Williamson County, TN: EPA's radon zone, explained

Williamson County is mapped as Zone 1 - Highest Potential by EPA — meaning the county-wide average indoor level is predicted above 4 pCi/L. Williamson County sits at the 97th percentile nationally for radon potential — above 97 of every 100 rated counties.

How Williamson County compares

Williamson County, Tennessee is home to about 269,136 people across 582.911 square miles. Population density there runs about 462 people per square mile. By population, Williamson County is the 84th largest of every county sharing its EPA zone — the 92nd percentile by population within that zone. Williamson County falls in EPA Radon Region 4, one of 736 counties nationally EPA assigns to that same geologic province.

Zone vs. an actual test

Where the national indoor average runs about 1.3 pCi/L, Williamson County's Zone 1 rating puts its predicted county average above EPA's 4 pCi/L action level itself. A Zone 1 rating means testing is worth doing sooner rather than later, but it is still county-wide average, not a reading for any single house — some homes in Williamson County will test well under the action level, and EPA's guidance is to test regardless of zone.

If Williamson County tests above the action level

For a Williamson 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 Tennessee's state radon program

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

The methodology guide shows exactly which EPA and Census sources feed Williamson County's numbers. If you're unsure why a low zone still deserves a test, the zone explainer covers that directly.