St. Mary Parish, LA: EPA's radon zone, explained

St. Mary Parish is mapped as Zone 3 - Low Potential by EPA — meaning the county-wide average indoor level is predicted below 2 pCi/L. Nationally that's the 21st percentile for radon potential — St. Mary Parish outranks 21% of every county EPA has rated.

St. Mary Parish by the numbers

St. Mary Parish, Louisiana is home to about 46,799 people across 558.528 square miles. Averaged across its land area, St. Mary Parish runs about 84 people per square mile. By population, St. Mary Parish is the 361st largest of every county sharing its EPA zone — the 65th percentile by population within that zone. EPA's geology-based Region 6 groups St. Mary Parish with 502 other counties nationally sharing its underlying rock and soil profile.

Why the zone isn't the answer by itself

Well clear of both the 1.3 pCi/L national indoor average and EPA's 4 pCi/L action level, St. Mary Parish's Zone 3 prediction is the low end of the scale. Even at Zone 3, St. Mary Parish homes are not guaranteed a low reading — zone is a county-wide average, and EPA recommends testing every home regardless of zone.

Testing and mitigation in St. Mary Parish

For a St. Mary Parish 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 Louisiana's state radon program

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

Where St. Mary Parish's figures come from

The methodology guide shows exactly which EPA and Census sources feed St. Mary Parish's numbers. The guide to what a radon zone actually means covers why testing matters in every zone, not just Zone 1.