Mercer County, OH: EPA's radon zone, explained
EPA places Mercer County in Zone 1 - Highest Potential — a predicted county-wide average indoor radon level above 4 pCi/L, based on the geology, soil and aerial radioactivity data behind EPA's national map. Rank it against the rest of the country and Mercer County lands in the 89th percentile for radon potential.
Mercer County by the numbers
Mercer County, Ohio is home to about 42,648 people across 462.447 square miles. That works out to roughly 92 people per square mile. By population, Mercer County is the 359th largest of every county sharing its EPA zone — the 66th percentile by population within that zone. Mercer County falls in EPA Radon Region 5, one of 524 counties nationally EPA assigns to that same geologic province.
Why the zone isn't the answer by itself
Fixing a home becomes an EPA recommendation past 4 pCi/L, and that is precisely the side of the line Mercer County's Zone 1 prediction lands on, well above the roughly 1.3 pCi/L most US homes average. That doesn't mean every Mercer County house tests high — a zone is a county-wide prediction, never a single-home reading — but it's a reason to test sooner rather than let it wait.
Testing and mitigation in Mercer County
For a Mercer 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 Ohio's state radon program
For Mercer County, an ongoing number beats a one-time test - a Airthings radon monitor tracks Mercer County's levels over time. RadonZoneCheck earns a commission there, at no extra cost to you. Code RADONZONE-10OFF takes 10% off at checkout.
Where Mercer County's figures come from
Every figure on this page traces to EPA's own Map of Radon Zones and Census data, documented in the methodology guide. The guide to what a radon zone actually means covers why testing matters in every zone, not just Zone 1.