Washington County, IN: EPA's radon zone, explained

EPA's own radon map rates Washington County zone 1 - highest potential, predicting a county-wide average indoor level above 4 pCi/L. Nationally that's the 85th percentile for radon potential — Washington County outranks 85% of every county EPA has rated.

Where Washington County sits nationally

Washington County, Indiana is home to about 28,345 people across 513.689 square miles. Averaged across its land area, Washington County runs about 55 people per square mile. By population, Washington County is the 482nd largest of every county sharing its EPA zone — the 55th percentile by population within that zone. Washington County falls in EPA Radon Region 5, one of 524 counties nationally EPA assigns to that same geologic province.

Zone vs. an actual test

Washington 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. Being in Zone 1 raises the odds Washington County homes test above the action level, but it does not mean any specific house does — EPA recommends every home be tested, in every zone, because the only way to know is to test.

If Washington County tests above the action level

For a Washington 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 Washington County, an ongoing number beats a one-time test - a Airthings radon monitor tracks Washington 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 Washington County's figures come from

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