Maryland: 24 counties covered
Of 24 rated Maryland counties, 8 are in EPA's highest radon-potential zone (Zone 1) — but every county page links to testing guidance regardless of zone, because a county rating is a county-wide average, not a reading for any one house.
- Montgomery County
- Prince George's County
- Baltimore County
- Anne Arundel County
- Baltimore city
- Howard County
- Frederick County
- Harford County
- Carroll County
- Charles County
- Washington County
- St. Mary's County
- Wicomico County
- Cecil County
- Calvert County
- Allegany County
- Worcester County
- Queen Anne's County
- Talbot County
- Caroline County
- Dorchester County
- Garrett County
- Somerset County
- Kent County