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Radon Levels in Weld County, Colorado

EPA puts Weld County in Zone 1, its highest radon zone, and high results are common there: 46.0% of the 663 radon tests reported to CDC (2023) 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

How to test this homeFind a certified tester

  • Share at 4+ pCi/L

    46.0%

    7.1 pts above CO

    Weld
    46.0%
    Colorado
    38.9%

    663 tests, 2023

  • Median result

    3.5pCi/L

    Below action level

    Weld
    3.5 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2023

  • Tests reported

    663

    10th of 38 by tests

    Weld
    663
    Jefferson
    3,068

    Reported to CDC before any fix, 2023

  • EPA radon zone

    Zone 1

    Highest potential

    1. Zone 1
    2. 2
    3. 3

    1 highest, 3 lowest

    EPA predicts over 4 pCi/L indoors

  • Rank in Colorado

    10 of 38

    Top quarter

    Weld
    46.0%
    Park
    57.5%

    By share, of 38 with 30+ tests

Read a result for Weld County

Starts at Weld County's median, 3.5 pCi/L

3.5pCi/L

EPA’s own thresholds; the figures are the area’s, never this home’s.

EPA’s reading of 3.5 pCi/L

Between 2 and 4 pCi/L

  1. Under 20 to 1.9 pCi/L
  2. 2 to 4consider a fix
  3. 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. Weld County's median reported result, 2023.

Interpret this readingWhat a fix costs

The data

Weld County, At a glance

Weld County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

EPA puts all of Weld County in and predicts an average indoor of greater than 4 pCi/L (greater than 150 Bq/m³) across the county. That is one forecast for the whole county, not a reading of any address in it; homes above EPA's turn up in all three zones.

USGS geologic potential: High across 66% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

Radon tests reported for Weld County, 2023

Share of reported tests at or above 4 pCi/L, by year, Weld County
Share of reported tests at or above 4 pCi/L, by year, Weld County
YearShare of reported tests at or above 4 pCi/L
201449.6%
201551.9%
201641.2%
201745.4%
201843.9%
201947.1%
202049.3%
202147.5%
202249.6%
202346.0%

2023: 46.0%. Across these years the share ran from 41.2% to 51.9%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

These are the 663 tests reported to CDC for Weld County over 2023, many from homes being sold or already suspected of a problem: they describe the county's reported tests, not this house.

Measured: CDC tracking data. Figure above: 2023. Measured data period: 2005 to 2023 (every CDC figure on file).

How to read these figures

Weld County, Compared

Weld County radon figures, compared

Calculated by Radon411

In Weld County, about 5 in 10 reported tests in 2023 (305 of 663) were at or above EPA's action level. Against the national pool of 33.4%, the gap is 12.6 points higher. Across 61 Colorado counties pooled together, 38.9% of tests reached that level, so Weld County runs 7.1 points above its state.

Of 5 neighbours with a figure, 2 reported a larger share at or above 4 pCi/L and 3 a smaller one; the spread is 34.2% (Boulder County) to 50.0% (Morgan County). By share, Weld County is 10th of 38 Colorado counties with 30 or more tests for the same period.

Year by year, 2014 opened at 49.6% and 2023 closed at 46.0% (down 3.6 points), with 2015 the high at 51.9%. Half the tests came in under 3.5 pCi/L and half over it; the single highest reached 74.2 pCi/L. Below the action level, 196 tests (29.6%) came in under 2 pCi/L, against 39.2% statewide, and 162 tests (24.4%) fell between 2 and 4, against 22.0% statewide.

Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 46.0% share point the same way here. USGS geology rates 66% of the county's land high radon potential. CDC also counts 210 tests taken after mitigation in Weld County for the same period: tests in homes where a fix was already in.

The busiest year on file was 2018, with 1,569 tests reported; the quietest, 2014, had 381.

