What EPA's radon map says about Williams County

EPA's own radon map rates Williams County zone 1 - highest potential, predicting a county-wide average indoor level above 4 pCi/L. Williams County sits at the 88th percentile nationally for radon potential — above 88 of every 100 rated counties.

How Williams County compares

About 40,763 people live in Williams County, North Dakota, spread over 2,077.6 square miles. That works out to roughly 20 people per square mile. Among counties sharing Williams County's EPA zone, it outranks 65% of them by population. Geologically, Williams County is grouped into EPA Region 8 alongside 290 other counties that share its rock and soil setting.

Zone vs. an actual test

Williams 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. EPA's guidance doesn't change because of a Zone 1 label: test the house. What changes is the reason to not put it off in Williams County specifically.

If Williams County tests above the action level

Mitigation cost doesn't depend on Williams 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 North Dakota's state radon program

Williams County's test kit answers today; a Airthings radon monitor keeps answering for Williams County over time. Affiliate link, RadonZoneCheck earns a commission there, at no extra cost to you. Use code RADONZONE-10OFF at checkout for 10% off.

More on how this number was built

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