Colusa County, CA: EPA's radon zone, explained

Colusa County is mapped as Zone 3 - Low Potential by EPA — meaning the county-wide average indoor level is predicted below 2 pCi/L. Colusa County sits at the 15th percentile nationally for radon potential — above 15 of every 100 rated counties.

Colusa County by the numbers

Colusa County, California is home to about 22,074 people across 1,150.639 square miles. Averaged across its land area, Colusa County runs about 19 people per square mile. By population, Colusa County is the 577th largest of every county sharing its EPA zone — the 44th percentile by population within that zone. Geologically, Colusa County is grouped into EPA Region 9 alongside 94 other counties that share its rock and soil setting.

What this actually means for a house in Colusa County

Colusa County's predicted average sits below the national indoor average of about 1.3 pCi/L, well under EPA's 4 pCi/L action level. Even at Zone 3, Colusa County homes are not guaranteed a low reading — zone is a county-wide average, and EPA recommends testing every home regardless of zone.

Next steps for a home in Colusa County

For a Colusa 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 California's state radon program

For Colusa County, an ongoing number beats a one-time test - a Airthings radon monitor tracks Colusa County's levels over time. RadonZoneCheck earns a commission there, at no extra cost to you. RADONZONE-10OFF at checkout knocks 10% off.

Where Colusa County's figures come from

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