Maricopa County, AZ: EPA's radon zone, explained

Maricopa County is mapped as Zone 2 - Moderate Potential by EPA — meaning the county-wide average indoor level is predicted between 2 and 4 pCi/L. That puts Maricopa County in the 66th percentile nationally for radon potential — higher than 66% of the counties EPA has rated.

Maricopa County by the numbers

Maricopa County, Arizona is home to about 4,673,096 people across 9,202.021 square miles. Averaged across its land area, Maricopa County runs about 508 people per square mile. By population, Maricopa County is the 3rd largest of every county sharing its EPA zone — the 100th percentile by population within that zone. Maricopa County falls in EPA Radon Region 9, one of 95 counties nationally EPA assigns to that same geologic province.

What this actually means for a house in Maricopa County

EPA's Zone 2 rating for Maricopa County sits below its own 4 pCi/L action level but above the national indoor average of about 1.3 pCi/L. Maricopa County's Zone 2 rating sits in the middle nationally; EPA's guidance is unchanged by that — test every home regardless of its zone.

Testing and mitigation in Maricopa County

For a Maricopa 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 Arizona's state radon program

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

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