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Radon Levels in Johnson County, Kansas

EPA puts Johnson County in Zone 1, its highest radon zone, and high results are common there: 41.2% of the 5,240 radon tests reported to CDC (2022) 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

    41.2%

    9.2 pts above KS

    Johnson
    41.2%
    Kansas
    32.0%

    5,240 tests, 2022

  • Median result

    3.0pCi/L

    Below action level

    Johnson
    3.0 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2022

  • Tests reported

    5,240

    Most tested in Kansas

    Johnson
    5,240

    Reported to CDC before any fix, 2022

  • 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 Kansas

    10 of 38

    Top quarter

    Johnson
    41.2%
    Mitchell
    57.4%

    By share, of 38 with 30+ tests

Read a result for Johnson County

Starts at Johnson County's median, 3.0 pCi/L

3.0pCi/L

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

EPA’s reading of 3.0 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. Johnson County's median reported result, 2022.

Interpret this readingWhat a fix costs

The data

Johnson County, At a glance

Johnson County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

EPA puts all of Johnson 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 98% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

Radon tests reported for Johnson County, 2022

Share of reported tests at or above 4 pCi/L, by year, Johnson County
Share of reported tests at or above 4 pCi/L, by year, Johnson County
YearShare of reported tests at or above 4 pCi/L
201640.1%
201742.0%
201841.4%
201938.2%
202039.7%
202139.6%
202241.2%

2022: 41.2%. Across these years the share ran from 38.2% to 42.0%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

These are the 5,240 tests reported to CDC for Johnson County over 2022, 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: 2022. Measured data period: 2016 to 2022 (every CDC figure on file).

How to read these figures

Johnson County, Compared

Johnson County radon figures, compared

Calculated by Radon411

In Johnson County, about 4 in 10 reported tests in 2022 (2,160 of 5,240) were at or above EPA's action level. Against the national pool of 23.9%, the gap is 17.3 points higher. Across 96 Kansas counties pooled together, 32.0% of tests reached that level, so Johnson County runs 9.2 points above its state.

Of 7 neighbours with a figure, 2 reported a larger share at or above 4 pCi/L and 5 a smaller one; the spread is 25.8% (Douglas County) to 61.3% (Cass County, MO). By share, Johnson County is 10th of 38 Kansas counties with 30 or more tests for the same period.

Year by year, 2016 opened at 40.1% and 2022 closed at 41.2% (about level), with 2017 the high at 42.0%. Half the tests came in under 3.0 pCi/L and half over it; the single highest reached 111.4 pCi/L. Below the action level, 1,872 tests (35.7%) came in under 2 pCi/L, against 45.8% statewide, and 1,208 tests (23.1%) fell between 2 and 4, against 22.1% statewide.

Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 41.2% share point the same way here. USGS geology rates 98% of the county's land high radon potential. CDC also counts 365 tests taken after mitigation in Johnson County for the same period: tests in homes where a fix was already in.

The busiest year on file was 2021, with 25,860 tests reported; the quietest, 2022, had 5,240.

