Is Delaware County, PA a radon risk area?

Delaware County is mapped as Zone 1 - Highest Potential by EPA — meaning the county-wide average indoor level is predicted above 4 pCi/L. Rank it against the rest of the country and Delaware County lands in the 99th percentile for radon potential.

How Delaware County compares

About 584,882 people live in Delaware County, Pennsylvania, spread over 183.808 square miles. That works out to roughly 3,182 people per square mile. Rank 31 by population within its EPA zone puts Delaware County at the 97th percentile among zone peers. Geologically, Delaware County is grouped into EPA Region 3 alongside 244 other counties that share its rock and soil setting.

Zone vs. an actual test

Delaware County's predicted average sits above EPA's own action level of 4 pCi/L — the concentration at which EPA recommends fixing a home — versus a national average around 1.3 pCi/L indoors and 0.4 pCi/L outdoors. 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 Delaware County will test well under the action level, and EPA's guidance is to test regardless of zone.

If Delaware County tests above the action level

Nothing about Delaware 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 Pennsylvania's state radon program

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

More on how this number was built

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