Illinois: 102 counties covered
Of 102 rated Illinois counties, 56 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.
- Cook County
- DuPage County
- Lake County
- Will County
- Kane County
- McHenry County
- Winnebago County
- Madison County
- St. Clair County
- Champaign County
- Sangamon County
- Peoria County
- McLean County
- Kendall County
- Rock Island County
- Tazewell County
- LaSalle County
- Kankakee County
- DeKalb County
- Macon County
- Vermilion County
- Williamson County
- Adams County
- Whiteside County
- Grundy County
- Boone County
- Jackson County
- Ogle County
- Knox County
- Henry County
- Coles County
- Macoupin County
- Stephenson County
- Woodford County
- Clinton County
- Franklin County
- Marion County
- Jefferson County
- Livingston County
- Monroe County
- Effingham County
- Lee County
- Christian County
- Morgan County
- Fulton County
- Bureau County
- Randolph County
- Montgomery County
- Logan County
- McDonough County
- Iroquois County
- Saline County
- Jo Daviess County
- Fayette County
- Jersey County
- Shelby County
- Perry County
- Douglas County
- Crawford County
- Hancock County
- Union County
- Piatt County
- Bond County
- Edgar County
- Warren County
- Wayne County
- Carroll County
- Richland County
- De Witt County
- Mercer County
- Clark County
- Lawrence County
- Moultrie County
- Pike County
- Massac County
- Washington County
- Ford County
- White County
- Johnson County
- Clay County
- Cass County
- Mason County
- Menard County
- Marshall County
- Greene County
- Wabash County
- Cumberland County
- Jasper County
- Hamilton County
- Schuyler County
- Brown County
- Henderson County
- Edwards County
- Putnam County
- Stark County
- Scott County
- Pulaski County
- Gallatin County
- Alexander County
- Calhoun County
- Pope County
- Hardin County