Marion County, IN: 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. That puts Marion County in the 100th percentile nationally for radon potential — higher than 100% of the counties EPA has rated.

How Marion County compares

Marion County, Indiana is home to about 981,628 people across 395.996 square miles. Population density there runs about 2,479 people per square mile. By population, Marion County is the 9th largest of every county sharing its EPA zone — the 99th percentile by population within that zone. EPA's geology-based Region 5 groups Marion County with 523 other counties nationally sharing its underlying rock and soil profile.

Zone vs. an actual test

Marion 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. 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.

Next steps for a home in Marion County

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 Indiana'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

Every figure on this page traces to EPA's own Map of Radon Zones and Census data, documented in the methodology guide. Whatever zone Marion County carries, the zone explainer covers what it does and doesn't tell you about any one house.