Long-term kit
RL416 long-term radon test kit, lab fee included
- Lab fee included
Method: Alpha track
In home: 90 days to 12 months
Locations and reports
Radon Levels in Douglas County, Colorado
EPA puts Douglas County in Zone 1, its highest radon zone, and 35.8% of its 1,635 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
Share at 4+ pCi/L
35.8%
3.1 pts below CO
- Douglas
- 35.8%
- Colorado
- 38.9%
1,635 tests, 2023
Median result
2.4pCi/L
Below action level
- Douglas
- 2.4 pCi/L
- EPA
- 4.0 pCi/L
Midpoint of the tests, 2023
Tests reported
1,635
7th of 38 by tests
- Douglas
- 1,635
- Jefferson
- 3,068
Reported to CDC before any fix, 2023
EPA radon zone
Zone 1
Highest potential
- Zone 1
- 2
- 3
1 highest, 3 lowest
EPA predicts over 4 pCi/L indoors
Rank in Colorado
28 of 38
Middle half
- Douglas
- 35.8%
- Park
- 57.5%
By share, of 38 with 30+ tests
Read a result for Douglas County
Starts at Douglas County's median, 2.4 pCi/L
EPA’s own thresholds; the figures are the area’s, never this home’s.
EPA’s reading of 2.4 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. Douglas County's median reported result, 2023.
The data
Douglas County, At a glance
Douglas County at a glance
Predicted: EPA radon zone
EPA Zone 1: highest predicted potential
EPA puts all of Douglas County in
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 Douglas County, 2023
| Year | Share of reported tests at or above 4 pCi/L |
|---|---|
| 2014 | 48.6% |
| 2015 | 45.3% |
| 2016 | 33.3% |
| 2017 | 41.8% |
| 2018 | 47.7% |
| 2019 | 46.3% |
| 2020 | 44.5% |
| 2021 | 47.1% |
| 2022 | 46.6% |
| 2023 | 35.8% |
2023: 35.8%. Across these years the share ran from 33.3% to 48.6%. Each column counts radon tests reported to CDC that year, not homes.
What these figures count
These are the 1,635 tests reported to CDC for Douglas 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).
Douglas County, Compared
Douglas County radon figures, compared
Calculated by Radon411In Douglas County, about 4 in 10 reported tests in 2023 (586 of 1,635) were at or above EPA's action level. Against the national pool of 33.4%, the gap is 2.4 points higher. Across 61 Colorado counties pooled together, 38.9% of tests reached that level, so Douglas County runs 3.1 points below its state.
Of 6 neighbours with a figure, 5 reported a larger share at or above 4 pCi/L and 1 a smaller one; the spread is 34.4% (El Paso County) to 57.5% (Park County). By share, Douglas County is 28th of 38 Colorado counties with 30 or more tests for the same period.
Year by year, 2014 opened at 48.6% and 2023 closed at 35.8% (down 12.8 points), with 2014 the high at 48.6%. Half the tests came in under 2.4 pCi/L and half over it; the single highest reached 60.9 pCi/L. Below the action level, 741 tests (45.3%) came in under 2 pCi/L, against 39.2% statewide, and 308 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 35.8% share point the same way here. USGS geology rates 100% of the county's land high radon potential. CDC also counts 480 tests taken after mitigation in Douglas County for the same period: tests in homes where a fix was already in.
The busiest year on file was 2018, with 2,479 tests reported; the quietest, 2014, had 597.
Douglas CountyColorado, pooled
| Year | Tests | Douglas County, at or above 4 pCi/L | Colorado, pooled |
|---|---|---|---|
| 2014 | 597 | 48.6% | 46.5% |
| 2015 | 751 | 45.3% | 47.2% |
| 2016 | 913 | 33.3% | 38.5% |
| 2017 | 1,017 | 41.8% | 41.0% |
| 2018 | 2,479 | 47.7% | 44.2% |
| 2019 | 2,298 | 46.3% | 44.1% |
| 2020 | 1,714 | 44.5% | 41.9% |
| 2021 | 2,086 | 47.1% | 47.3% |
| 2022 | 1,770 | 46.6% | 38.6% |
| 2023 | 1,635 | 35.8% | 38.9% |
Pooled shares: Radon411's sum of CDC's county counts for one period.
Douglas County, State data
Colorado's own radon test data
MeasuredWhat Colorado's own agencies publish about radon tests in Douglas 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 Douglas 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
46.4%
7,832 of 16,872 tests
Median result
3.5 pCi/L
Highest result
274.0 pCi/L
Tests behind these figures
16,872 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.
Douglas County, Geology
The ground under Douglas County
PredictedUSGS's geologic map puts all of Douglas County's mapped land in one class: high radon potential. The map puts 62.5% of Douglas County in the area it calls Dawson Arkose and 37.1% in Rocky Mountains. USGS gives the first a radon index of 14 and the second 15, both in the high band (12 to 17).
| Area | The rock, in USGS's words | Potential | Radon index | Share of land |
|---|---|---|---|---|
| Dawson Arkose | Consolidated alluvial fan and river deposits sourced in the granite mountains to the west. | High | 14 | 62.5% |
| Rocky Mountains | Precambrian granites, gneiss, amphibolite, variable uranium content, abundant shear zones.Sedimentary rocks shed from the granitic highlands. | High | 15 | 37.1% |
Names, descriptions and indexes: U.S. Geological Survey, Geologic Radon Potential Map (2025). Shares: Radon411, from the map cut to the boundary.