Share of reported tests at or above 4 pCi/L by year, Weld County against Colorado2014 to 2023, years with 30 or more tests: Weld County 49.6% to 46.0%, the state's pooled share 38.9% in 2023.
Weld CountyColorado, pooled
Share of reported tests at or above 4 pCi/L by year, Weld County against Colorado: data table
YearTestsWeld County, at or above 4 pCi/LColorado, pooled
201438149.6%46.5%
201550751.9%47.2%
201652041.2%38.5%
201764745.4%41.0%
20181,56943.9%44.2%
20191,56747.1%44.1%
20201,38249.3%41.9%
20211,43947.5%47.3%
20221,25749.6%38.6%
202366346.0%38.9%
Weld County against Colorado, the United States and nearby counties, 2023
AreaTestsAt or above 4 pCi/LMedianDifference from Weld County
Weld County66346.0%3.5 pCi/LThis county
Colorado, pooled61 counties, pooled21,80238.9%Not pooled−7.1 points
United States, pooled642 counties, pooled128,74033.4%Not pooled−12.6 points
Park Countyhighest in the state10657.5%4.6 pCi/L+11.5 points
Bent Countylowest in the state1083.7%0.1 pCi/L−42.3 points
Morgan County, COborders it3850.0%4.0 pCi/L+4.0 points
Larimer County, COborders it1,90249.4%3.8 pCi/L+3.4 points
Kimball County, NEborders it2040.0%3.4 pCi/L−6.0 points
Broomfield County, COborders it75836.9%2.3 pCi/L−9.1 points
Adams County, COborders it1,19136.6%2.3 pCi/L−9.4 points
Boulder County, COborders it1,96334.2%2.5 pCi/L−11.8 points
Logan County, COborders it2123.8%2.3 pCi/L−22.2 points
Where Weld County's reported results fell, 2023, beside Colorado and the United States
ResultWeld County testsWeld County shareColorado shareU.S. shareAgainst Colorado
Under 2 pCi/L19629.6%39.2%41.4%−9.6 points
2 to 4 pCi/L16224.4%22.0%25.2%+2.4 points
4 pCi/L and above30546.0%38.9%33.4%+7.1 points
Weld County ranked among the 38 Colorado counties with 30 or more tests, by share at or above 4 pCi/L, 2023
CountyRankTestsAt or above 4 pCi/LMedian
Chaffee County5 of 3816555.2%4.5 pCi/L
Lake County6 of 383554.3%4.7 pCi/L
Fremont County7 of 386650.0%4.0 pCi/L
Morgan County7 of 383850.0%4.0 pCi/L
Larimer County9 of 381,90249.4%3.8 pCi/L
Weld County10 of 3866346.0%3.5 pCi/L
Eagle County11 of 3831245.8%3.5 pCi/L
Elbert County11 of 3812045.8%2.9 pCi/L
Custer County13 of 383545.7%3.5 pCi/L
Pueblo County14 of 3834244.4%3.5 pCi/L
Routt County15 of 3817844.4%3.3 pCi/L
Weld County radon tests by year, against Colorado's pooled share
YearTestsAt or above 4 pCi/LMedianHighestColorado, pooledChange from the row before
202366346.0%3.5 pCi/L74.2 pCi/L38.9%−3.6 points
20221,25749.6%3.9 pCi/L50.7 pCi/L38.6%+2.1 points
20211,43947.5%3.7 pCi/L275.0 pCi/L47.3%−1.8 points
20201,38249.3%3.9 pCi/L70.8 pCi/L41.9%+2.2 points
20191,56747.1%3.7 pCi/L63.0 pCi/L44.1%+3.2 points
20181,56943.9%3.4 pCi/L94.0 pCi/L44.2%−1.5 points
201764745.4%3.7 pCi/L117.8 pCi/L41.0%+4.2 points
201652041.2%3.3 pCi/L41.3 pCi/L38.5%−10.7 points
201550751.9%4.1 pCi/L28.5 pCi/L47.2%+2.3 points
201438149.6%3.9 pCi/L48.1 pCi/L46.5%First on file

Pooled shares: Radon411's sum of CDC's county counts for one period.

Weld County, State data

Colorado's own radon test data

Measured

What Colorado's own agencies publish about radon tests in Weld County, with their period. These are not CDC's figures and are never added to them.

MeasuredState figures, not CDC's

What Colorado CDPHE reports for Weld County

Radon data (county results): radon test results received from radon testing labs, primarily short-term household tests taken by the homeowner. Period: 2005-2025.

Share over 4 pCi/L

48.6%

5,419 of 11,159 tests

Median result

3.8 pCi/L

Highest result

275.0 pCi/L

Tests behind these figures

11,159 tests

  • CDPHE says these data do not give actual radon levels in homes that have not been tested.
  • When possible, only tests conducted before a mitigation system was installed are used.
  • Some counties have very small numbers of test results, and their summaries may be less representative.
  • These figures describe tests reported to Colorado CDPHE in this area. They count a different set of tests than CDC's figures and are not a reading of any one home.

Measured: Colorado CDPHE data, 2005-2025.

Weld County, Geology

The ground under Weld County

Predicted

USGS's geologic map splits Weld County: 66.2% of its land is high radon potential and 33.9% moderate. The map puts 66.2% of Weld County in the area it calls High Plains and 33.1% in White River Formation, with one smaller area beside them. USGS gives the first a radon index of 12, in the high band (12 to 17), and the second 11, in the moderate band (9 to 11).

USGS's geologic areas under Weld County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
High PlainsQuaternary deposits and Cretaceous marine shales that locally have a high radiometric signature.High1266.2%
White River FormationSiltstone, sandstone, conglomerate, volcanic tuff.Moderate1133.1%
Tertiary RocksTertiary sandstone, siltstone, and volcanic ash.Moderate110.8%

Names, descriptions and indexes: U.S. Geological Survey, Geologic Radon Potential Map (2025). Shares: Radon411, from the map cut to the boundary.

Weld County beside the ground around it

On high-potential ground Weld County is close to Colorado as a whole: 66.2% of its land against 74.8%. Of the 8 counties that border it, 5 have more high-potential ground, Adams County the most at 100%.

