What EPA's radon map says about Marion County

EPA's own radon map rates Marion County zone 3 - low potential, predicting a county-wide average indoor level below 2 pCi/L. That puts Marion County in the 17th percentile nationally for radon potential — higher than 17% of the counties EPA has rated.

How Marion County compares

About 28,368 people live in Marion County, South Carolina, spread over 489.372 square miles. Population density there runs about 58 people per square mile. Among counties sharing Marion County's EPA zone, it outranks 51% of them by population. EPA's geology-based Region 4 groups Marion County with 735 other counties nationally sharing its underlying rock and soil profile.

Zone vs. an actual test

A Zone 3 county like Marion County is predicted closer to the 0.4 pCi/L typical outdoors than to EPA's 4 pCi/L action level. A Zone 3 label describes Marion County's average, not any one basement or slab in it — testing is still the only way an individual homeowner actually finds out.

If Marion County tests above the action level

Mitigation cost doesn't depend on Marion County's zone — it depends on the house's foundation. A typical system runs $800–$2,500 (about $1,200 typical), and testing first is what tells you whether that's needed at all.

Find South Carolina's state radon program

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

More on how this number was built

Marion County's zone, rank and region all trace back to named EPA and Census sources — the methodology guide lists exactly which ones. If you're unsure why a low zone still deserves a test, the zone explainer covers that directly.