Share of reported tests at or above 4 pCi/L by year, Johnson County against Kansas2016 to 2022, years with 30 or more tests: Johnson County 40.1% to 41.2%, the state's pooled share 32.0% in 2022.
Johnson CountyKansas, pooled
Share of reported tests at or above 4 pCi/L by year, Johnson County against Kansas: data table
YearTestsJohnson County, at or above 4 pCi/LKansas, pooled
20166,58640.1%35.4%
20179,46242.0%37.3%
20188,77541.4%37.3%
201910,60338.2%33.7%
20207,78139.7%31.3%
202125,86039.6%31.8%
20225,24041.2%32.0%
Johnson County against Kansas, the United States and nearby counties, 2022
AreaTestsAt or above 4 pCi/LMedianDifference from Johnson County
Johnson County5,24041.2%3.0 pCi/LThis county
Kansas, pooled96 counties, pooled18,38532.0%Not pooled−9.2 points
United States, pooled853 counties, pooled333,14023.9%Not pooled−17.3 points
Mitchell Countyhighest in the state4757.4%6.2 pCi/L+16.2 points
Crawford Countylowest in the state345.9%0.8 pCi/L−35.3 points
Cass County, MOborders it3161.3%6.2 pCi/L+20.1 points
Jackson County, MOborders it35754.6%4.4 pCi/L+13.4 points
Miami County, KSborders it21440.2%3.0 pCi/L−1.0 points
Franklin County, KSborders it10634.0%2.1 pCi/L−7.2 points
Wyandotte County, KSborders it68729.7%2.1 pCi/L−11.5 points
Leavenworth County, KSborders it51526.8%1.9 pCi/L−14.4 points
Douglas County, KSborders it86825.8%1.8 pCi/L−15.4 points
Where Johnson County's reported results fell, 2022, beside Kansas and the United States
ResultJohnson County testsJohnson County shareKansas shareU.S. shareAgainst Kansas
Under 2 pCi/L1,87235.7%45.8%54.5%−10.1 points
2 to 4 pCi/L1,20823.1%22.1%21.7%+1.0 points
4 pCi/L and above2,16041.2%32.0%23.9%+9.2 points
Johnson County ranked among the 38 Kansas counties with 30 or more tests, by share at or above 4 pCi/L, 2022
CountyRankTestsAt or above 4 pCi/LMedian
Republic County5 of 383452.9%4.3 pCi/L
Saline County6 of 3872952.3%4.2 pCi/L
McPherson County7 of 3821551.6%4.1 pCi/L
Pawnee County8 of 385343.4%3.7 pCi/L
Atchison County9 of 386041.7%3.3 pCi/L
Johnson County10 of 385,24041.2%3.0 pCi/L
Miami County11 of 3821440.2%3.0 pCi/L
Marion County12 of 384540.0%2.5 pCi/L
Cowley County13 of 3811839.8%3.3 pCi/L
Jefferson County14 of 385339.6%2.4 pCi/L
Dickinson County15 of 3811338.9%2.7 pCi/L
Johnson County radon tests by year, against Kansas' pooled share
YearTestsAt or above 4 pCi/LMedianHighestKansas, pooledChange from the row before
20225,24041.2%3.0 pCi/L111.4 pCi/L32.0%+1.6 points
202125,86039.6%2.9 pCi/L155.2 pCi/L31.8%−0.1 points
20207,78139.7%2.9 pCi/L92.5 pCi/L31.3%+1.5 points
201910,60338.2%2.9 pCi/L157.0 pCi/L33.7%−3.2 points
20188,77541.4%3.1 pCi/L108.1 pCi/L37.3%−0.6 points
20179,46242.0%3.1 pCi/L93.7 pCi/L37.3%+1.9 points
20166,58640.1%3.1 pCi/L59.7 pCi/L35.4%First on file

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

Johnson County, Geology

The ground under Johnson County

Predicted

Most of the ground under Johnson County is one class on USGS's geologic map: 97.6% of its land is high radon potential, with 2.4% moderate. 97.5% of it lies in the area the map calls Glacial Drift, to which USGS gives a radon index of 12, in the high band (12 to 17).

USGS's geologic areas under Johnson County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Glacial DriftGlacial drift and loess of varying thickness, largely locally derived from underlying and nearby Paleozoic sedimentary rocks.High1297.5%
Glaciated AreaGlacial till underlain by Pennsylvanian and Permian marine and nonmarine sedimentary shale, sandstone, limestone, coal.Moderate92.4%

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

Johnson County beside the ground around it

Johnson County has more high-potential ground than Kansas as a whole: 97.6% of its land against 59.9%. Of the 7 counties that border it, 1 has more high-potential ground, Wyandotte County the most at 100%.

