States Ranked by Radon Risk
Not all states face the same radon risk. Some have nearly every county in EPA Zone 1 (highest risk), while others have almost none. The table below ranks all states by the percentage of their counties classified as Zone 1 — where predicted average indoor radon levels exceed the EPA's 4 pCi/L action level.
States at the top of this list have widespread radon potential driven by underlying geology — typically granite, shale, or phosphate-bearing soils. States near the bottom still have radon in individual homes, which is why the EPA recommends testing all homes regardless of location.
| Rank | State | Zone 1 Counties | Zone 2 | Zone 3 | Total | % Zone 1 |
|---|---|---|---|---|---|---|
| 1 | Iowa | 99 | 0 | 0 | 99 | 100% |
| 2 | North Dakota | 53 | 0 | 0 | 53 | 100% |
| 3 | Wyoming | 21 | 2 | 0 | 23 | 91% |
| 4 | Montana | 50 | 7 | 0 | 57 | 88% |
| 5 | Colorado | 53 | 11 | 0 | 64 | 83% |
| 6 | Minnesota | 68 | 19 | 0 | 87 | 78% |
| 7 | Maine | 12 | 4 | 0 | 16 | 75% |
| 8 | Pennsylvania | 49 | 17 | 1 | 67 | 73% |
| 9 | South Dakota | 48 | 18 | 0 | 66 | 73% |
| 10 | Indiana | 57 | 35 | 0 | 92 | 62% |
| 11 | Kansas | 65 | 40 | 0 | 105 | 62% |
| 12 | Ohio | 53 | 35 | 0 | 88 | 60% |
| 13 | Nebraska | 53 | 24 | 16 | 93 | 57% |
| 14 | Illinois | 56 | 43 | 3 | 102 | 55% |
| 15 | New York | 34 | 13 | 15 | 62 | 55% |
| 16 | Nevada | 9 | 7 | 1 | 17 | 53% |
| 17 | Connecticut | 4 | 3 | 1 | 8 | 50% |
| 18 | Virginia | 64 | 28 | 44 | 136 | 47% |
| 19 | Idaho | 19 | 20 | 5 | 44 | 43% |
| 20 | Tennessee | 39 | 32 | 24 | 95 | 41% |
| 21 | Rhode Island | 2 | 2 | 1 | 5 | 40% |
| 22 | Wisconsin | 28 | 44 | 0 | 72 | 39% |
| 23 | West Virginia | 20 | 29 | 6 | 55 | 36% |
| 24 | Maryland | 8 | 9 | 7 | 24 | 33% |
| 25 | New Jersey | 7 | 11 | 3 | 21 | 33% |
| 26 | Kentucky | 30 | 82 | 8 | 120 | 25% |
| 27 | Utah | 7 | 22 | 0 | 29 | 24% |
| 28 | Massachusetts | 3 | 10 | 1 | 14 | 21% |
| 29 | New Mexico | 7 | 26 | 0 | 33 | 21% |
| 30 | Alabama | 13 | 33 | 21 | 67 | 19% |
| 31 | Washington | 7 | 14 | 18 | 39 | 18% |
| 32 | Michigan | 9 | 32 | 42 | 83 | 11% |
| 33 | New Hampshire | 1 | 9 | 0 | 10 | 10% |
| 34 | Missouri | 11 | 97 | 7 | 115 | 10% |
| 35 | North Carolina | 8 | 31 | 61 | 100 | 8% |
| 36 | California | 2 | 29 | 27 | 58 | 3% |
| 37 | Georgia | 4 | 55 | 100 | 159 | 3% |
| 38 | South Carolina | 1 | 8 | 37 | 46 | 2% |
| 39 | Texas | 0 | 39 | 215 | 254 | 0% |
| 40 | Alaska | 0 | 5 | 22 | 27 | 0% |
| 41 | Louisiana | 0 | 0 | 64 | 64 | 0% |
| 42 | Hawaii | 0 | 0 | 5 | 5 | 0% |
| 43 | Mississippi | 0 | 8 | 74 | 82 | 0% |
| 44 | Oregon | 0 | 21 | 15 | 36 | 0% |
| 45 | Florida | 0 | 9 | 58 | 67 | 0% |
| 46 | Vermont | 0 | 12 | 2 | 14 | 0% |
| 47 | Delaware | 0 | 1 | 2 | 3 | 0% |
| 48 | Oklahoma | 0 | 9 | 68 | 77 | 0% |
| 49 | Arizona | 0 | 15 | 0 | 15 | 0% |
| 50 | Arkansas | 0 | 14 | 61 | 75 | 0% |
The Highest-Risk States
Iowa leads with 99 of 99 counties (100%) in Zone 1. North Dakota leads with 53 of 53 counties (100%) in Zone 1. Wyoming leads with 21 of 23 counties (91%) in Zone 1. Montana leads with 50 of 57 counties (88%) in Zone 1. Colorado leads with 53 of 64 counties (83%) in Zone 1. These states have geology dominated by granite, shale, and glacial deposits that produce higher levels of uranium decay and therefore more radon gas. If you live in any of these states, testing your home should be a high priority.
The Lowest-Risk States
Arkansas has just 0 Zone 1 counties out of 75 (0%). Arizona has just 0 Zone 1 counties out of 15 (0%). Oklahoma has just 0 Zone 1 counties out of 77 (0%). Delaware has just 0 Zone 1 counties out of 3 (0%). Vermont has just 0 Zone 1 counties out of 14 (0%). Even in these states, individual homes can have elevated radon. The EPA's recommendation is universal: test every home, regardless of state or zone classification.