Douglas County beside the ground around it
Douglas County has more high-potential ground than Colorado as a whole: 100% of its land against 74.8%. Of the 6 counties that border it, none has as large a share of high-potential ground; Arapahoe County comes nearest at 100%.
The map rates the ground, not the air in any home.
Local help
Douglas County, Local resources
Radon testing and mitigation in Douglas County
- Colorado's radon programColorado Department of Public Health and EnvironmentColorado's radon program licenses or certifies radon professionals, offers free or discounted test kits and states the rule for disclosing radon when a home is sold, and answers questions about testing and mitigation in Colorado.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 Colorado25 professionals are listed in Colorado.
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
Douglas County, Questions
Questions about radon in Douglas County
Are radon levels high in Douglas County?
In 2023, 35.8% of the radon tests reported to CDC for Douglas County, Colorado were at or above 4 pCi/L, and the median reported pre-mitigation result was 2.4 pCi/L. Of 38 Colorado counties with 30 or more tests for the same period, 27 reported a higher share; pooled, the state comes to 38.9%.
Which counties border Douglas County?
Douglas County borders 6 counties: Jefferson County, Teller County, El Paso County, Elbert County, Arapahoe County and Park County.
Products
Douglas County, Products
Testing a home in Douglas County
In Douglas County, 35.8% of 1,635 tests reported to CDC in 2023 reached 4 pCi/L, against 38.9% pooled across Colorado. In Zone 1 the year-round level in Douglas 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
Monitor
EcoQube Wi-Fi continuous radon monitor
- C-NRPP 2024/25: Approved
- Plug-in
- Wi-Fi
- App only
Method: Ion chamber
First reading: Within 10 minutes
Short-term kit
Air Chek Short-Term radon test kit you hang, lab fee included
- Lab fee included
In home: 72 to 168 hours
- See every kit and monitor Test kits and monitors
Test kits and monitors: 1 of 3
Douglas County is in EPA Zone 1, the highest predicted radon potential. Best radon monitors for Zone 1 counties like Douglas County
Explore
Douglas County, Bordering counties
Counties bordering Douglas County
Calculated by Radon411- Radon levels in Park CountyEPA zone 1
57.5%at or above 4 pCi/L
Median 4.6 pCi/L · 106 tests · 21.7 points above Douglas County
- Radon levels in Teller CountyEPA zone 1
56.8%at or above 4 pCi/L
Median 4.9 pCi/L · 162 tests · 21.0 points above Douglas County
- Radon levels in Elbert CountyEPA zone 1
45.8%at or above 4 pCi/L
Median 2.9 pCi/L · 120 tests · 10.0 points above Douglas County
- Radon levels in Jefferson CountyEPA zone 1
41.8%at or above 4 pCi/L
Median 3.2 pCi/L · 3,068 tests · 6.0 points above Douglas County
- Radon levels in Arapahoe CountyEPA zone 1
37.9%at or above 4 pCi/L
Median 2.5 pCi/L · 1,990 tests · 2.1 points above Douglas County
- Radon levels in El Paso CountyEPA zone 1
34.4%at or above 4 pCi/L
Median 2.5 pCi/L · 1,844 tests · 1.4 points below Douglas County
Douglas County, Cities and towns
Cities and towns in Douglas County
Douglas County is growing: the Census Bureau puts it at 399,396 residents on July 1, 2025, up 10.8% from 360,315 in 2020. That works out to 475 people per square mile of land. Aurora, the largest place on its list (410,053 residents in all), extends beyond Douglas County, so no share of the total is worked out for it. CDC's 1,635 reported tests for 2023 work out to 4.1 per 1,000 residents (2025 estimate), close to Colorado's 3.6.
From Aurora to Larkspur, CDC publishes no figure by town: each reads Douglas County's.
| Place | Residents | Since 2020 | Share of residents |
|---|---|---|---|
| Auroraextends beyond Douglas County | 410,053 | +6.0% | Counted whole |
| Castle Rock | 83,815 | +13.4% | 21.0% |
| Parker | 65,985 | +12.3% | 16.5% |
| Littletonextends beyond Douglas County | 46,246 | +1.5% | Counted whole |
| Castle Pines | 15,583 | +39.0% | 3.9% |
| Lone Tree | 15,278 | +7.5% | 3.8% |
| Larkspur | 219 | +6.8% | Under 0.1% |
Places in Douglas County with a city page: 1 of 2
7 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 Douglas County is counted whole, so it has no share.
Douglas County, Homes
Homes in Douglas County
Of Douglas County's 140,542 homes, 74% are detached houses, against 62% across Colorado; 18% are in buildings of five or more homes. Much of Douglas County is recent building: 53% of its homes have gone up since 2000 and the middle year built is 2001, against 1987 for Colorado. Owners live in 78% of the occupied homes (Colorado: 66%), so for most households a test and a fix are their own to order.
| Homes | Douglas County | Colorado |
|---|---|---|
| Housing units | 140,542 | 2,545,124 |
| Detached houses | 74% | 62% |
| In buildings of 5 or more | 18% | 22% |
| Mobile homes | under 1% | 4% |
| Built before 1960 | 1% | 16% |
| Built since 2000 | 53% | 31% |
| Middle year built | 2001 | 1987 |
| Owner-occupied | 78% | 66% |
Census Bureau, American Community Survey, 2019 to 2023: estimates from a sample of homes, looser for a small place. Shares by Radon411.