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Radon Levels in Jefferson County, Colorado

EPA puts Jefferson County in Zone 1, its highest radon zone, and high results are common there: 41.8% of the 3,068 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

    41.8%

    2.9 pts above CO

    Jefferson
    41.8%
    Colorado
    38.9%

    3,068 tests, 2023

  • Median result

    3.2pCi/L

    Below action level

    Jefferson
    3.2 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2023

  • Tests reported

    3,068

    Most tested in Colorado

    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

    17 of 38

    Middle half

    Jefferson
    41.8%
    Park
    57.5%

    By share, of 38 with 30+ tests

Read a result for Jefferson County

Starts at Jefferson County's median, 3.2 pCi/L

3.2pCi/L

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

EPA’s reading of 3.2 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. Jefferson County's median reported result, 2023.

Interpret this readingWhat a fix costs

The data

Jefferson County, At a glance

Jefferson County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

EPA puts all of Jefferson 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 100% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

Radon tests reported for Jefferson County, 2023

Share of reported tests at or above 4 pCi/L, by year, Jefferson County
Share of reported tests at or above 4 pCi/L, by year, Jefferson County
YearShare of reported tests at or above 4 pCi/L
201448.5%
201547.9%
201638.9%
201743.0%
201848.6%
201949.5%
202047.7%
202153.8%
202244.4%
202341.8%

2023: 41.8%. Across these years the share ran from 38.9% to 53.8%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

These are the 3,068 tests reported to CDC for Jefferson 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

Jefferson County, Compared

Jefferson County radon figures, compared

Calculated by Radon411

In Jefferson County, about 4 in 10 reported tests in 2023 (1,282 of 3,068) were at or above EPA's action level. Against the national pool of 33.4%, the gap is 8.4 points higher. Across 61 Colorado counties pooled together, 38.9% of tests reached that level, so Jefferson County runs 2.9 points above its state.

Of 10 neighbours with a figure, 2 reported a larger share at or above 4 pCi/L and 8 a smaller one; the spread is 26.4% (Gilpin County) to 57.5% (Park County). By share, Jefferson County is 17th of 38 Colorado counties with 30 or more tests for the same period.

Year by year, 2014 opened at 48.5% and 2023 closed at 41.8% (down 6.7 points), with 2021 the high at 53.8%. Half the tests came in under 3.2 pCi/L and half over it; the single highest reached 73.0 pCi/L. Below the action level, 1,029 tests (33.5%) came in under 2 pCi/L, against 39.2% statewide, and 757 tests (24.7%) fell between 2 and 4, against 22.0% statewide.

Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 41.8% share point the same way here. USGS geology rates 100% of the county's land high radon potential. CDC also counts 820 tests taken after mitigation in Jefferson County for the same period: tests in homes where a fix was already in.

The busiest year on file was 2018, with 5,268 tests reported; the quietest, 2014, had 884.

