What EPA's radon map says about Franklin County

EPA places Franklin 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 Franklin County lands in the 100th percentile for radon potential.

Franklin County by the numbers

About 1,356,303 people live in Franklin County, Ohio, spread over 532.417 square miles. Averaged across its land area, Franklin County runs about 2,547 people per square mile. Among counties sharing Franklin County's EPA zone, it outranks 100% of them by population. Franklin County falls in EPA Radon Region 5, one of 524 counties nationally EPA assigns to that same geologic province.

Zone vs. an actual test

Franklin County's predicted average sits above EPA's own action level of 4 pCi/L — the concentration at which EPA recommends fixing a home — versus a national average around 1.3 pCi/L indoors and 0.4 pCi/L outdoors. That doesn't mean every Franklin 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 Franklin County

Mitigation cost doesn't depend on Franklin 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

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

Franklin 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.