Long-term kit
RL416 long-term radon test kit, lab fee included
- Lab fee included
Method: Alpha track
In home: 90 days to 12 months
Locations and reports
Radon Levels in Clark County, Idaho
EPA puts Clark County in Zone 1, its highest radon zone, and only 4 radon tests from it are on file with CDC (2008 to 2017, laboratory totals). Homes with elevated levels occur in all three zones, so only a test measures yours.
Data checked By Radon411Methodology
Highest bordering
67.1%
Jefferson County
- Jefferson
- 67.1%
- Fremont
- 49.5%
4 of 5 neighbours have 30+ tests, 2008 to 2017
Nearest county
67.1%
Jefferson County, 32 miles
- Jefferson
- 67.1%
- Idaho
- 41.5%
Share at 4+ pCi/L, 207 tests, 2008 to 2017
Ground above low
100.0%
8.0 pts above Idaho
- Clark
- 100.0%
- Idaho
- 92.0%
Share of land USGS rates moderate or high potential
Tests reported
4
Too few for a share
- Clark
- 4
- Needed
- 30
2008 to 2017, laboratory
EPA radon zone
Zone 1
Highest potential
- Zone 1
- 2
- 3
1 highest, 3 lowest
EPA predicts over 4 pCi/L indoors
Read a result for Clark County
Starts at EPA's action level
EPA’s own thresholds; the figures are the area’s, never this home’s.
EPA’s reading of 4.0 pCi/L
At or above 4 pCi/L
- Under 20 to 1.9 pCi/L
- 2 to 4consider a fix
- 4 and aboveEPA says fix it
This result is at or above 4 pCi/L. EPA recommends fixing a home when the radon level is 4 pCi/L or higher.
The data
Clark County, At a glance
Clark County at a glance
Predicted: EPA radon zone
EPA Zone 1: highest predicted potential
EPA puts all of Clark County in
USGS geologic potential: High across 67% of the county's area
Predicted: EPA Map of Radon Zones
Measured: tests reported to CDC
Radon tests reported for Clark County, 2008 to 2017, laboratory
CDC holds 4 reported tests for Clark County over the 2008 to 2017 laboratory totals, with a median of 14.1 pCi/L: too few to describe a county (Radon411 reads a share from 30 tests up). Clark County beside the counties around it is how this page reads it.
What these figures count
Only 4 tests were reported to CDC for Clark County over 2008 to 2017 (laboratory totals), and tests are often run on homes being sold or already suspected of a problem: so few describe neither the county nor this house.
Measured: CDC tracking data. Figure above: 2008 to 2017, laboratory. Measured data period: 2005 to 2017 (every CDC figure on file).
Clark County, Compared
Clark County radon figures, compared
Calculated by Radon411CDC's largest figure for Clark County rests on 4 tests over the 2008 to 2017 laboratory totals, 3 of them at or above 4 pCi/L, so each test moves its share by 25.0 points and no rank or comparison is drawn from it. Results reached as high as 26.4 pCi/L, with a middle value of 14.1 pCi/L.
Of the counties bordering it, 4 have a figure over the 2008 to 2017 laboratory totals: Jefferson County reported the largest share at or above 4 pCi/L, 67.1%, and Fremont County the smallest, 49.5%, with Lemhi County at 65.2% and Beaverhead County, MT at 59.5% between. Farther out, Madison County (49 miles away) at 63.7%, Teton County (68 miles away) at 53.3% and Bonneville County (72 miles away) at 40.1% have figures too. Pooled across 44 Idaho counties, 41.5% of reported tests reached that level; across every county in the country with a figure, 35.7%.
Statewide for the same period, Bear Lake County reported the highest share (78.1%) and Payette County the lowest (4.9%). EPA's 1993 map puts Clark County in Zone 1, the map's highest label: a predicted county average above 4 pCi/L. USGS geology rates 67% of the county's land high radon potential.
Pooled shares: Radon411's sum of CDC's county counts for one period.
Clark County, Geology
The ground under Clark County
PredictedUSGS's geologic map splits Clark County: 66.7% of its land is high radon potential and 33.3% moderate. The map puts 66.6% of Clark County in the area it calls Northern Rocky Mountain Province and 33.3% in Eastern Snake river plain and Malheur-Boise-King Section. USGS gives the first a radon index of 12, in the high band (12 to 17), and the second 10, in the moderate band (9 to 11).
| Area | The rock, in USGS's words | Potential | Radon index | Share of land |
|---|---|---|---|---|
| Northern Rocky Mountain Province | Diorite and granite of the Idaho batholith. | High | 12 | 66.6% |
| Eastern Snake river plain and Malheur-Boise-King Section | Basalt flows. | Moderate | 10 | 33.3% |
Names, descriptions and indexes: U.S. Geological Survey, Geologic Radon Potential Map (2025). Shares: Radon411, from the map cut to the boundary.
Clark County beside the ground around it
Clark County has more high-potential ground than Idaho as a whole: 66.7% of its land against 50.7%. Of the 5 counties that border it, 3 have more high-potential ground, Beaverhead County, MT the most at 100%.
The map rates the ground, not the air in any home.
