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

EPA places Arapahoe 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 Arapahoe County lands in the 99th percentile for radon potential.

Where Arapahoe County sits nationally

Arapahoe County, Colorado is home to about 666,918 people across 797.887 square miles. Population density there runs about 836 people per square mile. By population, Arapahoe County is the 25th largest of every county sharing its EPA zone — the 98th percentile by population within that zone. Arapahoe County falls in EPA Radon Region 8, one of 291 counties nationally EPA assigns to that same geologic province.

Why the zone isn't the answer by itself

Arapahoe 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. 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 Arapahoe County will test well under the action level, and EPA's guidance is to test regardless of zone.

If Arapahoe County tests above the action level

For a Arapahoe 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 Arapahoe County, an ongoing number beats a one-time test - a Airthings radon monitor tracks Arapahoe 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 Arapahoe 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.