Johnston County, OK: EPA's radon zone, explained

Johnston County is mapped as Zone 3 - Low Potential by EPA — meaning the county-wide average indoor level is predicted below 2 pCi/L. Nationally that's the 7th percentile for radon potential — Johnston County outranks 7% of every county EPA has rated.

How Johnston County compares

Johnston County, Oklahoma is home to about 10,293 people across 643.05 square miles. That works out to roughly 16 people per square mile. By population, Johnston County is the 814th largest of every county sharing its EPA zone — the 21st percentile by population within that zone. Johnston County falls in EPA Radon Region 6, one of 503 counties nationally EPA assigns to that same geologic province.

Zone vs. an actual test

Johnston County's Zone 3 prediction undercuts the roughly 1.3 pCi/L most US homes average, nowhere near EPA's 4 pCi/L threshold for recommending mitigation. A Zone 3 rating means Johnston County's county-wide average is predicted low, but EPA is explicit that a meaningful share of homes in low-potential zones still test above the action level — a low zone is not a reason to skip testing.

If Johnston County tests above the action level

For a Johnston County homeowner, the order is simple: test first, since a kit costs a small fraction of what mitigation would. Only if the result clears 4 pCi/L does a system — typically $800–$2,500, about $1,200 for a common install — become worth pricing out.

Find Oklahoma's state radon program

For Johnston County, an ongoing number beats a one-time test - a Airthings radon monitor tracks Johnston County's levels over time. RadonZoneCheck earns a commission there, at no extra cost to you. Use code RADONZONE-10OFF at checkout for 10% off.

Where Johnston County's figures come from

The methodology guide shows exactly which EPA and Census sources feed Johnston County's numbers. Whatever zone Johnston County carries, the zone explainer covers what it does and doesn't tell you about any one house.