Johnson County, MO: EPA's radon zone, explained

EPA's own radon map rates Johnson County zone 2 - moderate potential, predicting a county-wide average indoor level between 2 and 4 pCi/L. Nationally, Johnson County ranks higher in radon potential than 56% of rated counties (the 56th percentile).

Where Johnson County sits nationally

Johnson County, Missouri is home to about 55,880 people across 829.262 square miles. Population density there runs about 67 people per square mile. By population, Johnson County is the 323rd largest of every county sharing its EPA zone — the 69th percentile by population within that zone. Geologically, Johnson County is grouped into EPA Region 7 alongside 411 other counties that share its rock and soil setting.

Why the zone isn't the answer by itself

Call it a middle case: above the 1.3 pCi/L most US homes average, below the 4 pCi/L line EPA uses to recommend mitigation — that's Johnson County's Zone 2 prediction. Johnson County's Zone 2 rating sits in the middle nationally; EPA's guidance is unchanged by that — test every home regardless of its zone.

Next steps for a home in Johnson County

For a Johnson 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 Missouri's state radon program

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

Where Johnson County's figures come from

The methodology guide shows exactly which EPA and Census sources feed Johnson County's numbers. The guide to what a radon zone actually means covers why testing matters in every zone, not just Zone 1.