Foster County, ND: EPA's radon zone, explained

EPA places Foster 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. Rank it against the rest of the country and Foster County lands in the 69th percentile for radon potential.

How Foster County compares

Foster County, North Dakota is home to about 3,323 people across 634.08 square miles. Averaged across its land area, Foster County runs about 5 people per square mile. By population, Foster County is the 974th largest of every county sharing its EPA zone — the 9th percentile by population within that zone. Geologically, Foster County is grouped into EPA Region 8 alongside 290 other counties that share its rock and soil setting.

Why the zone isn't the answer by itself

Foster County carries a Zone 1 label because its predicted average clears 4 pCi/L outright — three-plus times the 0.4 pCi/L typical outdoors and above the 1.3 pCi/L most homes see. That doesn't mean every Foster County house tests high — a zone is a county-wide prediction, never a single-home reading — but it's a reason to test sooner rather than let it wait.

If Foster County tests above the action level

For a Foster 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 North Dakota's state radon program

For Foster County, an ongoing number beats a one-time test - a Airthings radon monitor tracks Foster 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 Foster 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. The guide to what a radon zone actually means covers why testing matters in every zone, not just Zone 1.