%. Retest within 30 days to confirm.

Where this information comes from

• U.S. EPA, A Citizen's Guide to Radon (2016 PDF, includes the myths and facts list)
• U.S. EPA, Health Risk of Radon (lifetime risk chart, 21,000 deaths estimate)
• World Health Organization, Radon fact sheet
• U.S. EPA, Consumer's Guide to Radon Reduction (2016 PDF)
• EPA Map of Radon Zones, county data (Rutherford County, TN: Zone 1)
• Tennessee Department of Environment and Conservation: Tennessee Radon Program
• CDC Environmental Public Health Tracking Network, Radon Tests from Labs 2008-2017 (county summary via lung.com)
• Angi, How Much Does Radon Mitigation Cost? (2026 data)