Harford County, MD: EPA's radon zone, explained

EPA places Harford County in Zone 1 - Highest Potential — a predicted county-wide average indoor radon level above 4 pCi/L, based on the geology, soil and aerial radioactivity data behind EPA's national map. Rank it against the rest of the country and Harford County lands in the 97th percentile for radon potential.

Harford County by the numbers

Harford County, Maryland is home to about 265,514 people across 437.125 square miles. That works out to roughly 607 people per square mile. By population, Harford County is the 85th largest of every county sharing its EPA zone — the 92nd percentile by population within that zone. EPA's geology-based Region 3 groups Harford County with 244 other counties nationally sharing its underlying rock and soil profile.

Zone vs. an actual test

Outdoor air nationally averages about 0.4 pCi/L, indoor air about 1.3 pCi/L — Harford County's Zone 1 prediction clears both of those and EPA's fix-it threshold besides. 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 Harford County will test well under the action level, and EPA's guidance is to test regardless of zone.

Next steps for a home in Harford County

For a Harford 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 Maryland's state radon program

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

Where Harford County's figures come from

Every figure on this page traces to EPA's own Map of Radon Zones and Census data, documented in the methodology guide. The guide to what a radon zone actually means covers why testing matters in every zone, not just Zone 1.