Skip to content
RADON411

Locations and reports

Radon Levels in Jefferson County, Missouri

EPA puts Jefferson County in Zone 2, its middle radon zone, and 38.9% of its 36 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

How to test this homeFind a certified tester

  • Share at 4+ pCi/L

    38.9%

    3.2 pts above MO

    Jefferson
    38.9%
    Missouri
    35.7%

    36 tests, 2023

  • Median result

    3.4pCi/L

    Below action level

    Jefferson
    3.4 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2023

  • Tests reported

    36

    7th of 7 by tests

    Jefferson
    36
    St. Louis
    443

    Reported to CDC before any fix, 2023

  • 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

    5 of 7

    Middle half

    Jefferson
    38.9%
    Clay
    44.6%

    By share, of 7 with 30+ tests

Read a result for Jefferson County

Starts at Jefferson County's median, 3.4 pCi/L

3.4pCi/L

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

EPA’s reading of 3.4 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 2: moderate predicted potential

Jefferson 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 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
201444.1%
201537.8%
201641.7%
201747.4%
201860.8%
201939.4%
202042.2%
202158.7%
202254.5%
202338.9%

2023: 38.9%. Across these years the share ran from 37.8% to 60.8%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

These are the 36 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 (14 of 36) were at or above EPA's action level. Against the national pool of 33.4%, the gap is 5.5 points higher. Across 91 Missouri counties pooled together, 35.7% of tests reached that level, so Jefferson County runs 3.2 points above its state.

St. Louis County, the one bordering county with a figure for the same period, reported 22.6%. By share, Jefferson County is 5th of 7 Missouri counties with 30 or more tests for the same period.

Year by year, 2014 opened at 44.1% and 2023 closed at 38.9% (down 5.2 points), with 2018 the high at 60.8%. Half the tests came in under 3.4 pCi/L and half over it; the single highest reached 9.3 pCi/L. Below the action level, 9 tests (25.0%) came in under 2 pCi/L, against 36.6% statewide, and 13 tests (36.1%) fell between 2 and 4, against 27.7% statewide.

Zone 2 on EPA's map (predicted average between 2 and 4 pCi/L) and a 38.9% share point the same way here. USGS geology rates 100% of the county's land moderate radon potential.

The busiest year on file was 2016, with 278 tests reported; the quietest, 2019, had 33.

Share of reported tests at or above 4 pCi/L by year, Jefferson County against Missouri2014 to 2023, years with 30 or more tests: Jefferson County 44.1% to 38.9%, the state's pooled share 35.7% in 2023.
Jefferson CountyMissouri, pooled
Share of reported tests at or above 4 pCi/L by year, Jefferson County against Missouri: data table
YearTestsJefferson County, at or above 4 pCi/LMissouri, pooled
201413644.1%34.4%
201516437.8%27.6%
201627841.7%29.8%
201713347.4%37.0%
20185160.8%42.1%
20193339.4%33.5%
20206442.2%33.2%
20217558.7%31.8%
202214554.5%40.8%
20233638.9%35.7%
Jefferson County against Missouri, the United States and nearby counties, 2023
AreaTestsAt or above 4 pCi/LMedianDifference from Jefferson County
Jefferson County3638.9%3.4 pCi/LThis county
Missouri, pooled91 counties, pooled1,41235.7%Not pooled−3.2 points
United States, pooled642 counties, pooled128,74033.4%Not pooled−5.5 points
Clay Countyhighest in the state8344.6%3.6 pCi/L+5.7 points
Greene Countylowest in the state4827.1%2.9 pCi/L−11.8 points
Washington County, MOborders it2100.0%10.4 pCi/L+61.1 points
Ste. Genevieve County, MOborders it366.7%5.6 pCi/L+27.8 points
Franklin County, MOborders it2045.0%3.8 pCi/L+6.1 points
St. Louis County, MOborders it44322.6%2.0 pCi/L−16.3 points
St. Francois County, MOborders it10.0%3.8 pCi/L−38.9 points
Where Jefferson County's reported results fell, 2023, beside Missouri and the United States
ResultJefferson County testsJefferson County shareMissouri shareU.S. shareAgainst Missouri
Under 2 pCi/L925.0%36.6%41.4%−11.6 points
2 to 4 pCi/L1336.1%27.7%25.2%+8.4 points
4 pCi/L and above1438.9%35.7%33.4%+3.2 points
Jefferson County ranked among the 7 Missouri counties with 30 or more tests, by share at or above 4 pCi/L, 2023
CountyRankTestsAt or above 4 pCi/LMedian
Clay County1 of 78344.6%3.6 pCi/L
Jackson County2 of 720444.1%3.4 pCi/L
Boone County3 of 76342.9%3.6 pCi/L
St. Charles County4 of 710340.8%3.4 pCi/L
Jefferson County5 of 73638.9%3.4 pCi/L
Greene County6 of 74827.1%2.9 pCi/L
St. Louis County7 of 744322.6%2.0 pCi/L
Jefferson County radon tests by year, against Missouri's pooled share
YearTestsAt or above 4 pCi/LMedianHighestMissouri, pooledChange from the row before
20233638.9%3.4 pCi/L9.3 pCi/L35.7%−15.6 points
202214554.5%4.7 pCi/L72.8 pCi/L40.8%−4.2 points
20217558.7%4.6 pCi/L74.3 pCi/L31.8%+16.5 points
20206442.2%3.1 pCi/L19.6 pCi/L33.2%+2.8 points
20193339.4%3.3 pCi/L80.9 pCi/L33.5%−21.4 points
20185160.8%5.0 pCi/L80.4 pCi/L42.1%+13.4 points
201713347.4%3.6 pCi/L99.7 pCi/L37.0%+5.7 points
201627841.7%3.2 pCi/L245.5 pCi/L29.8%+3.9 points
201516437.8%2.8 pCi/L50.5 pCi/L27.6%−6.3 points
201413644.1%3.6 pCi/L77.7 pCi/L34.4%First on file

