Honolulu County, HI: EPA's radon zone, explained

EPA's own radon map rates Honolulu County zone 3 - low potential, predicting a county-wide average indoor level below 2 pCi/L. Honolulu County sits at the 32nd percentile nationally for radon potential — above 32 of every 100 rated counties.

Where Honolulu County sits nationally

Honolulu County, Hawaii is home to about 998,747 people across 600.791 square miles. Population density there runs about 1,662 people per square mile. By population, Honolulu County is the 27th largest of every county sharing its EPA zone — the 97th percentile by population within that zone. Honolulu 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 Honolulu County

A Zone 3 county like Honolulu County is predicted closer to the 0.4 pCi/L typical outdoors than to EPA's 4 pCi/L action level. Low doesn't mean zero: EPA's own data shows homes in Zone 3 counties like Honolulu County testing above the action level often enough that skipping a test isn't the safe assumption.

Testing and mitigation in Honolulu County

For a Honolulu 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 Hawaii's state radon program

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

Where Honolulu County's figures come from

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