Share of land by USGS radon potential: Johnson County, the 7 counties bordering it and Kansas
AreaEPA zoneLow potentialModerate potentialHigh potential
Johnson CountyZone 1None2.4%97.6%
Wyandotte CountyZone 1NoneNone100%
Leavenworth CountyZone 1None28.8%71.2%
Miami CountyZone 2None68.6%31.4%
Douglas CountyZone 1None100%None
Jackson County, MOZone 1None30.5%69.5%
Cass County, MOZone 1None64.5%35.5%
Franklin CountyZone 2None100%None
Kansas105 counties, pooled by land areaVariesNone40.1%59.9%

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

Local help

Johnson County, Local resources

Radon testing and mitigation in Johnson County

Listed radon businesses with an address in Johnson County:

Listed radon professionals by town: Overland Park, Olathe, Shawnee, Lenexa, Leawood, Gardner, Merriam, Bonner Springs

What Kansas's program says
Phone
(800) 693-5343

What it means

Johnson County, Questions

Questions about radon in Johnson County

Are radon levels high in Johnson County?

In 2022, 41.2% of the radon tests reported to CDC for Johnson County, Kansas were at or above 4 pCi/L, and the median reported pre-mitigation result was 3.0 pCi/L. Of 38 Kansas counties with 30 or more tests for the same period, 9 reported a higher share; pooled, the state comes to 32.0%.

Which counties border Johnson County?

Johnson County borders 7 counties: Wyandotte County, KS, Leavenworth County, KS, Miami County, KS, Douglas County, KS, Jackson County, MO, Cass County, MO and Franklin County, KS.

Products

Johnson County, Products

Testing a home in Johnson County

In Johnson County, 41.2% of 5,240 tests reported to CDC in 2022 reached 4 pCi/L, against 32.0% pooled across Kansas. In Zone 1 the year-round level in Johnson 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

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

Explore

Johnson County, Bordering counties

Counties bordering Johnson County

Calculated by Radon411

Johnson County, Cities and towns

Cities and towns in Johnson County

The Census Bureau estimates 636,906 residents in Johnson County for July 1, 2025, 25,777 more than in 2020 (+4.2%). Packed into its land, that is 1,345 people per square mile. Its largest town, Overland Park, has 203,677 residents, 32.0% of the total. CDC's 5,240 reported tests for 2022 work out to 8.2 per 1,000 residents (2025 estimate), close to Kansas' 6.2.

From Overland Park to Mission Woods, CDC publishes no figure by town: each reads Johnson County's.

The 12 largest cities and towns in Johnson County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Overland Park203,677+3.0%32.0%
Olathe150,025+6.0%23.6%
Shawnee69,848+3.6%11.0%
Lenexa60,506+5.2%9.5%
Leawood34,093+0.6%5.4%
Gardner26,154+11.6%4.1%
Prairie Village23,008+0.2%3.6%
Merriam11,509+3.8%1.8%
Spring Hillextends beyond Johnson County10,777+31.6%Counted whole
Mission10,125+1.8%1.6%
Bonner Springsextends beyond Johnson County7,962+1.8%Counted whole
De Sotoextends beyond Johnson County7,083+15.1%Counted whole
  • Roeland Park
  • Fairway
  • Mission Hills
  • Edgerton
  • Westwood
  • Lake Quivira (part)
  • Westwood Hills
  • Mission Woods

20 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 Johnson County is counted whole, so it has no share.

Johnson County, Homes

Homes in Johnson County

Of Johnson County's 255,320 homes, 66% are detached houses, against 72% across Kansas; 20% are in buildings of five or more homes. Much of Johnson County is recent building: 30% of its homes have gone up since 2000 and the middle year built is 1988, against 1975 for Kansas. 69% of occupied homes are lived in by their owner (Kansas: 67%); in the rest a tenant can run a test, but a fix is the landlord's to make.

Homes in Johnson County, Kansas, 2019 to 2023
HomesJohnson CountyKansas
Housing units255,3201,285,221
Detached houses66%72%
In buildings of 5 or more20%13%
Mobile homes1%4%
Built before 196014%33%
Built since 200030%20%
Middle year built19881975
Owner-occupied69%67%

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