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Radon Levels in Johnson County, Missouri

EPA puts Johnson County in Zone 2, its middle radon zone, and only 20 radon tests from it are on file with CDC (2023). 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

    25.8%

    4.8 pts below MO

    Johnson
    25.8%
    Missouri
    30.6%

    264 tests, 2008 to 2017

  • Median result

    1.9pCi/L

    Below action level

    Johnson
    1.9 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    264

    19th of 72 by tests

    Johnson
    264
    St. Louis
    17,747

    Reported to CDC before any fix, 2008 to 2017

  • EPA radon zone

    Zone 2

    Moderate potential

    1. 1
    2. Zone 2
    3. 3

    1 highest, 3 lowest

    EPA predicts 2 to 4 pCi/L indoors

  • Rank in Missouri

    49 of 72

    Middle half

    Johnson
    25.8%
    Atchison
    69.9%

    By share, of 72 with 30+ tests

Read a result for Johnson County

Starts at Johnson County's median, 1.9 pCi/L

1.9pCi/L

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

EPA’s reading of 1.9 pCi/L

Below 2 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 below 2 pCi/L, the level at which EPA says a fix should be considered. Johnson County's median reported result, 2008 to 2017.

Interpret this reading

The data

Johnson County, At a glance

Johnson County at a glance

Predicted: EPA radon zone

EPA Zone 2: moderate predicted potential

Johnson County is a county: EPA's forecast for it as a whole is an average indoor of 2 to 4 pCi/L (75 to 150 Bq/m³). It describes no single address, and homes above EPA's are found in every zone, this one included.

USGS geologic potential: Moderate across 100% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

Radon tests reported for Johnson County, 2023

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
201444.4%
201537.5%
201620.0%
201750.0%
201833.3%
201950.0%
202016.7%
202116.7%
202233.3%
202345.0%

2023: 45.0%. Across these years the share ran from 16.7% to 50.0%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

Only 20 tests were reported to CDC for Johnson County over 2023, and tests are often run on homes being sold or already suspected of a problem: so few describe neither the county nor 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

Johnson County, Compared

Johnson County radon figures, compared

Calculated by Radon411

Johnson County's 264 reported tests over the 2008 to 2017 laboratory totals had a middle value of 1.9 pCi/L and a maximum of 83.1 pCi/L, and 25.8% reached 4 pCi/L. Missouri as a whole (115 Missouri counties, pooled) comes to 30.6%: 4.8 points higher than here.

The newest figure, in 2023, counts only 20 tests (45.0% at or above 4 pCi/L), too few to compare, so this reading uses the 264-test figure.

Among its state's counties it ranks 49th of 72 Missouri counties with 30 or more tests for the same period. Statewide for the same period, Atchison County reported the highest share (69.9%) and Vernon County the lowest (8.8%).

Below the action level, 133 tests (50.4%) came in under 2 pCi/L, against 41.2% statewide, and 63 tests (23.9%) fell between 2 and 4, against 28.1% statewide. The busiest year on file was 2016, with 45 tests reported; the quietest, 2019, had 2.

EPA's 1993 zone map puts Johnson County in Zone 2, a predicted county average between 2 and 4 pCi/L. USGS geology rates 100% of the county's land moderate radon potential.

Next door, the shares run from 37.9% in Pettis County to 46.7% in Lafayette County.

CDC also counts 11 tests taken after mitigation in Johnson County for the same period: tests in homes where a fix was already in.

Johnson County against Missouri, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Johnson County
Johnson County26425.8%1.9 pCi/LThis county
Missouri, pooled115 counties, pooled58,52530.6%Not pooled+4.8 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled+9.9 points
Atchison Countyhighest in the state10369.9%5.6 pCi/L+44.1 points
Vernon Countylowest in the state348.8%0.9 pCi/L−17.0 points
Lafayette County, MOborders it18446.7%3.5 pCi/L+20.9 points
Cass County, MOborders it98845.5%3.5 pCi/L+19.7 points
Jackson County, MOborders it6,61841.3%3.1 pCi/L+15.5 points
Henry County, MOborders it8138.3%3.4 pCi/L+12.5 points
Pettis County, MOborders it13237.9%2.8 pCi/L+12.1 points
Where Johnson County's reported results fell, 2008 to 2017, laboratory, beside Missouri and the United States
ResultJohnson County testsJohnson County shareMissouri shareU.S. shareAgainst Missouri
Under 2 pCi/L13350.4%41.2%39.6%+9.2 points
2 to 4 pCi/L6323.9%28.1%24.7%−4.2 points
4 pCi/L and above6825.8%30.6%35.7%−4.8 points
Johnson County ranked among the 72 Missouri counties with 30 or more tests, by share at or above 4 pCi/L, 2008 to 2017, laboratory
CountyRankTestsAt or above 4 pCi/LMedian
Callaway County44 of 7219227.1%1.7 pCi/L
Daviess County45 of 723026.7%2.0 pCi/L
St. Charles County46 of 727,00126.5%2.2 pCi/L
St. Louis County47 of 7217,74726.3%2.3 pCi/L
Washington County48 of 723826.3%1.5 pCi/L
Johnson County49 of 7226425.8%1.9 pCi/L
Moniteau County50 of 726825.0%2.2 pCi/L
Cole County51 of 7276824.6%2.1 pCi/L
DeKalb County52 of 723324.2%2.3 pCi/L
Monroe County53 of 723023.3%1.5 pCi/L
Camden County54 of 7276322.9%2.2 pCi/L
Johnson County radon tests by year, against Missouri's pooled share
YearTestsAt or above 4 pCi/LMedianHighestMissouri, pooledChange from the row before
20232045.0%3.4 pCi/L29.9 pCi/L35.7%+11.7 points
2022933.3%3.4 pCi/L15.7 pCi/L40.8%+16.6 points
2021616.7%2.0 pCi/L21.2 pCi/L31.8%Level
2020616.7%1.7 pCi/L7.1 pCi/L33.2%−33.3 points
2019250.0%6.9 pCi/L12.1 pCi/L33.5%+16.7 points
20181833.3%2.4 pCi/L13.1 pCi/L42.1%−16.7 points
20171450.0%3.4 pCi/L12.6 pCi/L37.0%+30.0 points
20164520.0%2.0 pCi/L31.3 pCi/L29.8%−17.5 points
2015837.5%2.7 pCi/L8.7 pCi/L27.6%−6.9 points
20141844.4%2.8 pCi/L37.0 pCi/L34.4%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

