National data
Radon levels by state
How often tested homes come back over the EPA action level, across 47 states and 2,661 counties. Sort the table, then drill into a state for what mitigation costs there.
| State | Median county % ≥4 | Worst county | Counties w/ data |
|---|---|---|---|
| Iowa | 64.5% | Sioux (84.2%) | 99 |
| South Dakota | 63.7% | Faulk (100%) | 62 |
| North Dakota | 63.6% | Sioux (100%) | 53 |
| Nebraska | 58.6% | Clay (85.1%) | 93 |
| Minnesota | 55.9% | Rock (83.8%) | 87 |
| Ohio | 53.6% | Pickaway (81.4%) | 88 |
| Colorado | 51.6% | Crowley (100%) | 63 |
| Montana | 50% | Treasure (100%) | 51 |
| Pennsylvania | 49% | Fulton (69.7%) | 67 |
| Wisconsin | 46.1% | Menominee (100%) | 72 |
| Wyoming | 43.9% | Sheridan (69.5%) | 23 |
| Illinois | 43% | Stark (80.5%) | 102 |
| Indiana | 42.8% | Fayette (76.9%) | 92 |
| Utah | 42.7% | Beaver (100%) | 28 |
| Idaho | 40% | Bear Lake (78.1%) | 44 |
| Maine | 34% | York (48.4%) | 16 |
| New Hampshire | 33% | Carroll (48.2%) | 10 |
| Kentucky | 30.3% | Meade (68.4%) | 120 |
| Virginia | 30% | Buena Vista (85.7%) | 131 |
| Nevada | 29.9% | Pershing (52%) | 16 |
| Connecticut | 27.7% | Windham (33.7%) | 8 |
| Missouri | 27.3% | Atchison (69.9%) | 115 |
| New Mexico | 27.3% | Quay (100%) | 29 |
| Tennessee | 27.3% | Jefferson (56.8%) | 95 |
| New York | 25.3% | Chemung (52.3%) | 62 |
| Vermont | 24.8% | Orange (38.1%) | 14 |
| Rhode Island | 24.4% | Washington (39.1%) | 5 |
| Alaska | 23.3% | Kodiak Island (50%) | 9 |
| West Virginia | 23.1% | Morgan (58.1%) | 55 |
| Massachusetts | 20.9% | Worcester (33.5%) | 14 |
| Maryland | 20% | Washington (56%) | 24 |
| Georgia | 17.6% | Peach (66.7%) | 129 |
| Delaware | 16.9% | New Castle (17%) | 3 |
| Michigan | 16.6% | Lenawee (56.7%) | 83 |
| North Carolina | 14.7% | Transylvania (47.8%) | 99 |
| Washington | 14.3% | Columbia (100%) | 39 |
| Oregon | 11.5% | Union (42%) | 34 |
| Alabama | 11.1% | Lamar (75%) | 65 |
| California | 10.8% | Alpine (63.2%) | 56 |
| Arizona | 8.9% | Gila (64.7%) | 13 |
| District of Columbia | 8.7% | District of Columbia (8.7%) | 1 |
| Florida | 6.1% | Dixie (100%) | 62 |
| South Carolina | 5% | Oconee (42.6%) | 46 |
| Arkansas | 0% | Miller (100%) | 58 |
| Louisiana | 0% | Franklin (100%) | 36 |
| Oklahoma | 0% | Alfalfa (100%) | 52 |
| Texas | 0% | Carson (100%) | 138 |
How to read this
Each row shows the median of its counties’ “percent over 4 pCi/L” — a single number for how radon-prone a typical county in that state is — alongside the single worst county and how many counties have enough tests to report. Iowa leads the states with published data, but the more useful view is your own: a state’s median hides big county-to-county swings driven by local geology.
The EPA’s action level is 4 pCi/L (picocuries per liter); at or above it, the EPA recommends installing a mitigation system. There is no known safe level of radon. A high state or county figure doesn’t tell you about a specific home — only a test can — but it tells you how seriously to take testing where you live.
The highest-radon states also publish the least
The three states at the very top of the table — Iowa, South Dakota, and North Dakota, each with a median county well over 60% of tested homes above the action level — are also the ones with the thinnest public data. The figures you see for them here come from the CDC’s national baseline; unlike Pennsylvania or Minnesota, these states don’t publish their own fresher, more detailed radon datasets. Iowa collects lab reports by rule but doesn’t release them. So they rank as some of the most radon-prone places in the country on the data that exists — and there’s less of it. If you’re in one of them, treat radon as a serious risk and test: the gaps in the public record are not gaps in the radon.
Source: CDC EPHT (labs), 2008–2017. Centers for Disease Control and Prevention, National Environmental Public Health Tracking Network. Accessed July 2026. ephtracking.cdc.gov. Full methodology. Looking for a specific address? Radon by ZIP code.