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Radon Levels in Arapahoe County, Colorado

EPA puts Arapahoe County in Zone 1, its highest radon zone, and 37.9% of its 1,990 reported radon tests (2023) were 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

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  • Share at 4+ pCi/L

    37.9%

    1.0 pts below CO

    Arapahoe
    37.9%
    Colorado
    38.9%

    1,990 tests, 2023

  • Median result

    2.5pCi/L

    Below action level

    Arapahoe
    2.5 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2023

  • Tests reported

    1,990

    3rd of 38 by tests

    Arapahoe
    1,990
    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

    23 of 38

    Middle half

    Arapahoe
    37.9%
    Park
    57.5%

    By share, of 38 with 30+ tests

Read a result for Arapahoe County

Starts at Arapahoe County's median, 2.5 pCi/L

2.5pCi/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

  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. Arapahoe County's median reported result, 2023.

Interpret this readingWhat a fix costs

The data

Arapahoe County, At a glance

Arapahoe County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

EPA puts all of Arapahoe 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 Arapahoe County, 2023

Share of reported tests at or above 4 pCi/L, by year, Arapahoe County
Share of reported tests at or above 4 pCi/L, by year, Arapahoe County
YearShare of reported tests at or above 4 pCi/L
201442.1%
201546.0%
201638.8%
201738.9%
201844.5%
201944.4%
202038.3%
202148.9%
202238.6%
202337.9%

2023: 37.9%. Across these years the share ran from 37.9% to 48.9%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

These are the 1,990 tests reported to CDC for Arapahoe 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

Arapahoe County, Compared

Arapahoe County radon figures, compared

Calculated by Radon411

In Arapahoe County, about 4 in 10 reported tests in 2023 (755 of 1,990) were at or above EPA's action level. Against the national pool of 33.4%, the gap is 4.5 points higher. Across 61 Colorado counties pooled together, 38.9% of tests reached that level, so Arapahoe County runs 1.0 points below 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 35.4% (Denver County) to 45.8% (Elbert County). By share, Arapahoe County is 23rd of 38 Colorado counties with 30 or more tests for the same period.

Year by year, 2014 opened at 42.1% and 2023 closed at 37.9% (down 4.2 points), with 2021 the high at 48.9%. Half the tests came in under 2.5 pCi/L and half over it; the single highest reached 76.0 pCi/L. Below the action level, 861 tests (43.3%) came in under 2 pCi/L, against 39.2% statewide, and 374 tests (18.8%) fell between 2 and 4, against 22.0% statewide.

Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 37.9% share point the same way here. USGS geology rates 100% of the county's land high radon potential. CDC also counts 553 tests taken after mitigation in Arapahoe County for the same period: tests in homes where a fix was already in.

The busiest year on file was 2018, with 3,420 tests reported; the quietest, 2014, had 978.

Share of reported tests at or above 4 pCi/L by year, Arapahoe County against Colorado2014 to 2023, years with 30 or more tests: Arapahoe County 42.1% to 37.9%, the state's pooled share 38.9% in 2023.
Arapahoe CountyColorado, pooled
Share of reported tests at or above 4 pCi/L by year, Arapahoe County against Colorado: data table
YearTestsArapahoe County, at or above 4 pCi/LColorado, pooled
201497842.1%46.5%
20151,13146.0%47.2%
20161,08938.8%38.5%
20171,47438.9%41.0%
20183,42044.5%44.2%
20193,31944.4%44.1%
20202,57338.3%41.9%
20212,30348.9%47.3%
20222,45838.6%38.6%
20231,99037.9%38.9%
Arapahoe County against Colorado, the United States and nearby counties, 2023
AreaTestsAt or above 4 pCi/LMedianDifference from Arapahoe County
Arapahoe County1,99037.9%2.5 pCi/LThis county
Colorado, pooled61 counties, pooled21,80238.9%Not pooled+1.0 points
United States, pooled642 counties, pooled128,74033.4%Not pooled−4.5 points
Park Countyhighest in the state10657.5%4.6 pCi/L+19.6 points
Bent Countylowest in the state1083.7%0.1 pCi/L−34.2 points
Washington County, COborders it2100.0%5.3 pCi/L+62.1 points
Lincoln County, COborders it757.1%4.0 pCi/L+19.2 points
Elbert County, COborders it12045.8%2.9 pCi/L+7.9 points
Jefferson County, COborders it3,06841.8%3.2 pCi/L+3.9 points
Adams County, COborders it1,19136.6%2.3 pCi/L−1.3 points
Douglas County, COborders it1,63535.8%2.4 pCi/L−2.1 points
Denver County, COborders it2,27735.4%2.8 pCi/L−2.5 points
Where Arapahoe County's reported results fell, 2023, beside Colorado and the United States
ResultArapahoe County testsArapahoe County shareColorado shareU.S. shareAgainst Colorado
Under 2 pCi/L86143.3%39.2%41.4%+4.1 points
2 to 4 pCi/L37418.8%22.0%25.2%−3.2 points
4 pCi/L and above75537.9%38.9%33.4%−1.0 points
Arapahoe 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
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
Arapahoe County23 of 381,99037.9%2.5 pCi/L
Ouray County24 of 384337.2%1.2 pCi/L
Broomfield County25 of 3875836.9%2.3 pCi/L
Adams County26 of 381,19136.6%2.3 pCi/L
Pitkin County27 of 3816436.6%2.5 pCi/L
Douglas County28 of 381,63535.8%2.4 pCi/L
Arapahoe County radon tests by year, against Colorado's pooled share
YearTestsAt or above 4 pCi/LMedianHighestColorado, pooledChange from the row before
20231,99037.9%2.5 pCi/L76.0 pCi/L38.9%−0.7 points
20222,45838.6%2.6 pCi/L112.2 pCi/L38.6%−10.3 points
20212,30348.9%3.8 pCi/L279.0 pCi/L47.3%+10.6 points
20202,57338.3%2.4 pCi/L115.6 pCi/L41.9%−6.1 points
20193,31944.4%3.2 pCi/L147.0 pCi/L44.1%−0.1 points
20183,42044.5%3.2 pCi/L63.9 pCi/L44.2%+5.6 points
20171,47438.9%2.8 pCi/L56.3 pCi/L41.0%+0.1 points
20161,08938.8%2.7 pCi/L80.8 pCi/L38.5%−7.2 points
20151,13146.0%3.4 pCi/L60.0 pCi/L47.2%+3.9 points
201497842.1%3.0 pCi/L63.7 pCi/L46.5%First on file

