Radon Levels Explained (pCi/L & the 4.0 Action Level)

Radon is measured in picocuries per liter of air (pCi/L). The higher the number, the more radon you’re breathing. Here’s how to read your result and know when to act.

The EPA Action Level: 4.0 pCi/L

The EPA recommends that any home testing at or above 4.0 pCi/L have a radon mitigation system installed. They also suggest considering mitigation between 2.0 and 4.0 pCi/L, because there is no truly “safe” level of radon, less is always better. For reference, the World Health Organization recommends acting at about 2.7 pCi/L.

What Different Radon Levels Mean

• Below 2.0 pCi/L: relatively low; retest periodically (levels change over time).
• 2.0–3.9 pCi/L: the EPA suggests considering mitigation; many families choose to mitigate.
• 4.0–9.9 pCi/L: above the action level; mitigation is recommended.
• 10+ pCi/L: well above the action level; mitigate as soon as possible.

Average outdoor radon is around 0.4 pCi/L, and the average indoor level in the U.S. is about 1.3 pCi/L, but in Central Illinois, many homes test far higher. See why Illinois levels run high →

Why Your Level Can Change

Radon readings vary with the seasons, weather, and how your home is sealed and ventilated. Winter often produces higher readings as homes are closed up. That’s why a single test is a snapshot, and why, once you mitigate, a post-install test confirms the system is doing its job year-round.

Got a number at or above 4.0? We can bring it down, guaranteed. Request a free quote →