Johnson County, KS radon potential

Johnson County is mapped as Zone 1 - Highest Potential by EPA — meaning the county-wide average indoor level is predicted above 4 pCi/L. Johnson County sits at the 99th percentile nationally for radon potential — above 99 of every 100 rated counties.

Where Johnson County sits nationally

Johnson County, Kansas is home to about 632,276 people across 473.629 square miles. Averaged across its land area, Johnson County runs about 1,335 people per square mile. Within its own EPA zone, Johnson County is bigger by population than 97% of the counties that share it. Geologically, Johnson County is grouped into EPA Region 7 alongside 411 other counties that share its rock and soil setting.

What this actually means for a house in Johnson County

Outdoor air nationally averages about 0.4 pCi/L, indoor air about 1.3 pCi/L — Johnson County's Zone 1 prediction clears both of those and EPA's fix-it threshold besides. EPA's guidance doesn't change because of a Zone 1 label: test the house. What changes is the reason to not put it off in Johnson County specifically.

Next steps for a home in Johnson County

A short-term test kit is the standard first step and typically costs far less than mitigation itself. If a test comes back at or above 4 pCi/L, a radon mitigation system for a typical home runs roughly $800–$2,500 (around $1,200 for a common single-family install), driven mainly by foundation type, not by county.

Find Kansas'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

Johnson County's zone, rank and region all trace back to named EPA and Census sources — the methodology guide lists exactly which ones. If you're unsure why a low zone still deserves a test, the zone explainer covers that directly.