Locations and reports
Radon in Longmont, Colorado
Longmont lies in 2 counties, in EPA Zone 1; 81.6% of it is in Boulder County, where 34.2% of 1,963 reported tests (2023) were at or above 4 pCi/L. Homes with elevated levels occur in all three zones, so the only way to know your home's level is to test it.
Data checked By Radon411Methodology
Share at 4+ pCi/L
34.2%
4.7 pts below CO
- Boulder
- 34.2%
- Colorado
- 38.9%
Boulder, 32nd of 38 in CO, 2023
Median result
2.5pCi/L
Below action level
- Boulder
- 2.5 pCi/L
- EPA
- 4.0 pCi/L
Midpoint of Boulder's tests, 2023
Highest county
46.0%
Weld
- Weld
- 46.0%
- Boulder
- 34.2%
2 counties, 2 with 30+ tests, 2023
EPA radon zone
Zone 1
Highest potential
- Zone 1
- 2
- 3
1 highest, 3 lowest
Boulder; Weld Zone 1
Tests reported
1,963
In Boulder
- Boulder
- 1,963
- Weld
- 663
Reported to CDC before any fix, 2023
Read a result for Longmont
Starts at Boulder County's median, 2.5 pCi/L
EPA’s own thresholds; the figures are the area’s, never this home’s.
EPA’s reading of 2.5 pCi/L
Between 2 and 4 pCi/L
- Under 20 to 1.9 pCi/L
- 2 to 4consider a fix
- 4 and aboveEPA says fix it
This result is between 2 and 4 pCi/L. EPA recommends considering a fix when the level is between 2 and 4 pCi/L. Boulder County's median reported result, 2023.
The data
Longmont, What is published
What is published for Longmont
Every figure for Longmont is a county's
Radon data is published by county, not by city, and Longmont reaches into 2 counties: Boulder County and Weld County. Each has its own EPA zone and its own CDC test figures, set side by side below and each labelled with its county. Only a test tells you about one home.
Longmont covers 28.8 square miles of land: 81.6% of it lies in Boulder County and 18.4% in Weld County. About 100,109 people live there, 3,479 to the square mile.
Longmont, Counties
Longmont's counties, side by side
MeasuredEach county's own EPA zone and its own published CDC figures for 2023, the newest period any of them has. A county without a figure for that period is shown without one, never with a figure from another year.
- Boulder County
- EPA zone
- Zone 1: Highest predicted potential
- At or above 4 pCi/L
- 34.2%
- Median result
- 2.5 pCi/L
- Tests reported
- 1,963 tests
- Annual years on file
- 10
- Weld County
- EPA zone
- Zone 1: Highest predicted potential
- At or above 4 pCi/L
- 46.0%
- Median result
- 3.5 pCi/L
- Tests reported
- 663 tests
- Annual years on file
- 10
| County | EPA zone | Reported tests at or above 4 pCi/L | Median reported result | Tests reported | Annual years on file |
|---|---|---|---|---|---|
| Boulder County | Zone 1: Highest predicted potential | 34.2% | 2.5 pCi/L | 1,963 tests | 10 |
| Weld County | Zone 1: Highest predicted potential | 46.0% | 3.5 pCi/L | 663 tests | 10 |
Longmont spans 2 counties, each with its own EPA zone and its own CDC figure: Boulder County (Zone 1, 34.2% of 1,963 tests) and Weld County (Zone 1, 46.0% of 663 tests).
Boulder County ranks 32nd of 38 Colorado counties with 30 or more tests for that period, 4.7 points below the 38.9% pooled across Colorado's counties, 0.8 points above the national pool of 33.4%. Of the city's other county, Weld County reported 46.0%.
The counties nearest Longmont with a CDC figure for 2023, measured from the middle of the city: Broomfield County (15 miles, 36.9% of 758 tests), Denver County (30 miles, 35.4% of 2,277 tests) and Gilpin County (31 miles, 26.4% of 129 tests). Across the 8 nearest, the share at or above 4 pCi/L runs from 26.4% to 49.4%; the farthest of them, Arapahoe County, is 54 miles away at 37.9%.
No business in the directory gives Longmont as its address; the Colorado list covers it. Colorado's other cities with a page of their own: Colorado Springs, Fort Collins, Grand Junction and Aurora.
