St. Croix County, WI: EPA's radon zone, explained

EPA places St. Croix 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. Nationally, St. Croix County ranks higher in radon potential than 94% of rated counties (the 94th percentile).

Where St. Croix County sits nationally

St. Croix County, Wisconsin is home to about 97,954 people across 722.158 square miles. Averaged across its land area, St. Croix County runs about 136 people per square mile. By population, St. Croix County is the 200th largest of every county sharing its EPA zone — the 81st percentile by population within that zone. EPA's geology-based Region 5 groups St. Croix County with 523 other counties nationally sharing its underlying rock and soil profile.

Why the zone isn't the answer by itself

Outdoor air nationally averages about 0.4 pCi/L, indoor air about 1.3 pCi/L — St. Croix County's Zone 1 prediction clears both of those and EPA's fix-it threshold besides. Some St. Croix 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.

If St. Croix County tests above the action level

For a St. Croix 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 Wisconsin's state radon program

For St. Croix County, an ongoing number beats a one-time test - a Airthings radon monitor tracks St. Croix 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 St. Croix 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 St. Croix County carries, the zone explainer covers what it does and doesn't tell you about any one house.