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Radon Levels in St. Charles County, Missouri

EPA puts St. Charles County in Zone 2, its middle radon zone, and high results are common there: 40.8% of the 103 radon tests reported to CDC (2023) 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

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  • Share at 4+ pCi/L

    40.8%

    5.1 pts above MO

    St. Charles
    40.8%
    Missouri
    35.7%

    103 tests, 2023

  • Median result

    3.4pCi/L

    Below action level

    St. Charles
    3.4 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2023

  • Tests reported

    103

    3rd of 7 by tests

    St. Charles
    103
    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

    4 of 7

    Middle half

    St. Charles
    40.8%
    Clay
    44.6%

    By share, of 7 with 30+ tests

Read a result for St. Charles County

Starts at St. Charles 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. St. Charles County's median reported result, 2023.

Interpret this readingWhat a fix costs

The data

St. Charles County, At a glance

St. Charles County at a glance

Predicted: EPA radon zone

EPA Zone 2: moderate predicted potential

St. Charles 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. Charles County, 2023

Share of reported tests at or above 4 pCi/L, by year, St. Charles County
Share of reported tests at or above 4 pCi/L, by year, St. Charles County
YearShare of reported tests at or above 4 pCi/L
201424.2%
201530.6%
201626.2%
201735.9%
201841.9%
201941.7%
202033.6%
202127.9%
202243.7%
202340.8%

2023: 40.8%. Across these years the share ran from 24.2% to 43.7%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

These are the 103 tests reported to CDC for St. Charles 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. Charles County, Compared

St. Charles County radon figures, compared

Calculated by Radon411

In St. Charles County, about 4 in 10 reported tests in 2023 (42 of 103) were at or above EPA's action level. Against the national pool of 33.4%, the gap is 7.4 points higher. Across 91 Missouri counties pooled together, 35.7% of tests reached that level, so St. Charles County runs 5.1 points above its state.

St. Louis County, the one bordering county with a figure for the same period, reported 22.6%. By share, St. Charles County is 4th of 7 Missouri counties with 30 or more tests for the same period.

Year by year, 2014 opened at 24.2% and 2023 closed at 40.8% (up 16.6 points), with 2022 the high at 43.7%. Half the tests came in under 3.4 pCi/L and half over it; the single highest reached 19.0 pCi/L. Below the action level, 35 tests (34.0%) came in under 2 pCi/L, against 36.6% statewide, and 26 tests (25.2%) 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 40.8% share point the same way here. USGS geology rates 100% of the county's land moderate radon potential. CDC also counts 8 tests taken after mitigation in St. Charles County for the same period: tests in homes where a fix was already in.

The busiest year on file was 2016, with 657 tests reported; the quietest, 2023, had 103.

Share of reported tests at or above 4 pCi/L by year, St. Charles County against Missouri2014 to 2023, years with 30 or more tests: St. Charles County 24.2% to 40.8%, the state's pooled share 35.7% in 2023.
St. Charles CountyMissouri, pooled
Share of reported tests at or above 4 pCi/L by year, St. Charles County against Missouri: data table
YearTestsSt. Charles County, at or above 4 pCi/LMissouri, pooled
201431424.2%34.4%
201540230.6%27.6%
201665726.2%29.8%
201739835.9%37.0%
201822941.9%42.1%
201911541.7%33.5%
202012233.6%33.2%
202122927.9%31.8%
202226143.7%40.8%
202310340.8%35.7%
St. Charles County against Missouri, the United States and nearby counties, 2023
AreaTestsAt or above 4 pCi/LMedianDifference from St. Charles County
St. Charles County10340.8%3.4 pCi/LThis county
Missouri, pooled91 counties, pooled1,41235.7%Not pooled−5.1 points
United States, pooled642 counties, pooled128,74033.4%Not pooled−7.4 points
Clay Countyhighest in the state8344.6%3.6 pCi/L+3.8 points
Greene Countylowest in the state4827.1%2.9 pCi/L−13.7 points
Warren County, MOborders it757.1%5.7 pCi/L+16.3 points
Franklin County, MOborders it2045.0%3.8 pCi/L+4.2 points
Lincoln County, MOborders it1643.8%2.8 pCi/L+3.0 points
St. Louis County, MOborders it44322.6%2.0 pCi/L−18.2 points
Where St. Charles County's reported results fell, 2023, beside Missouri and the United States
ResultSt. Charles County testsSt. Charles County shareMissouri shareU.S. shareAgainst Missouri
Under 2 pCi/L3534.0%36.6%41.4%−2.6 points
2 to 4 pCi/L2625.2%27.7%25.2%−2.5 points
4 pCi/L and above4240.8%35.7%33.4%+5.1 points
St. Charles 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. Charles County radon tests by year, against Missouri's pooled share
YearTestsAt or above 4 pCi/LMedianHighestMissouri, pooledChange from the row before
202310340.8%3.4 pCi/L19.0 pCi/L35.7%−2.9 points
202226143.7%3.5 pCi/L34.9 pCi/L40.8%+15.8 points
202122927.9%2.4 pCi/L13.9 pCi/L31.8%−5.7 points
202012233.6%2.8 pCi/L37.4 pCi/L33.2%−8.1 points
201911541.7%3.2 pCi/L22.2 pCi/L33.5%−0.2 points
201822941.9%3.2 pCi/L31.2 pCi/L42.1%+6.0 points
201739835.9%2.8 pCi/L27.5 pCi/L37.0%+9.7 points
201665726.2%2.2 pCi/L36.3 pCi/L29.8%−4.4 points
201540230.6%2.5 pCi/L39.7 pCi/L27.6%+6.4 points
201431424.2%2.2 pCi/L26.9 pCi/L34.4%First on file