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

Arapahoe County, State data

Colorado's own radon test data

Measured

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

48.0%

10,282 of 21,419 tests

Median result

3.7 pCi/L

Highest result

369.6 pCi/L

Tests behind these figures

21,419 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.

Arapahoe County, Geology

The ground under Arapahoe County

Predicted

USGS's geologic map puts all of Arapahoe County's mapped land in one class: high radon potential. The map puts 57.1% of Arapahoe County in the area it calls High Plains and 42.9% in Dawson Arkose. USGS gives the first a radon index of 12 and the second 14, both in the high band (12 to 17).

USGS's geologic areas under Arapahoe County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
High PlainsQuaternary deposits and Cretaceous marine shales that locally have a high radiometric signature.High1257.1%
Dawson ArkoseConsolidated alluvial fan and river deposits sourced in the granite mountains to the west.High1442.9%

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

Arapahoe County beside the ground around it

Arapahoe County has more high-potential ground than Colorado as a whole: 100% of its land against 74.8%. Of the 7 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: Arapahoe County, the 7 counties bordering it and Colorado
AreaEPA zoneLow potentialModerate potentialHigh potential
Arapahoe CountyZone 1NoneNone100%
Adams CountyZone 1NoneNone100%
Elbert CountyZone 1NoneNone100%
Denver CountyZone 1NoneNone100%
Douglas CountyZone 1NoneNone100%
Jefferson CountyZone 1NoneNone100%
Lincoln CountyZone 1NoneNone100%
Washington CountyZone 1None9.6%90.4%
Colorado64 counties, pooled by land areaVariesNone25.2%74.8%

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

Local help

Arapahoe County, Local resources

Radon testing and mitigation in Arapahoe County

Listed radon professionals by town: Littleton

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

Arapahoe County, Questions

Questions about radon in Arapahoe County

Are radon levels high in Arapahoe County?

In 2023, 37.9% of the radon tests reported to CDC for Arapahoe County, Colorado were at or above 4 pCi/L, and the median reported pre-mitigation result was 2.5 pCi/L. Of 38 Colorado counties with 30 or more tests for the same period, 22 reported a higher share; pooled, the state comes to 38.9%.

Which counties border Arapahoe County?

Arapahoe County borders 7 counties: Adams County, Elbert County, Denver County, Douglas County, Jefferson County, Lincoln County and Washington County.

Products

Arapahoe County, Products

Testing a home in Arapahoe County

In Arapahoe County, 37.9% of 1,990 tests reported to CDC in 2023 reached 4 pCi/L, against 38.9% pooled across Colorado. In Zone 1 the year-round level in Arapahoe 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

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

Explore

Arapahoe County, Bordering counties

Counties bordering Arapahoe County

Calculated by Radon411

Arapahoe County, Cities and towns

Cities and towns in Arapahoe County

The Census Bureau estimates 673,820 residents in Arapahoe County for July 1, 2025, 18,551 more than in 2020 (+2.8%). That works out to 845 people per square mile of land. Aurora, the largest place on its list (410,053 residents in all), extends beyond Arapahoe County, so no share of the total is worked out for it. CDC's 1,990 reported tests for 2023 work out to 3.0 per 1,000 residents (2025 estimate), close to Colorado's 3.6.

From Aurora to Foxfield, CDC publishes no figure by town: each reads Arapahoe County's.

The 12 largest cities and towns in Arapahoe County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Auroraextends beyond Arapahoe County410,053+6.0%Counted whole
Centennial108,658+0.9%16.1%
Littletonextends beyond Arapahoe County46,246+1.5%Counted whole
Englewood35,305+5.0%5.2%
Greenwood Village15,323−2.0%2.3%
Cherry Hills Village6,389−0.6%0.9%
Sheridan5,972−2.1%0.9%
Glendale4,475−2.7%0.7%
Bennettextends beyond Arapahoe County4,224+45.3%Counted whole
Deer Trail1,755+58.7%0.3%
Columbine Valley1,684+12.1%0.2%
Bow Marextends beyond Arapahoe County832−2.3%Counted whole
  • Foxfield

13 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 Arapahoe County is counted whole, so it has no share.

Arapahoe County, Homes

Homes in Arapahoe County

Of Arapahoe County's 265,980 homes, 56% are detached houses, against 62% across Colorado; 29% are in buildings of five or more homes. Half of its homes were built before 1985, close to Colorado's 1987: 9% date from before 1960 and 26% from 2000 or later. 65% 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 Arapahoe County, Colorado, 2019 to 2023
HomesArapahoe CountyColorado
Housing units265,9802,545,124
Detached houses56%62%
In buildings of 5 or more29%22%
Mobile homes1%4%
Built before 19609%16%
Built since 200026%31%
Middle year built19851987
Owner-occupied65%66%

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