Is Marlboro County, SC a radon risk area?

EPA places Marlboro County in Zone 3 - Low Potential — a predicted county-wide average indoor radon level below 2 pCi/L, based on the geology, soil and aerial radioactivity data behind EPA's national map. That puts Marlboro County in the 16th percentile nationally for radon potential — higher than 16% of the counties EPA has rated.

Marlboro County by the numbers

About 25,498 people live in Marlboro County, South Carolina, spread over 479.874 square miles. Population density there runs about 53 people per square mile. Rank 537 by population within its EPA zone puts Marlboro County at the 48th percentile among zone peers. EPA's geology-based Region 4 groups Marlboro County with 735 other counties nationally sharing its underlying rock and soil profile.

Zone vs. an actual test

Marlboro County's Zone 3 prediction undercuts the roughly 1.3 pCi/L most US homes average, nowhere near EPA's 4 pCi/L threshold for recommending mitigation. Low doesn't mean zero: EPA's own data shows homes in Zone 3 counties like Marlboro County testing above the action level often enough that skipping a test isn't the safe assumption.

Next steps for a home in Marlboro County

Nothing about Marlboro 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 South Carolina's state radon program

Marlboro County's test kit answers today; a Airthings radon monitor keeps answering for Marlboro 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

Marlboro County's zone, rank and region all trace back to named EPA and Census sources — the methodology guide lists exactly which ones. The guide to what a radon zone actually means covers why testing matters in every zone, not just Zone 1.