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

Jefferson County, Geology

The ground under Jefferson County

Predicted

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

USGS's geologic areas under Jefferson County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Salem PlateauOrdovician 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.

Jefferson County beside the ground around it

Across Missouri 8.2% of the land is rated high; none of Jefferson County's is. Of the 6 counties that border it, 2 have more high-potential ground, St. Francois County the most at 31.2%.

Share of land by USGS radon potential: Jefferson County, the 6 counties bordering it and Missouri
AreaEPA zoneLow potentialModerate potentialHigh potential
Jefferson CountyZone 2None100%None
Monroe County, ILZone 2None100%None
Washington CountyZone 2None94.9%5.1%
St. Louis CountyZone 2None100%None
Franklin CountyZone 2None100%None
St. Francois CountyZone 2None68.8%31.2%
Ste. Genevieve CountyZone 2None100%None
Missouri115 counties, pooled by land areaVaries5.8%86.0%8.2%

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

Local help

Jefferson County, Local resources

Radon testing and mitigation in Jefferson County

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

What it means

Jefferson County, Questions

Questions about radon in Jefferson County

Are radon levels high in Jefferson County?

In 2023, 38.9% of the radon tests reported to CDC for Jefferson County, Missouri were at or above 4 pCi/L, and the median reported pre-mitigation result was 3.4 pCi/L. Of 7 Missouri counties with 30 or more tests for the same period, 4 reported a higher share; pooled, the state comes to 35.7%.

Which counties border Jefferson County?

Jefferson County borders 6 counties: Monroe County, IL, Washington County, MO, St. Louis County, MO, Franklin County, MO, St. Francois County, MO and Ste. Genevieve County, MO.

Products

Jefferson County, Products

Testing a home in Jefferson County

In Jefferson County, 38.9% of 36 tests reported to CDC in 2023 reached 4 pCi/L, against 35.7% pooled across Missouri. In Zone 2 a short-term kit (2 to 90 days) is the quick first check in Jefferson County; a long-term test tells the year-round average.

Test kits and monitors3

See every kit and monitor

Best radon monitors for Jefferson County homes, compared

Explore

Jefferson County, Bordering counties

Counties bordering Jefferson County

Calculated by Radon411

Jefferson County, Cities and towns

Cities and towns in Jefferson County

The Census Bureau estimates 233,132 residents in Jefferson County for July 1, 2025, 6,188 more than in 2020 (+2.7%). That works out to 355 people per square mile of land. No town dominates: Arnold, the largest, has 20,994 residents, 9.0% of the total. CDC's 36 reported tests for 2023 work out to 0.2 per 1,000 residents (2025 estimate), close to Missouri's 0.2.

From Arnold to Peaceful Village, CDC publishes no figure by town: each reads Jefferson County's.

The 12 largest cities and towns in Jefferson County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Arnold20,994+0.4%9.0%
Festus14,066+10.0%6.0%
Eurekaextends beyond Jefferson County13,422+14.0%Counted whole
De Soto6,527+1.1%2.8%
Pevely6,239+3.0%2.7%
Herculaneum5,402+15.1%2.3%
Crystal City4,694−0.9%2.0%
Hillsboro3,493+0.1%1.5%
Byrnes Mill3,183+0.2%1.4%
Olympian Village710−1.3%0.3%
Lake Tekakwitha233+1.7%Under 0.1%
Cedar Hill Lakes203−1.9%Under 0.1%
  • Scotsdale
  • Parkdale
  • Kimmswick
  • Peaceful Village

16 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 Jefferson County is counted whole, so it has no share.

Jefferson County, Homes

Homes in Jefferson County

Jefferson County is a place of houses on their own lots: 80% of its 93,276 homes are detached single-family houses, against 70% across Missouri. Much of Jefferson County is recent building: 27% of its homes have gone up since 2000 and the middle year built is 1986, against 1978 for Missouri. Owners live in 81% of the occupied homes (Missouri: 68%), so for most households a test and a fix are their own to order.

Homes in Jefferson County, Missouri, 2019 to 2023
HomesJefferson CountyMissouri
Housing units93,2762,809,501
Detached houses80%70%
In buildings of 5 or more5%13%
Mobile homes10%5%
Built before 196015%28%
Built since 200027%21%
Middle year built19861978
Owner-occupied81%68%

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