Share of land by USGS radon potential: Weld County, the 8 counties bordering it and Colorado
AreaEPA zoneLow potentialModerate potentialHigh potential
Weld CountyZone 1None33.9%66.2%
Broomfield CountyZone 1NoneNone100%
Morgan CountyZone 1NoneNone100%
Adams CountyZone 1NoneNone100%
Boulder CountyZone 1NoneNone100%
Larimer CountyZone 1None1.6%98.4%
Laramie County, WYZone 1None93.8%6.2%
Kimball County, NEZone 2None100%None
Logan CountyZone 1None40.7%59.3%
Colorado64 counties, pooled by land areaVariesNone25.2%74.8%

The map rates the ground, not the air in any home.

Local help

Weld County, Local resources

Radon testing and mitigation in Weld County

Listed radon businesses with an address in Weld County:

Listed radon professionals by town: Frederick

What Colorado's program says
Licensing of radon professionals
Listed contractors are certified by the National Radon Proficiency Program or the National Radon Safety Board, and must be licensed by the Colorado Department of Regulatory Agencies.
Disclosure when a home is sold or rented
As of August 7, 2023, landlords in Colorado are required to disclose radon information when a tenant signs a lease agreement.

What it means

Weld County, Questions

Questions about radon in Weld County

Are radon levels high in Weld County?

In 2023, 46.0% of the radon tests reported to CDC for Weld County, Colorado were at or above 4 pCi/L, and the median reported pre-mitigation result was 3.5 pCi/L. Of 38 Colorado counties with 30 or more tests for the same period, 9 reported a higher share; pooled, the state comes to 38.9%.

Which counties border Weld County?

Weld County borders 8 counties: Broomfield County, CO, Morgan County, CO, Adams County, CO, Boulder County, CO, Larimer County, CO, Laramie County, WY, Kimball County, NE and Logan County, CO.

Products

Weld County, Products

Testing a home in Weld County

In Weld County, 46.0% of 663 tests reported to CDC in 2023 reached 4 pCi/L, against 38.9% pooled across Colorado. In Zone 1 the year-round level in Weld 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

Weld County is in EPA Zone 1, the highest predicted radon potential. Best radon monitors for Zone 1 counties like Weld County

Explore

Weld County, Bordering counties

Counties bordering Weld County

Calculated by Radon411

Weld County, Cities and towns

Cities and towns in Weld County

Weld County is growing: the Census Bureau puts it at 378,426 residents on July 1, 2025, up 14.2% from 331,434 in 2020. That works out to 95 people per square mile of land. Thornton, the largest place on its list (147,766 residents in all), extends beyond Weld County, so no share of the total is worked out for it. Reported testing is thin here: 663 tests over 2023, or 1.8 for every 1,000 residents (2025 estimate), where Colorado as a whole comes to 3.6.

From Thornton to Raymer (New Raymer), CDC publishes no figure by town: each reads Weld County's.

The 12 largest cities and towns in Weld County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Thorntonextends beyond Weld County147,766+3.9%Counted whole
Greeley115,073+5.3%30.4%
Longmontextends beyond Weld County100,109+0.9%Counted whole
Brightonextends beyond Weld County45,395+12.7%Counted whole
Windsorextends beyond Weld County43,840+31.9%Counted whole
Erieextends beyond Weld County40,904+34.5%Counted whole
Northglennextends beyond Weld County38,893+2.3%Counted whole
Evans22,500+0.9%5.9%
Johnstownextends beyond Weld County22,433+29.2%Counted whole
Firestone20,129+20.9%5.3%
Frederick18,241+24.0%4.8%
Berthoudextends beyond Weld County14,080+32.8%Counted whole
  • Severance
  • Timnath (part)
  • Fort Lupton
  • Milliken
  • Lochbuie (part)
  • Mead
  • Dacono
  • Eaton
  • Platteville
  • Ault
  • La Salle
  • Keenesburg
  • Hudson
  • Kersey
  • Pierce
  • Gilcrest
  • Nunn
  • Garden City
  • Grover
  • Raymer (New Raymer)

32 incorporated places, as listed in the U.S. Census Bureau’s 2020 place reference files. “(part)” marks a place that extends into another county. Residents: U.S. Census Bureau population estimates for July 1, 2025 (Vintage 2025). A place that extends beyond Weld County is counted whole, so it has no share.

Weld County, Homes

Homes in Weld County

Of Weld County's 125,033 homes, 73% are detached houses, against 62% across Colorado; 11% are in buildings of five or more homes. Much of Weld County is recent building: 49% of its homes have gone up since 2000 and the middle year built is 2000, against 1987 for Colorado. Owners live in 76% of the occupied homes (Colorado: 66%), so for most households a test and a fix are their own to order.

Homes in Weld County, Colorado, 2019 to 2023
HomesWeld CountyColorado
Housing units125,0332,545,124
Detached houses73%62%
In buildings of 5 or more11%22%
Mobile homes7%4%
Built before 196012%16%
Built since 200049%31%
Middle year built20001987
Owner-occupied76%66%

Census Bureau, American Community Survey, 2019 to 2023: estimates from a sample of homes, looser for a small place. Shares by Radon411.