Radon Test Results in Boise and the Treasure Valley, by County and ZIP Code

1. 1

   In Ada County, 25.2% of radon tests reported to CDC by national laboratories for 2008 to 2017 (763 of 3,030 tests) were at or above EPA's action level of 4 pCi/L. The median was 2.4 pCi/L and the highest reading was 67.3 pCi/L. Source [1], [2]

2. 2

   A further 35.4% of Ada County tests in the same data (1,074 of 3,030) fell between 2 and 4 pCi/L, the range in which EPA says to consider fixing a home. Source [1], [8]

3. 3

   Radon varies sharply inside Boise: in ZIP code 83702, 563 of 1,471 home radon tests on record with the state (38%) were over 3.9 pCi/L, compared with 166 of 1,120 (15%) in ZIP code 83709. Source [4], [5]

4. 4

   Elmore County had the highest share of elevated tests in the Treasure Valley in the CDC data, 48.1% (76 of 158 tests), and the state's records for Mountain Home ZIP code 83647 show 202 of 425 tests (48%) over 3.9 pCi/L. Source [1], [4]

5. 5

   Canyon County tested lower than Ada: 10.7% of tests (60 of 563) were at or above 4 pCi/L in the CDC data, though one Canyon County test reached 62.8 pCi/L. Source [1], [2]

6. 6

   In the suburbs the state's ZIP-level shares are lower but not zero: Meridian 83642, 137 of 873 tests over 3.9 pCi/L (16%); Meridian 83646, 84 of 683 (12%); Eagle 83616, 83 of 644 (13%); Nampa 83686, 65 of 534 (12%). Source [4]

7. 7

   EPA maps Gem County as Zone 3, its lowest radon potential, yet 20.2% of Gem County tests in the CDC data (20 of 99) were at or above 4 pCi/L, and state records for Emmett ZIP code 83617 show 47 of 235 tests (20%) over 3.9 pCi/L. Source [7], [1], [4]

8. 8

   EPA assigns 19 of Idaho's 44 counties to Zone 1 (highest radon potential), 20 to Zone 2 and 5 to Zone 3. Ada and Canyon counties are Zone 2; Elmore, Boise and Valley counties are Zone 1. Source [7]

9. 9

   The Idaho Department of Health and Welfare states that "2 out of 5 Idaho homes have elevated radon levels" and its radon map reports 46,406 tests as of May 2026. Source [6], [5]

10. 10

    Idaho has no statute that mentions radon: a search of the Idaho Statutes on the Legislature's website returns zero results for the word, and the state health department says radon control methods in the residential building code "are not required by law in Idaho unless a city or county has specifically adopted them." Source [11], [6]

11. 11

    The Idaho Department of Health and Welfare says professional radon mitigation "typically costs $1,500-$3,500" and radon-resistant features in a new home "$300-$500." Source [6]

12. 12

    EPA's Citizen's Guide to Radon says to test before converting an unfinished basement to living space and, because major renovations can change radon levels, to "always test again after work is completed." Source [10]

Shares are the count over 3.9 pCi/L divided by tests, rounded to a whole percent. All 271 Idaho ZIP codes are in the download.

• Voluntary tests, not a random sample of homes. Homes that were tested may differ from homes that were not.
• CDC's Idaho lab data ends in 2017.
• The state does not publish when its ZIP counts begin, which laboratories report, or average and maximum readings by ZIP code.
• The state map's stated total (46,406) is 348 more than its ZIP rows add up to (46,058). The state does not explain the difference.
• The state's "2 out of 5 Idaho homes" wording describes tested homes; its own ZIP data works out to 37.3% of tests over 3.9 pCi/L (our arithmetic).
• CDC's data service returns the three radon bands without labels. Their order was established by checking counts against percentages across all 44 counties.
• The statute search covers Idaho Statutes only. Whether Boise, Meridian, Nampa or other jurisdictions have locally adopted radon construction rules was not checked.
• EPA's zone map was developed in 1993 and predicts county averages; EPA says it is not meant to show whether an individual home should be tested.