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

St. Charles County, Geology

The ground under St. Charles County

Predicted

USGS's geologic map puts all of St. Charles County's mapped land in one class: moderate radon potential. The map puts 82.7% of St. Charles County in the area it calls Salem Plateau and 17.3% in Dissected Till Plains. USGS gives both a radon index of 11, in the moderate band (9 to 11).

USGS's geologic areas under St. Charles County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Salem PlateauOrdovician dolomite, sandstone, limestone, and shale.Moderate1182.7%
Dissected Till PlainsTill-covered Pennsylvanian sandstone, shale, and limestone.Moderate1117.3%

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

St. Charles County beside the ground around it

Across Missouri 8.2% of the land is rated high; none of St. Charles County's is. Of the 7 counties that border it, 4 have more high-potential ground, Jersey County, IL the most at 99.6%.

Share of land by USGS radon potential: St. Charles County, the 7 counties bordering it and Missouri
AreaEPA zoneLow potentialModerate potentialHigh potential
St. Charles CountyZone 2None100%None
St. Louis CountyZone 2None100%None
Lincoln CountyZone 2None97.4%2.6%
Calhoun County, ILZone 1None0.6%99.4%
Warren CountyZone 2None100%None
Jersey County, ILZone 1None0.4%99.6%
Franklin CountyZone 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. Charles County, Local resources

Radon testing and mitigation in St. Charles County

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

What it means

St. Charles County, Questions

Questions about radon in St. Charles County

Are radon levels high in St. Charles County?

In 2023, 40.8% of the radon tests reported to CDC for St. Charles 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, 3 reported a higher share; pooled, the state comes to 35.7%.

Which counties border St. Charles County?

St. Charles County borders 7 counties: St. Louis County, MO, Lincoln County, MO, Calhoun County, IL, Warren County, MO, Jersey County, IL, Franklin County, MO and Madison County, IL.

Products

St. Charles County, Products

Testing a home in St. Charles County

In St. Charles County, 40.8% of 103 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. Charles County; a long-term test tells the year-round average.

Test kits and monitors3

See every kit and monitor

Best radon monitors for St. Charles County homes, compared

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St. Charles County, Bordering counties

Counties bordering St. Charles County

Calculated by Radon411

St. Charles County, Cities and towns

Cities and towns in St. Charles County

The Census Bureau estimates 426,499 residents in St. Charles County for July 1, 2025, 20,092 more than in 2020 (+4.9%). That works out to 761 people per square mile of land. Its largest town, O'Fallon, has 96,101 residents, 22.5% of the total. CDC's 103 reported tests for 2023 work out to 0.2 per 1,000 residents (2025 estimate), close to Missouri's 0.2.

From O'Fallon to Weldon Spring Heights, CDC publishes no figure by town: each reads St. Charles County's.

The 12 largest cities and towns in St. Charles County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
O'Fallon96,101+4.8%22.5%
St. Charles72,561+2.4%17.0%
St. Peters61,479+6.5%14.4%
Wentzville49,495+9.3%11.6%
Lake St. Louis19,129+13.8%4.5%
Dardenne Prairie15,032+17.6%3.5%
Cottleville6,717+17.9%1.6%
Weldon Spring5,301−0.6%1.2%
St. Paul3,849+25.8%0.9%
Foristellextends beyond St. Charles County919+68.3%Counted whole
Flint Hill566+12.7%0.1%
New Melle550−0.4%0.1%
  • Josephville
  • West Alton
  • Portage Des Sioux
  • Augusta
  • Weldon Spring Heights

17 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 St. Charles County is counted whole, so it has no share.

St. Charles County, Homes

Homes in St. Charles County

Of St. Charles County's 164,594 homes, 75% are detached houses, against 70% across Missouri; 11% are in buildings of five or more homes. Much of St. Charles County is recent building: 38% of its homes have gone up since 2000 and the middle year built is 1994, 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 St. Charles County, Missouri, 2019 to 2023
HomesSt. Charles CountyMissouri
Housing units164,5942,809,501
Detached houses75%70%
In buildings of 5 or more11%13%
Mobile homes3%5%
Built before 19607%28%
Built since 200038%21%
Middle year built19941978
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.