Is Jefferson County, CO a radon risk area?

EPA places Jefferson 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 that's the 99th percentile for radon potential — Jefferson County outranks 99% of every county EPA has rated.

Jefferson County by the numbers

About 578,533 people live in Jefferson County, Colorado, spread over 764.424 square miles. That works out to roughly 757 people per square mile. Rank 32 by population within its EPA zone puts Jefferson County at the 97th percentile among zone peers. Geologically, Jefferson 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

Where the national indoor average runs about 1.3 pCi/L, Jefferson 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 Jefferson County will test well under the action level, and EPA's guidance is to test regardless of zone.

If Jefferson County tests above the action level

Nothing about Jefferson County's EPA zone changes the mitigation number: $800–$2,500 nationally (around $1,200 typical), set by the house's foundation rather than the county it sits in — and only relevant once a test actually clears 4 pCi/L.

Find Colorado's state radon program

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

More on how this number was built

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