Share of reported tests at or above 4 pCi/L by year, Jefferson County against Colorado2014 to 2023, years with 30 or more tests: Jefferson County 48.5% to 41.8%, the state's pooled share 38.9% in 2023.
Jefferson CountyColorado, pooled
Share of reported tests at or above 4 pCi/L by year, Jefferson County against Colorado: data table
YearTestsJefferson County, at or above 4 pCi/LColorado, pooled
201488448.5%46.5%
20151,23847.9%47.2%
20161,48038.9%38.5%
20171,62343.0%41.0%
20185,26848.6%44.2%
20194,71749.5%44.1%
20203,48147.7%41.9%
20213,65053.8%47.3%
20223,79344.4%38.6%
20233,06841.8%38.9%
Jefferson County against Colorado, the United States and nearby counties, 2023
AreaTestsAt or above 4 pCi/LMedianDifference from Jefferson County
Jefferson County3,06841.8%3.2 pCi/LThis county
Colorado, pooled61 counties, pooled21,80238.9%Not pooled−2.9 points
United States, pooled642 counties, pooled128,74033.4%Not pooled−8.4 points
Delta Countyhighest in the state12455.6%4.6 pCi/L+13.8 points
Bent Countylowest in the state1083.7%0.1 pCi/L−38.1 points
Park County, COborders it10657.5%4.6 pCi/L+15.7 points
Teller County, COborders it16256.8%4.9 pCi/L+15.0 points
Clear Creek County, COborders it5141.2%3.0 pCi/L−0.6 points
Arapahoe County, COborders it1,99037.9%2.5 pCi/L−3.9 points
Broomfield County, COborders it75836.9%2.3 pCi/L−4.9 points
Adams County, COborders it1,19136.6%2.3 pCi/L−5.2 points
Douglas County, COborders it1,63535.8%2.4 pCi/L−6.0 points
Denver County, COborders it2,27735.4%2.8 pCi/L−6.4 points
Boulder County, COborders it1,96334.2%2.5 pCi/L−7.6 points
Gilpin County, COborders it12926.4%1.1 pCi/L−15.4 points
Where Jefferson County's reported results fell, 2023, beside Colorado and the United States
ResultJefferson County testsJefferson County shareColorado shareU.S. shareAgainst Colorado
Under 2 pCi/L1,02933.5%39.2%41.4%−5.7 points
2 to 4 pCi/L75724.7%22.0%25.2%+2.7 points
4 pCi/L and above1,28241.8%38.9%33.4%+2.9 points
Jefferson 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
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
Grand County16 of 3813644.1%3.3 pCi/L
Jefferson County17 of 383,06841.8%3.2 pCi/L
Clear Creek County18 of 385141.2%3.0 pCi/L
Archuleta County19 of 389240.2%2.3 pCi/L
Garfield County20 of 3822439.3%2.9 pCi/L
La Plata County20 of 3822439.3%3.1 pCi/L
Summit County22 of 3829538.3%2.0 pCi/L
Jefferson County radon tests by year, against Colorado's pooled share
YearTestsAt or above 4 pCi/LMedianHighestColorado, pooledChange from the row before
20233,06841.8%3.2 pCi/L73.0 pCi/L38.9%−2.6 points
20223,79344.4%3.4 pCi/L127.0 pCi/L38.6%−9.4 points
20213,65053.8%4.4 pCi/L510.7 pCi/L47.3%+6.1 points
20203,48147.7%3.7 pCi/L129.3 pCi/L41.9%−1.8 points
20194,71749.5%3.9 pCi/L149.5 pCi/L44.1%+0.9 points
20185,26848.6%3.8 pCi/L338.3 pCi/L44.2%+5.6 points
20171,62343.0%3.3 pCi/L168.6 pCi/L41.0%+4.1 points
20161,48038.9%2.9 pCi/L2259.6 pCi/L38.5%−9.0 points
20151,23847.9%3.8 pCi/L171.9 pCi/L47.2%−0.6 points
201488448.5%3.8 pCi/L142.9 pCi/L46.5%First on file

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

Jefferson County, State data

Colorado's own radon test data

Measured

What Colorado's own agencies publish about radon tests in Jefferson 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 Jefferson 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

50.3%

15,212 of 30,229 tests

Median result

4.0 pCi/L

Highest result

510.7 pCi/L

Tests behind these figures

30,229 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.

Jefferson County, Geology

The ground under Jefferson County

Predicted

USGS's geologic map puts all of Jefferson County's mapped land in one class: high radon potential. The map puts 75.1% of Jefferson County in the area it calls Rocky Mountains and 22.7% in High Plains, with one smaller area beside them. USGS gives the first a radon index of 15 and the second 12, both in the high band (12 to 17).

USGS's geologic areas under Jefferson County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Rocky MountainsPrecambrian granites, gneiss, amphibolite, variable uranium content, abundant shear zones.Sedimentary rocks shed from the granitic highlands.High1575.1%
High PlainsQuaternary deposits and Cretaceous marine shales that locally have a high radiometric signature.High1222.7%
Dawson ArkoseConsolidated alluvial fan and river deposits sourced in the granite mountains to the west.High142.2%

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

Jefferson County beside the ground around it

Jefferson County has more high-potential ground than Colorado as a whole: 100% of its land against 74.8%. Of the 10 counties that border it, none has as large a share of high-potential ground; Adams County comes nearest at 100%.

