Radon in Idaho: radon map by county
EPA's Map of Radon Zones puts 19 of Idaho's 44 counties in Zone 1 (predicted average indoor radon above 4 pCi/L), 20 in Zone 2 and 5 in Zone 3. Of 9,531 building tests that radon laboratories reported for 2008 to 2017, 40.3% were at or above EPA's 4 pCi/L action level, against 36.5% nationally (number 15 of 42 states with 1,000+ tests). The highest county share is Bear Lake County (78.1% of 27 tests).
Radon by county in Idaho
| County | EPA zone | Lab tests | Median | 4+ pCi/L |
|---|---|---|---|---|
| Ada County | Zone 2 | 2,395 | 2.4 | 25.2% |
| Adams County | Zone 3 | 18 | 4.0 | 51.6% |
| Bannock County | Zone 2 | 543 | 3.2 | 41.9% |
| Bear Lake County | Zone 2 | 27 | 7.8 | 78.1% |
| Benewah County | Zone 1 | 21 | 2.4 | 31% |
| Bingham County | Zone 2 | 321 | 4.2 | 52% |
| Blaine County | Zone 1 | 218 | 5.3 | 57.7% |
| Boise County | Zone 1 | 43 | 2.0 | 22.4% |
| Bonner County | Zone 1 | 227 | 2.3 | 33% |
| Bonneville County | Zone 2 | 751 | 3.4 | 40.1% |
| Boundary County | Zone 1 | 38 | 2.1 | 20% |
| Butte County | Zone 1 | 19 | 12.8 | 75.9% |
| Camas County | Zone 1 | 2 | 3.3 | 27.3% |
| Canyon County | Zone 2 | 499 | 1.6 | 10.7% |
| Caribou County | Zone 2 | 44 | 3.4 | 43.9% |
| Cassia County | Zone 2 | 82 | 5.4 | 59.5% |
| Clark County | Zone 1 | 1 | 14.1 | 75% |
| Clearwater County | Zone 1 | 28 | 1.8 | 30.3% |
| Custer County | Zone 1 | 19 | 6.8 | 72.7% |
| Elmore County | Zone 1 | 119 | 3.8 | 48.1% |
| Franklin County | Zone 2 | 58 | 3.1 | 34.7% |
| Fremont County | Zone 1 | 65 | 3.9 | 49.5% |
| Gem County | Zone 3 | 75 | 1.9 | 20.2% |
| Gooding County | Zone 1 | 54 | 2.4 | 28.4% |
| Idaho County | Zone 1 | 61 | 2.9 | 40% |
| Jefferson County | Zone 2 | 144 | 5.7 | 67.1% |
| Jerome County | Zone 2 | 57 | 1.9 | 16.9% |
| Kootenai County | Zone 1 | 2,103 | 6.0 | 61% |
| Latah County | Zone 1 | 108 | 1.7 | 26% |
| Lemhi County | Zone 1 | 78 | 5.2 | 65.2% |
| Lewis County | Zone 3 | 6 | 3.8 | 50% |
| Lincoln County | Zone 2 | 15 | 3.3 | 40% |
| Madison County | Zone 2 | 200 | 6.2 | 63.7% |
| Minidoka County | Zone 2 | 65 | 1.8 | 15.3% |
| Nez Perce County | Zone 3 | 124 | 2.3 | 22.1% |
| Oneida County | Zone 2 | 21 | 6.5 | 74.1% |
| Owyhee County | Zone 2 | 26 | 2.1 | 23.3% |
| Payette County | Zone 2 | 55 | 1.5 | 4.9% |
| Power County | Zone 2 | 34 | 2.5 | 26.8% |
| Shoshone County | Zone 1 | 57 | 5.7 | 63% |
| Teton County | Zone 2 | 283 | 4.4 | 53.3% |
| Twin Falls County | Zone 2 | 286 | 2.4 | 24.1% |
| Valley County | Zone 1 | 85 | 2.3 | 27.3% |
| Washington County | Zone 3 | 56 | 2.1 | 20.9% |
Lab tests: buildings tested by radon laboratories, 2008 to 2017; median in pCi/L; “4+ pCi/L” is the share of tests at or above EPA's action level. n/a: no value on file (no tests, or too few to report).
What the zones mean
Zone 1 counties have a predicted average indoor radon screening level above 4 pCi/L, Zone 2 from 2 to 4 pCi/L and Zone 3 below 2 pCi/L. The prediction is from 1993 indoor measurements, geology, aerial radioactivity, soils and foundation types. EPA: “This map should not be used to determine if individual homes need to be tested.” EPA recommends fixing a home at 4 pCi/L or more and considering a fix between 2 and 4 pCi/L, because there is no known safe level of exposure.
Radon in Idaho: questions
What radon zone is Idaho?
EPA's 1993 map does not give states one zone; it zones each county. In Idaho, 19 counties are Zone 1, 20 Zone 2 and 5 Zone 3. EPA says the map "should not be used to determine if individual homes need to be tested".
Which Idaho counties have the highest radon levels?
By the share of laboratory tests at or above 4 pCi/L for 2008 to 2017 (counties with 20 or more tests): Bear Lake County 78.1% (median 7.8 pCi/L, 27 tests); Oneida County 74.1% (median 6.5 pCi/L, 21 tests); Jefferson County 67.1% (median 5.7 pCi/L, 144 tests); Lemhi County 65.2% (median 5.2 pCi/L, 78 tests); Madison County 63.7% (median 6.2 pCi/L, 200 tests).
How common is high radon in Idaho?
40.3% of the 9,531 tests in the laboratory data for Idaho (2008 to 2017) were at or above 4 pCi/L; the national share was 36.5%. Buildings are tested by choice, often because of a sale or a concern, so the share describes the tests on file, not every home.
Should I test for radon in Idaho?
Yes. EPA recommends testing every home no matter where it is, fixing it at 4 pCi/L or more and considering a fix between 2 and 4 pCi/L, because there is no known safe level of radon exposure.
States bordering Idaho
Other free hazard lookups
- Radon by county and ZIP codeEPA radon zone and the radon test results on file for any county, ZIP code or address.
- Fault lines near an addressUSGS-mapped earthquake faults within 5 miles, their age and slip rate, and the USGS seismic hazard value.
- Power lines near an addressHigh-voltage transmission lines within 1 mile from federal maps, with voltage and distance.
- Flood zone lookupThe FEMA flood zone mapped at any US address. Not an official flood zone determination.
- Superfund sites near an addressThe nearest EPA Superfund (NPL) sites and their status.
All of these, plus wetlands, FEMA hazard ratings and the federal contamination lists, are in the Environmental Records Report for any US address ($29).
Superfund sites in Idaho · Radon map of every state
Data compiled 2026-10-20. County-level screening information, not a radon measurement of any building.