What EPA's radon map says about Clark County

Clark County is mapped as Zone 1 - Highest Potential by EPA — meaning the county-wide average indoor level is predicted above 4 pCi/L. That puts Clark County in the 95th percentile nationally for radon potential — higher than 95% of the counties EPA has rated.

How Clark County compares

About 134,985 people live in Clark County, Ohio, spread over 395.879 square miles. Population density there runs about 341 people per square mile. Among counties sharing Clark County's EPA zone, it outranks 86% of them by population. EPA's geology-based Region 5 groups Clark County with 523 other counties nationally sharing its underlying rock and soil profile.

Why the zone isn't the answer by itself

Outdoor air nationally averages about 0.4 pCi/L, indoor air about 1.3 pCi/L — Clark 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 Clark County specifically.

Next steps for a home in Clark County

Mitigation cost doesn't depend on Clark 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 Ohio's state radon program

Clark County's test kit answers today; a Airthings radon monitor keeps answering for Clark 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

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