Share of land by USGS radon potential: Jefferson County, the 10 counties bordering it and Colorado
AreaEPA zoneLow potentialModerate potentialHigh potential
Jefferson CountyZone 1NoneNone100%
Denver CountyZone 1NoneNone100%
Douglas CountyZone 1NoneNone100%
Gilpin CountyZone 1NoneNone100%
Clear Creek CountyZone 1NoneNone100%
Broomfield CountyZone 1NoneNone100%
Teller CountyZone 1NoneNone100%
Boulder CountyZone 1NoneNone100%
Park CountyZone 1None0.3%99.7%
Arapahoe CountyZone 1NoneNone100%
Adams CountyZone 1NoneNone100%
Colorado64 counties, pooled by land areaVariesNone25.2%74.8%

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

Local help

Jefferson County, Local resources

Radon testing and mitigation in Jefferson County

Listed radon businesses with an address in Jefferson County:

Listed radon professionals by town: Lakewood, Westminster, Littleton, Morrison

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

Jefferson County, Questions

Questions about radon in Jefferson County

Are radon levels high in Jefferson County?

In 2023, 41.8% of the radon tests reported to CDC for Jefferson County, Colorado were at or above 4 pCi/L, and the median reported pre-mitigation result was 3.2 pCi/L. Of 38 Colorado counties with 30 or more tests for the same period, 16 reported a higher share; pooled, the state comes to 38.9%.

Which counties border Jefferson County?

Jefferson County borders 10 counties: Denver County, Douglas County, Gilpin County, Clear Creek County, Broomfield County, Teller County, Boulder County, Park County, Arapahoe County and Adams County.

Products

Jefferson County, Products

Testing a home in Jefferson County

In Jefferson County, 41.8% of 3,068 tests reported to CDC in 2023 reached 4 pCi/L, against 38.9% pooled across Colorado. In Zone 1 the year-round level in Jefferson 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

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

Explore

Jefferson County, Bordering counties

Counties bordering Jefferson County

Calculated by Radon411

Jefferson County, Cities and towns

Cities and towns in Jefferson County

Jefferson County's population has barely moved since 2020: 580,451 residents on July 1, 2025 against 583,131 then, by the Census Bureau's estimates. That works out to 759 people per square mile of land. Its largest town, Lakewood, has 156,927 residents, 27.0% of the total. CDC's 3,068 reported tests for 2023 work out to 5.3 per 1,000 residents (2025 estimate), close to Colorado's 3.6.

From Lakewood to Lakeside, CDC publishes no figure by town: each reads Jefferson County's.

The cities and towns in Jefferson County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Lakewood156,927+0.4%27.0%
Arvadaextends beyond Jefferson County122,901−1.2%Counted whole
Westminsterextends beyond Jefferson County116,182−0.4%Counted whole
Littletonextends beyond Jefferson County46,246+1.5%Counted whole
Wheat Ridge32,081−0.9%5.5%
Golden20,390Level3.5%
Superiorextends beyond Jefferson County13,637+4.1%Counted whole
Edgewater4,870−2.7%0.8%
Bow Marextends beyond Jefferson County832−2.3%Counted whole
Mountain View517−4.1%Under 0.1%
Morrison364−8.3%Under 0.1%
Lakeside16−5.9%Under 0.1%

12 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). A place that extends beyond Jefferson County is counted whole, so it has no share.

Jefferson County, Homes

Homes in Jefferson County

Of Jefferson County's 251,183 homes, 64% are detached houses, against 62% across Colorado; 22% are in buildings of five or more homes. Half of its homes were built before 1980, close to Colorado's 1987: 14% date from before 1960 and 19% from 2000 or later. 71% of occupied homes are lived in by their owner (Colorado: 66%); in the rest a tenant can run a test, but a fix is the landlord's to make.

Homes in Jefferson County, Colorado, 2019 to 2023
HomesJefferson CountyColorado
Housing units251,1832,545,124
Detached houses64%62%
In buildings of 5 or more22%22%
Mobile homes1%4%
Built before 196014%16%
Built since 200019%31%
Middle year built19801987
Owner-occupied71%66%

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