What EPA's radon map says about Johnston County

EPA's own radon map rates Johnston County zone 3 - low potential, predicting a county-wide average indoor level below 2 pCi/L. Nationally that's the 30th percentile for radon potential — Johnston County outranks 30% of every county EPA has rated.

How Johnston County compares

About 249,794 people live in Johnston County, North Carolina, spread over 792.017 square miles. That works out to roughly 315 people per square mile. Among counties sharing Johnston County's EPA zone, it outranks 90% of them by population. EPA's geology-based Region 4 groups Johnston County with 735 other counties nationally sharing its underlying rock and soil profile.

Zone vs. an actual test

Johnston County's predicted average sits below the national indoor average of about 1.3 pCi/L, well under EPA's 4 pCi/L action level. Don't read Zone 3 as a pass for Johnston County — EPA's guidance to test every home makes no exception for counties predicted low.

If Johnston County tests above the action level

Mitigation cost doesn't depend on Johnston County's zone — it depends on the house's foundation. A typical system runs $800–$2,500 (about $1,200 typical), and testing first is what tells you whether that's needed at all.

Find North Carolina's state radon program

Johnston County's test kit answers today; a Airthings radon monitor keeps answering for Johnston County over time. Affiliate link, RadonZoneCheck earns a commission there, at no extra cost to you. RADONZONE-10OFF at checkout knocks 10% off.

More on how this number was built

Johnston County's zone, rank and region all trace back to named EPA and Census sources — the methodology guide lists exactly which ones. The guide to what a radon zone actually means covers why testing matters in every zone, not just Zone 1.