Local help
Clark County, Local resources
Radon testing and mitigation in Clark County
- DHW Environmental Health ProgramDHW Environmental Health Program, run by Idaho Department of Health and Welfare, offers free or discounted test kits, and answers questions about testing and mitigation in Idaho.Visit the program’s page(opens in a new tab)
- Free and low-cost kitsThe program says:
Order a free radon test kit.
- Compare kits
- See the Idaho directoryNo professionals are listed in Idaho yet. The state program can point you to registered testers and mitigators.
What Idaho's program says
- Phone
- 800-445-8647
- radon@dhw.idaho.gov
What it means
Clark County, Questions
Questions about radon in Clark County
Are radon levels high in Clark County?
Over the 2008 to 2017 laboratory totals, 75.0% of the radon tests reported to CDC for Clark County, Idaho were at or above 4 pCi/L, and the median reported pre-mitigation result was 14.1 pCi/L. Of the counties bordering it, 4 have a figure over the 2008 to 2017 laboratory totals: Jefferson County reported the largest share at or above 4 pCi/L, 67.1%, and Fremont County the smallest, 49.5%, with Lemhi County at 65.2% and Beaverhead County, MT at 59.5% between.
Which counties border Clark County?
Clark County borders 5 counties: Jefferson County, ID, Fremont County, ID, Butte County, ID, Beaverhead County, MT and Lemhi County, ID.
Products
Clark County, Products
Testing a home in Clark County
No CDC figure names Clark County; the nearest county with one, Jefferson County (32 miles away), reported 67.1% of its tests at or above 4 pCi/L over the 2008 to 2017 laboratory totals, the only reading nearby until a test of this home. In Zone 1 the year-round level in Clark County is the one to know: a long-term test (more than 90 days) or a monitor, with a short-term kit for a first answer.
Test kits and monitors3
See every kit and monitor
Monitor
Hello Radon C battery radon monitor with an e-ink screen
- C-NRPP 2024/25: Not included
- Battery
- Screen
Method: Alpha spectrometry
First reading: Within 24 hours
Short-term kitFirst Alert
RD1 short-term charcoal radon test kit
- Lab fee included
Method: Charcoal
In home: 2 to 4 days
- See every kit and monitor Test kits and monitors
Test kits and monitors: 1 of 3
Clark County is in EPA Zone 1, the highest predicted radon potential. Best radon monitors for Zone 1 counties like Clark County
Explore
Clark County, Bordering counties
Counties bordering Clark County
Calculated by Radon411- Radon levels in Butte County, IDEPA zone 1
75.9%at or above 4 pCi/L
Median 12.8 pCi/L · 29 tests
- Radon levels in Jefferson County, IDEPA zone 2
67.1%at or above 4 pCi/L
Median 5.7 pCi/L · 207 tests
- Radon levels in Lemhi County, IDEPA zone 1
65.2%at or above 4 pCi/L
Median 5.2 pCi/L · 112 tests
- Radon levels in Beaverhead County, MTEPA zone 1
59.5%at or above 4 pCi/L
Median 5.0 pCi/L · 37 tests
- Radon levels in Fremont County, IDEPA zone 1
49.5%at or above 4 pCi/L
Median 3.9 pCi/L · 101 tests
Clark County, Cities and towns
Cities and towns in Clark County
About 783 people live in Clark County by the Census Bureau's estimate for July 1, 2025, 8 fewer than in 2020 (−1.0%). Spread over its land, that is 0.4 people to the square mile. Dubois alone is home to 516 of them, 65.9% of the total. Together its 2 incorporated places hold 69.6% of those residents; the other 30.4% live outside any of them. CDC's 4 reported tests for 2008 to 2017 work out to 5.1 per 1,000 residents (2025 estimate), close to Idaho's 6.4.
From Dubois to Spencer, CDC publishes no figure by town: each reads Clark County's.
| Place | Residents | Since 2020 | Share of residents |
|---|---|---|---|
| Dubois | 516 | −1.1% | 65.9% |
| Spencer | 29 | Level | 3.7% |
2 incorporated places, as listed in the U.S. Census Bureau’s 2020 place reference files. Residents: U.S. Census Bureau population estimates for July 1, 2025 (Vintage 2025).
Clark County, Homes
Homes in Clark County
Clark County is a place of houses on their own lots: 77% of its 442 homes are detached single-family houses, against 74% across Idaho. The homes are older than Idaho's as a whole: half were built before 1978 (1990 for Idaho), and 27% date from before 1960. 64% of occupied homes are lived in by their owner (Idaho: 72%); in the rest a tenant can run a test, but a fix is the landlord's to make.
| Homes | Clark County | Idaho |
|---|---|---|
| Housing units | 442 | 776,683 |
| Detached houses | 77% | 74% |
| In buildings of 5 or more | 1% | 9% |
| Mobile homes | 15% | 7% |
| Built before 1960 | 27% | 18% |
| Built since 2000 | 18% | 35% |
| Middle year built | 1978 | 1990 |
| Owner-occupied | 64% | 72% |
Census Bureau, American Community Survey, 2019 to 2023: estimates from a sample of homes, looser for a small place. Shares by Radon411.