USGS's geologic map puts all of Johnson County's mapped land in one class: moderate radon potential. All of it lies in the area the map calls Osage Plains, to which USGS gives a radon index of 11, in the moderate band (9 to 11).

USGS's geologic areas under Johnson County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Osage PlainsOrdovician dolomite, sandstone, limestone, and shale.Moderate11100%

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

Across Missouri 8.2% of the land is rated high; none of Johnson County's is. Of the 5 counties that border it, 2 have more high-potential ground, Jackson County the most at 69.5%.

Share of land by USGS radon potential: Johnson County, the 5 counties bordering it and Missouri
AreaEPA zoneLow potentialModerate potentialHigh potential
Johnson CountyZone 2None100%None
Lafayette CountyZone 2None100%None
Pettis CountyZone 2None100%None
Henry CountyZone 2None100%None
Cass CountyZone 1None64.5%35.5%
Jackson CountyZone 1None30.5%69.5%
Missouri115 counties, pooled by land areaVaries5.8%86.0%8.2%

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

Local help

Johnson County, Local resources

Radon testing and mitigation in Johnson County

What Missouri's program says
Phone
573-751-6102
Email
radon@health.mo.gov

What it means

Johnson County, Questions

Questions about radon in Johnson County

Are radon levels high in Johnson County?

In 2023, 45.0% of the radon tests reported to CDC for Johnson County, Missouri were at or above 4 pCi/L, and the median reported pre-mitigation result was 3.4 pCi/L. Of 72 Missouri counties with 30 or more tests for the same period, 48 reported a higher share; pooled, the state comes to 30.6%.

Which counties border Johnson County?

Johnson County borders 5 counties: Lafayette County, Pettis County, Henry County, Cass County and Jackson County.

Products

Johnson County, Products

Testing a home in Johnson County

In Johnson County, 25.8% of 264 tests reported to CDC over the 2008 to 2017 laboratory totals reached 4 pCi/L, against 30.6% pooled across Missouri. In Zone 2 a short-term kit (2 to 90 days) is the quick first check in Johnson County; a long-term test tells the year-round average.

Test kits and monitors3

See every kit and monitor

Best radon monitors for Johnson County homes, compared

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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 56,670 residents in Johnson County for July 1, 2025, 2,647 more than in 2020 (+4.9%). That works out to 68 people per square mile of land. Its largest town, Warrensburg, has 20,283 residents, 35.8% of the total. Together its 7 incorporated places hold 47.5% of those residents; the other 52.5% live outside any of them. Reported testing is thin here: 264 tests over 2008 to 2017, or 4.7 for every 1,000 residents (2025 estimate), where Missouri as a whole comes to 9.3.

From Warrensburg to Centerview, CDC publishes no figure by town: each reads Johnson County's.

The cities and towns in Johnson County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Warrensburg20,283+5.0%35.8%
Knob Noster2,997+8.7%5.3%
Holden2,347+6.4%4.1%
Leeton565+6.4%1.0%
Chilhowee295+2.8%0.5%
Kingsville251+4.6%0.4%
Centerview198+4.8%0.3%

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).

Johnson County, Homes

Homes in Johnson County

Of Johnson County's 22,663 homes, 67% are detached houses, against 70% across Missouri; 9% are mobile homes. Much of Johnson County is recent building: 34% of its homes have gone up since 2000 and the middle year built is 1991, against 1978 for Missouri. 64% of occupied homes are lived in by their owner (Missouri: 68%); in the rest a tenant can run a test, but a fix is the landlord's to make.

Homes in Johnson County, Missouri, 2019 to 2023
HomesJohnson CountyMissouri
Housing units22,6632,809,501
Detached houses67%70%
In buildings of 5 or more8%13%
Mobile homes9%5%
Built before 196018%28%
Built since 200034%21%
Middle year built19911978
Owner-occupied64%68%

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