Boulder County, CO: EPA's radon zone, explained

Boulder County is mapped as Zone 1 - Highest Potential by EPA — meaning the county-wide average indoor level is predicted above 4 pCi/L. Rank it against the rest of the country and Boulder County lands in the 98th percentile for radon potential.

How Boulder County compares

Boulder County, Colorado is home to about 330,262 people across 726.416 square miles. That works out to roughly 455 people per square mile. By population, Boulder County is the 71st largest of every county sharing its EPA zone — the 93rd percentile by population within that zone. EPA's geology-based Region 8 groups Boulder County with 290 other counties nationally sharing its underlying rock and soil profile.

Zone vs. an actual test

Boulder 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 Boulder 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 Boulder County tests above the action level

For a Boulder 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 Colorado's state radon program

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

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