Locations and reports
Radon in Helena, Montana
EPA puts Lewis and Clark County, where Helena is, in Zone 1, its highest radon zone, and high results are common there: 61.8% of the 468 radon tests reported to CDC (2008 to 2017, laboratory totals) came back at or above 4 pCi/L. Homes with elevated levels occur in all three zones, so only a test measures yours.
Data checked By Radon411Methodology
Share at 4+ pCi/L
61.8%
16.3 pts above MT
- Lewis and Clark
- 61.8%
- Montana
- 45.5%
Lewis and Clark, 7th of 23 in MT, 2008 to 2017
Median result
5.1pCi/L
Action level reached
- Lewis and Clark
- 5.1 pCi/L
- EPA
- 4.0 pCi/L
Midpoint of Lewis and Clark's tests, 2008 to 2017
Nearest county
71.0%
Jefferson, 31 miles
- Jefferson
- 71.0%
- Lewis and Clark
- 61.8%
62 tests, 2008 to 2017
EPA radon zone
Zone 1
Highest potential
- Zone 1
- 2
- 3
1 highest, 3 lowest
Lewis and Clark, where Helena is
Tests reported
468
In Lewis and Clark
- Lewis and Clark
- 468
- Jefferson
- 62
Reported to CDC before any fix, 2008 to 2017
Read a result for Helena
Starts at Lewis and Clark County's median, 5.1 pCi/L
EPA’s own thresholds; the figures are the area’s, never this home’s.
EPA’s reading of 5.1 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. Lewis and Clark County's median reported result, 2008 to 2017.
The data
Helena, What is published
What is published for Helena
Every figure for Helena is a county's
Radon data is published by county, not by city, so for Helena it is Lewis and Clark County's: its EPA zone (Zone 1) and the 468 tests reported to CDC for 2008 to 2017, laboratory. Only a test tells you about one home.
Helena covers 16.8 square miles of land, all of it in Lewis and Clark County. About 35,138 people live there, 2,089 to the square mile.
Helena, Counties
The county Helena lies in
MeasuredLewis and Clark County's EPA zone and the figures CDC publishes for it, 2008 to 2017, laboratory.
Lewis and Clark CountyZone 1Highest predicted potential
- 61.8%
- Reported tests at or above 4 pCi/L
- 5.1 pCi/L
- Median reported result
- 468 tests
- Tests reported to CDC
- None
- Yearly figures on file
Lewis and Clark County ranks 7th of 23 Montana counties with 30 or more tests for that period, 16.3 points above the 45.5% pooled across Montana's counties, 26.1 points above the national pool of 35.7%.
The counties nearest Helena with a CDC figure for 2008 to 2017, laboratory, measured from the middle of the city: Jefferson County (31 miles, 71.0% of 62 tests), Silver Bow County (57 miles, 70.1% of 174 tests) and Cascade County (59 miles, 29.4% of 235 tests). Across the 8 nearest, the share at or above 4 pCi/L runs from 29.4% to 71.0%; the farthest of them, Park County, is 105 miles away at 60.0%.
No business in the directory gives Helena as its address; the Montana list covers it. Montana's other cities with a page of their own: Bozeman, Kalispell, Billings and Boulder.
The counties around Helena
By distance from the middle of Helena; counties with 30 or more tests.
Predicted: EPA Map of Radon Zones, by county. Measured: CDC tracking data, by county, 2008 to 2017, laboratory.
Helena, Geology
The ground under Helena
PredictedUSGS's geologic map puts all of Helena's mapped land in one class: high radon potential. All of it lies in the area the map calls Northern Rocky Mountains, to which USGS gives a radon index of 13, in the high band (12 to 17).
| Area | The rock, in USGS's words | Potential | Radon index | Share of land |
|---|---|---|---|---|
| Northern Rocky Mountains | Granitic, metaigneous, metasedimentary rocks, valley fill colluvium and alluvium. | High | 13 | 100% |
Names, descriptions and indexes: U.S. Geological Survey, Geologic Radon Potential Map (2025). Shares: Radon411, from the map cut to the boundary.
Helena beside its county and Montana
All of Helena, like all of Lewis and Clark County, is rated high. Helena has more high-potential ground than Montana as a whole: 100% of its land against 78.5%.
The map rates the ground, not the air in any home.
Helena, Homes
Homes in Helena
Fewer than six in ten of Helena's 16,574 homes are detached houses (52%, against 66% across Lewis and Clark County); the rest are attached, in shared buildings or mobile. Half of its homes were built before 1975, close to Lewis and Clark County's 1982: 36% date from before 1960 and 23% from 2000 or later. 54% of occupied homes are lived in by their owner (Lewis and Clark County: 70%); in the rest a tenant can run a test, but a fix is the landlord's to make.
| Homes | Helena | Lewis and Clark County | Montana |
|---|---|---|---|
| Housing units | 16,574 | 33,837 | 522,939 |
| Detached houses | 52% | 66% | 69% |
| In buildings of 5 or more | 17% | 8% | 9% |
| Mobile homes | 3% | 10% | 10% |
| Built before 1960 | 36% | 23% | 26% |
| Built since 2000 | 23% | 27% | 27% |
| Middle year built | 1975 | 1982 | 1980 |
| Owner-occupied | 54% | 70% | 69% |
Census Bureau, American Community Survey, 2019 to 2023: estimates from a sample of homes, looser for a small place. Shares by Radon411.
Local help
Helena, Local resources
Local resources for Helena
Who to ask in Montana: its radon program, test kits and professionals.
- Radon Control ProgramRun by Montana DEQ, phone 1-800-546-0483.Visit the program’s page(opens in a new tab)
- Free and low-cost kitsThe program says:
Energy Bureau staff manage the radon information toll-free hotline and distribute subsidized radon test kits to Montana residents.
- Compare kits
- Professionals in MontanaRadon testers and mitigators in Montana, each with how its listing was checked.
The program's own words on licensing are on the Montana page.
What it means
Helena, Questions
Questions about radon in Helena
What radon zone is Helena in?
Helena takes its zone from Lewis and Clark County, which EPA puts in Zone 1, the map's highest class: a predicted county average above 4 pCi/L.
What are radon levels in Helena?
61.8% of the 468 tests reported from Lewis and Clark County for 2008 to 2017 (laboratory totals) were at or above 4 pCi/L, the level at which EPA says to fix a home: high results are ordinary around Helena. The median reported result was 5.1 pCi/L.
Should I test my home in Helena?
Yes. About 289 of the 468 tests reported from Lewis and Clark County were at or above 4 pCi/L. That is above Montana's pooled 45.5%, and ranks Lewis and Clark County 7th of the 23 Montana counties with 30 or more tests. No map says which homes those are. How to test a home.
What ground is Helena built on, as far as radon goes?
USGS's geologic map puts all of Helena on one named area, "Northern Rocky Mountains", rated high for radon potential, radon index 13.
Is radon higher in the counties around Helena?
Some are and some are not. Against Lewis and Clark County's 61.8%, 3 of the 8 nearest counties with a figure report more and 5 less, from Cascade County (29.4%, 59 miles off) to Jefferson County (71.0%, 31 miles off).
The answers use the published figures of Lewis and Clark County, the county Helena lies in.
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