Marion County, OH: EPA's radon zone, explained

EPA places Marion 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. Nationally, Marion County ranks higher in radon potential than 92% of rated counties (the 92nd percentile).

How Marion County compares

Marion County, Ohio is home to about 64,976 people across 403.807 square miles. Averaged across its land area, Marion County runs about 161 people per square mile. By population, Marion County is the 258th largest of every county sharing its EPA zone — the 76th percentile by population within that zone. Marion County falls in EPA Radon Region 5, one of 524 counties nationally EPA assigns to that same geologic province.

Zone vs. an actual test

Outdoor air nationally averages about 0.4 pCi/L, indoor air about 1.3 pCi/L — Marion County's Zone 1 prediction clears both of those and EPA's fix-it threshold besides. Some Marion County homes will test comfortably under the action level despite the county's Zone 1 rating — the only way to know which kind of home you have is an actual test.

If Marion County tests above the action level

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

Where Marion County's figures come from

The methodology guide shows exactly which EPA and Census sources feed Marion County's numbers. Whatever zone Marion County carries, the zone explainer covers what it does and doesn't tell you about any one house.