Ohio: 88 counties covered
Of 88 rated Ohio counties, 53 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.
- Franklin County
- Cuyahoga County
- Hamilton County
- Summit County
- Montgomery County
- Lucas County
- Butler County
- Stark County
- Lorain County
- Warren County
- Delaware County
- Lake County
- Mahoning County
- Clermont County
- Trumbull County
- Licking County
- Medina County
- Greene County
- Fairfield County
- Portage County
- Clark County
- Wood County
- Richland County
- Wayne County
- Miami County
- Allen County
- Columbiana County
- Ashtabula County
- Geauga County
- Tuscarawas County
- Muskingum County
- Ross County
- Hancock County
- Erie County
- Scioto County
- Union County
- Marion County
- Belmont County
- Jefferson County
- Knox County
- Athens County
- Pickaway County
- Sandusky County
- Washington County
- Huron County
- Lawrence County
- Seneca County
- Ashland County
- Darke County
- Shelby County
- Logan County
- Auglaize County
- Madison County
- Holmes County
- Brown County
- Highland County
- Mercer County
- Fulton County
- Clinton County
- Crawford County
- Preble County
- Ottawa County
- Champaign County
- Defiance County
- Guernsey County
- Coshocton County
- Williams County
- Morrow County
- Perry County
- Putnam County
- Jackson County
- Hardin County
- Van Wert County
- Gallia County
- Fayette County
- Adams County
- Henry County
- Hocking County
- Pike County
- Carroll County
- Meigs County
- Wyandot County
- Paulding County
- Noble County
- Harrison County
- Morgan County
- Monroe County
- Vinton County