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Radon Levels in St. Louis County, Missouri

EPA puts St. Louis County in Zone 2, its middle radon zone, and 22.6% of its 443 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

    22.6%

    13.1 pts below MO

    St. Louis
    22.6%
    Missouri
    35.7%

    443 tests, 2023

  • Median result

    2.0pCi/L

    Below action level

    St. Louis
    2.0 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2023

  • Tests reported

    443

    Most tested in Missouri

    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

    7 of 7

    Lowest quarter

    St. Louis
    22.6%
    Clay
    44.6%

    By share, of 7 with 30+ tests

Read a result for St. Louis County

Starts at St. Louis County's median, 2.0 pCi/L

2.0pCi/L

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

EPA’s reading of 2.0 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. St. Louis County's median reported result, 2023.

Interpret this readingWhat a fix costs

The data

St. Louis County, At a glance

St. Louis County at a glance

Predicted: EPA radon zone

EPA Zone 2: moderate predicted potential

St. Louis 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 St. Louis County, 2023

Share of reported tests at or above 4 pCi/L, by year, St. Louis County
Share of reported tests at or above 4 pCi/L, by year, St. Louis County
YearShare of reported tests at or above 4 pCi/L
201425.7%
201522.7%
201623.7%
201730.1%
201837.0%
201928.8%
202026.8%
202129.1%
202236.7%
202322.6%

2023: 22.6%. Across these years the share ran from 22.6% to 37.0%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

These are the 443 tests reported to CDC for St. Louis 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

St. Louis County, Compared

St. Louis County radon figures, compared

Calculated by Radon411

St. Louis County's 443 reported tests in 2023 had a middle value of 2.0 pCi/L and a maximum of 112.2 pCi/L, and 22.6% reached 4 pCi/L. Missouri as a whole (91 Missouri counties, pooled) comes to 35.7%: 13.1 points higher than here.

Among its state's counties it ranks 7th of 7 Missouri counties with 30 or more tests for the same period. Statewide for the same period, Clay County reported the highest share (44.6%) and Greene County the lowest (27.1%).

Below the action level, 212 tests (47.9%) came in under 2 pCi/L, against 36.6% statewide, and 131 tests (29.6%) fell between 2 and 4, against 27.7% statewide. The busiest year on file was 2016, with 1,356 tests reported; the quietest, 2019, had 208.

EPA's 1993 zone map puts St. Louis 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 38.9% in Jefferson County to 40.8% in St. Charles County. Over 10 years of annual figures the share fell 3.1 points, from 25.7% in 2014 to 22.6% in 2023; its peak was 37.0% in 2018.

CDC also counts 20 tests taken after mitigation in St. Louis County for the same period: tests in homes where a fix was already in.

Share of reported tests at or above 4 pCi/L by year, St. Louis County against Missouri2014 to 2023, years with 30 or more tests: St. Louis County 25.7% to 22.6%, the state's pooled share 35.7% in 2023.
St. Louis CountyMissouri, pooled
Share of reported tests at or above 4 pCi/L by year, St. Louis County against Missouri: data table
YearTestsSt. Louis County, at or above 4 pCi/LMissouri, pooled
201478325.7%34.4%
201597522.7%27.6%
20161,35623.7%29.8%
201745930.1%37.0%
201823837.0%42.1%
201920828.8%33.5%
202029826.8%33.2%
202133729.1%31.8%
202244136.7%40.8%
202344322.6%35.7%
St. Louis County against Missouri, the United States and nearby counties, 2023
AreaTestsAt or above 4 pCi/LMedianDifference from St. Louis County
St. Louis County44322.6%2.0 pCi/LThis county
Missouri, pooled91 counties, pooled1,41235.7%Not pooled+13.1 points
United States, pooled642 counties, pooled128,74033.4%Not pooled+10.8 points
Clay Countyhighest in the state8344.6%3.6 pCi/L+22.0 points
Greene Countylowest in the state4827.1%2.9 pCi/L+4.5 points
Franklin County, MOborders it2045.0%3.8 pCi/L+22.4 points
St. Charles County, MOborders it10340.8%3.4 pCi/L+18.2 points
Jefferson County, MOborders it3638.9%3.4 pCi/L+16.3 points
St. Louis city, MOborders it933.3%1.7 pCi/L+10.7 points
Where St. Louis County's reported results fell, 2023, beside Missouri and the United States
ResultSt. Louis County testsSt. Louis County shareMissouri shareU.S. shareAgainst Missouri
Under 2 pCi/L21247.9%36.6%41.4%+11.3 points
2 to 4 pCi/L13129.6%27.7%25.2%+1.9 points
4 pCi/L and above10022.6%35.7%33.4%−13.1 points
St. Louis 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
St. Louis County radon tests by year, against Missouri's pooled share
YearTestsAt or above 4 pCi/LMedianHighestMissouri, pooledChange from the row before
202344322.6%2.0 pCi/L112.2 pCi/L35.7%−14.1 points
202244136.7%2.9 pCi/L95.5 pCi/L40.8%+7.6 points
202133729.1%2.3 pCi/L53.1 pCi/L31.8%+2.3 points
202029826.8%2.2 pCi/L40.6 pCi/L33.2%−2.0 points
201920828.8%2.5 pCi/L38.8 pCi/L33.5%−8.2 points
201823837.0%2.7 pCi/L38.7 pCi/L42.1%+6.9 points
201745930.1%2.6 pCi/L41.6 pCi/L37.0%+6.4 points
20161,35623.7%2.2 pCi/L54.3 pCi/L29.8%+1.0 points
201597522.7%2.1 pCi/L26.1 pCi/L27.6%−3.0 points
201478325.7%2.2 pCi/L59.5 pCi/L34.4%First on file

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

St. Louis County, Geology

The ground under St. Louis County

Predicted

USGS's geologic map puts all of St. Louis 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 St. Louis County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Salem PlateauOrdovician dolomite, sandstone, limestone, and shale.Moderate1199.9%

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

St. Louis County beside the ground around it

Across Missouri 8.2% of the land is rated high; none of St. Louis County's is. Of the 7 counties that border it, 1 has more high-potential ground, Madison County, IL the most at 8.3%.