Against Colorado and its other counties
Shares of tests people chose to run and report, by county (2023); the state's is its counties' counts added up, calculated by Radon411. Only a test measures a home.
The counties around Longmont
By distance from the middle of Longmont; counties with 30 or more tests.
The median reported pre-mitigation result for 2023 was 2.5 pCi/L in Boulder County and 3.5 pCi/L in Weld County.
Predicted: EPA Map of Radon Zones, by county. Measured: CDC tracking data, by county, 2023.
Longmont, Geology
The ground under Longmont
PredictedUSGS's geologic map puts all of Longmont's mapped land in one class: high radon potential. All of it lies in the area the map calls High Plains, to which USGS gives a radon index of 12, in the high band (12 to 17).
| Area | The rock, in USGS's words | Potential | Radon index | Share of land |
|---|---|---|---|---|
| High Plains | Quaternary deposits and Cretaceous marine shales that locally have a high radiometric signature. | High | 12 | 100% |
Names, descriptions and indexes: U.S. Geological Survey, Geologic Radon Potential Map (2025). Shares: Radon411, from the map cut to the boundary.
Longmont beside its counties and Colorado
All of Longmont, like all of Boulder County, is rated high. Longmont has more high-potential ground than Colorado as a whole: 100% of its land against 74.8%.
The map rates the ground, not the air in any home.
Longmont, Homes
Homes in Longmont
Of Longmont's 42,435 homes, 60% are detached houses, against 58% across Boulder County; 22% are in buildings of five or more homes. Half of its homes were built before 1990, close to Boulder County's 1986: 10% date from before 1960 and 33% from 2000 or later. 63% of occupied homes are lived in by their owner (Boulder County: 62%); in the rest a tenant can run a test, but a fix is the landlord's to make.
| Homes | Longmont | Boulder County | Colorado |
|---|---|---|---|
| Housing units | 42,435 | 142,809 | 2,545,124 |
| Detached houses | 60% | 58% | 62% |
| In buildings of 5 or more | 22% | 25% | 22% |
| Mobile homes | 2% | 2% | 4% |
| Built before 1960 | 10% | 13% | 16% |
| Built since 2000 | 33% | 25% | 31% |
| Middle year built | 1990 | 1986 | 1987 |
| Owner-occupied | 63% | 62% | 66% |
Census Bureau, American Community Survey, 2019 to 2023: estimates from a sample of homes, looser for a small place. Shares by Radon411.
Local help
Longmont, Local resources
Local resources for Longmont
Who to ask in Colorado: its radon program, test kits and professionals.
- Colorado's radon programColorado Department of Public Health and Environment.Visit the program’s page(opens in a new tab)
- Free and low-cost kitsThe program says:
Order a discounted radon test kit
- Compare kits
- Professionals in ColoradoRadon testers and mitigators in Colorado, each with how its listing was checked.
The program's own words on licensing and disclosure are on the Colorado page.
What it means
Longmont, Questions
Questions about radon in Longmont
What radon zone is Longmont in?
Both counties Longmont reaches into (Boulder County and Weld County) are in Zone 1, the map's highest class: a predicted county average above 4 pCi/L. EPA zones counties, not cities, so that is the zone for every part of Longmont.
What are radon levels in Longmont?
A high result is no surprise around Longmont: 34.2% of 1,963 tests reported from Boulder County for 2023 reached 4 pCi/L or more. The median reported result was 2.5 pCi/L. Weld County reported 46.0% of 663 tests; the counties are kept apart.
Should I test my home in Longmont?
Yes. About 671 of the 1,963 tests reported from Boulder County were at or above 4 pCi/L. That is below Colorado's pooled 38.9%, and still not a reason to skip it, and ranks Boulder County 32nd of the 38 Colorado counties with 30 or more tests. No map says which homes those are. How to test a home.
What ground is Longmont built on, as far as radon goes?
USGS's geologic map puts all of Longmont on one named area, "High Plains", rated high for radon potential, radon index 12.
Is radon higher in the counties around Longmont?
Some are and some are not. Against Boulder County's 34.2%, 7 of the 8 nearest counties with a figure report more and 1 less, from Gilpin County (26.4%, 31 miles off) to Larimer County (49.4%, 39 miles off).
The answers use the published figures of Boulder County and Weld County, the counties Longmont lies in.
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