Share of land by USGS radon potential: St. Louis County, the 7 counties bordering it and Missouri
AreaEPA zoneLow potentialModerate potentialHigh potential
St. Louis CountyZone 2None100%None
St. Louis cityZone 2None100%None
St. Charles CountyZone 2None100%None
Jefferson CountyZone 2None100%None
Monroe County, ILZone 2None100%None
Franklin CountyZone 2None100%None
St. Clair County, ILZone 2None100%None
Madison County, ILZone 2None91.7%8.3%
Missouri115 counties, pooled by land areaVaries5.8%86.0%8.2%

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

Local help

St. Louis County, Local resources

Radon testing and mitigation in St. Louis County

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

What it means

St. Louis County, Questions

Questions about radon in St. Louis County

Are radon levels high in St. Louis County?

In 2023, 22.6% of the radon tests reported to CDC for St. Louis County, Missouri were at or above 4 pCi/L, and the median reported pre-mitigation result was 2.0 pCi/L. Of 7 Missouri counties with 30 or more tests for the same period, 6 reported a higher share; pooled, the state comes to 35.7%.

Which counties border St. Louis County?

St. Louis County borders 7 counties: St. Louis city, MO, St. Charles County, MO, Jefferson County, MO, Monroe County, IL, Franklin County, MO, St. Clair County, IL and Madison County, IL.

Products

St. Louis County, Products

Testing a home in St. Louis County

In St. Louis County, 22.6% of 443 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 St. Louis County; a long-term test tells the year-round average.

Test kits and monitors3

See every kit and monitor

Best radon monitors for St. Louis County homes, compared

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St. Louis County, Bordering counties

Counties bordering St. Louis County

Calculated by Radon411

St. Louis County, Cities and towns

Cities and towns in St. Louis County

About 990,911 people live in St. Louis County by the Census Bureau's estimate for July 1, 2025, 12,454 fewer than in 2020 (−1.2%). Packed into its land, that is 1,951 people per square mile. No town dominates: Florissant, the largest, has 51,124 residents, 5.2% of the total. Testing is reported here more often than across the state: CDC's 443 reported tests for 2023 come to 0.4 for every 1,000 residents (2025 estimate), against 0.2 for Missouri.

From Florissant to Glen Echo Park, CDC publishes no figure by town: each reads St. Louis County's.

The 12 largest cities and towns in St. Louis County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Florissant51,124−2.5%5.2%
Chesterfield49,309−1.3%5.0%
Wildwood34,955−1.2%3.5%
University City34,657−1.4%3.5%
Ballwin30,901−2.4%3.1%
Kirkwood29,331−0.4%3.0%
Maryland Heights27,559−2.5%2.8%
Hazelwood24,849−2.2%2.5%
Webster Groves23,450−2.3%2.4%
Creve Coeur18,480−1.8%1.9%
Manchester18,069−1.4%1.8%
Ferguson17,953−2.9%1.8%
  • Clayton
  • Overland
  • Eureka (part)
  • Jennings
  • St. Ann
  • Crestwood
  • Town and Country
  • Bridgeton
  • Bellefontaine Neighbors
  • Ellisville
  • Des Peres
  • Richmond Heights
  • Sunset Hills
  • Olivette
  • Ladue
  • Brentwood
  • Maplewood
  • Berkeley
  • Pacific (part)
  • Valley Park
  • Black Jack
  • St. John
  • Shrewsbury
  • Glendale
The other 52 places in St. Louis County
  • Dellwood
  • Rock Hill
  • Normandy
  • Breckenridge Hills
  • Fenton
  • Woodson Terrace
  • Frontenac
  • Northwoods
  • Pine Lawn
  • Green Park
  • Clarkson Valley
  • Pagedale
  • Riverview
  • Marlborough
  • Moline Acres
  • Bel-Ridge
  • Warson Woods
  • Hanley Hills
  • Vinita Park
  • Wellston
  • Lakeshire
  • Winchester
  • Oakland
  • Bel-Nor
  • Velda City
  • Charlack
  • Hillsdale
  • Calverton Park
  • Cool Valley
  • Country Club Hills
  • Grantwood Village
  • Pasadena Hills
  • Norwood Court
  • Velda Village Hills
  • Edmundson
  • Bella Villa
  • Flordell Hills
  • Greendale
  • Twin Oaks
  • Sycamore Hills
  • Crystal Lake Park
  • Beverly Hills
  • Pasadena Park
  • Wilbur Park
  • Huntleigh
  • Westwood
  • Uplands Park
  • Kinloch
  • Bellerive Acres
  • Country Life Acres
  • Champ
  • Glen Echo Park

88 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).

St. Louis County, Homes

Homes in St. Louis County

Of St. Louis County's 445,472 homes, 72% are detached houses, against 70% across Missouri; 17% are in buildings of five or more homes. The homes are older than Missouri's as a whole: half were built before 1968 (1978 for Missouri), and 35% date from before 1960. 69% 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 St. Louis County, Missouri, 2019 to 2023
HomesSt. Louis CountyMissouri
Housing units445,4722,809,501
Detached houses72%70%
In buildings of 5 or more17%13%
Mobile homesunder 1%5%
Built before 196035%28%
Built since 200010%21%
Middle year built19681978
Owner-occupied69%68%

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