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Radon Levels in St. Clair County, Illinois

EPA puts St. Clair County in Zone 2, its middle radon zone, and 33.7% of its 5,193 reported radon tests (2017) 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

    33.7%

    0.0 pts above IL

    St. Clair
    33.7%
    Illinois
    33.7%

    5,193 tests, 2017

  • Median result

    2.6pCi/L

    Below action level

    St. Clair
    2.6 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2017

  • Tests reported

    5,193

    8th of 70 by tests

    St. Clair
    5,193
    Cook
    26,937

    Reported to CDC before any fix, 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 Illinois

    52 of 70

    Middle half

    St. Clair
    33.7%
    Stark
    69.2%

    By share, of 70 with 30+ tests

Read a result for St. Clair County

Starts at St. Clair County's median, 2.6 pCi/L

2.6pCi/L

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

EPA’s reading of 2.6 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. Clair County's median reported result, 2017.

Interpret this readingWhat a fix costs

The data

St. Clair County, At a glance

St. Clair County at a glance

Predicted: EPA radon zone

EPA Zone 2: moderate predicted potential

St. Clair 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. Clair County, 2017

Share of reported tests at or above 4 pCi/L, by year, St. Clair County
Share of reported tests at or above 4 pCi/L, by year, St. Clair County
YearShare of reported tests at or above 4 pCi/L
201436.8%
201534.0%
201633.3%
201733.7%

2017: 33.7%. Across these years the share ran from 33.3% to 36.8%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

These are the 5,193 tests reported to CDC for St. Clair County over 2017, 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: 2017. Measured data period: 2005 to 2017 (every CDC figure on file).

How to read these figures

St. Clair County, Compared

St. Clair County radon figures, compared

Calculated by Radon411

In St. Clair County, about 3 in 10 reported tests in 2017 (1,749 of 5,193) were at or above EPA's action level. Against the national pool of 31.7%, the gap is 2.0 points higher. Pooled across 93 Illinois counties, the share is the same 33.7%.

Of 7 neighbours with a figure, 3 reported a larger share at or above 4 pCi/L and 4 a smaller one; the spread is 11.8% (Washington County) to 39.9% (Monroe County). By share, St. Clair County is 52nd of 70 Illinois counties with 30 or more tests for the same period.

Year by year, 2014 opened at 36.8% and 2017 closed at 33.7% (down 3.1 points), with 2014 the high at 36.8%. Half the tests came in under 2.6 pCi/L and half over it; the single highest reached 73.6 pCi/L. Below the action level, 1,974 tests (38.0%) came in under 2 pCi/L, against 37.7% statewide, and 1,470 tests (28.3%) fell between 2 and 4, against 28.6% statewide.

Zone 2 on EPA's map (predicted average between 2 and 4 pCi/L) and a 33.7% share point the same way here. USGS geology rates 100% of the county's land moderate radon potential.

The busiest year on file was 2017, with 5,193 tests reported; the quietest, 2016, had 108.

Share of reported tests at or above 4 pCi/L by year, St. Clair County against Illinois2014 to 2017, years with 30 or more tests: St. Clair County 36.8% to 33.7%, the state's pooled share 33.7% in 2017.
St. Clair CountyIllinois, pooled
Share of reported tests at or above 4 pCi/L by year, St. Clair County against Illinois: data table
YearTestsSt. Clair County, at or above 4 pCi/LIllinois, pooled
20145,11536.8%37.8%
20153,32134.0%36.6%
201610833.3%17.4%
20175,19333.7%33.7%
St. Clair County against Illinois, the United States and nearby counties, 2017
AreaTestsAt or above 4 pCi/LMedianDifference from St. Clair County
St. Clair County5,19333.7%2.6 pCi/LThis county
Illinois, pooled93 counties, pooled138,00633.7%Not pooledLevel
United States, pooled937 counties, pooled528,06231.7%Not pooled−2.0 points
Stark Countyhighest in the state3969.2%5.8 pCi/L+35.5 points
Williamson Countylowest in the state2498.4%0.9 pCi/L−25.3 points
Monroe County, ILborders it71439.9%3.0 pCi/L+6.2 points
Clinton County, ILborders it58237.6%3.0 pCi/L+3.9 points
Randolph County, ILborders it6035.0%2.9 pCi/L+1.3 points
St. Louis County, MOborders it45930.1%2.6 pCi/L−3.6 points
Madison County, ILborders it5,21429.5%2.6 pCi/L−4.2 points
St. Louis city, MOborders it8225.6%2.1 pCi/L−8.1 points
Washington County, ILborders it5111.8%2.0 pCi/L−21.9 points
Where St. Clair County's reported results fell, 2017, beside Illinois and the United States
ResultSt. Clair County testsSt. Clair County shareIllinois shareU.S. shareAgainst Illinois
Under 2 pCi/L1,97438.0%37.7%43.5%+0.3 points
2 to 4 pCi/L1,47028.3%28.6%24.8%−0.3 points
4 pCi/L and above1,74933.7%33.7%31.7%Level
St. Clair County ranked among the 70 Illinois counties with 30 or more tests, by share at or above 4 pCi/L, 2017
CountyRankTestsAt or above 4 pCi/LMedian
Kane County47 of 708,68236.6%2.8 pCi/L
McHenry County48 of 706,47435.1%2.9 pCi/L
Randolph County49 of 706035.0%2.9 pCi/L
Kendall County50 of 703,04535.0%2.8 pCi/L
Winnebago County51 of 702,70334.9%2.9 pCi/L
St. Clair County52 of 705,19333.7%2.6 pCi/L
DuPage County53 of 7019,40133.7%2.7 pCi/L
Hancock County54 of 706333.3%2.8 pCi/L
Grundy County55 of 7039032.3%2.4 pCi/L
Bond County56 of 707532.0%2.6 pCi/L
Kankakee County57 of 7052231.6%2.7 pCi/L
St. Clair County radon tests by year, against Illinois' pooled share
YearTestsAt or above 4 pCi/LMedianHighestIllinois, pooledChange from the row before
20175,19333.7%2.6 pCi/L73.6 pCi/L33.7%+0.4 points
201610833.3%2.8 pCi/L6.1 pCi/L17.4%−0.7 points
20153,32134.0%2.8 pCi/L89.3 pCi/L36.6%−2.8 points
20145,11536.8%3.0 pCi/L120.9 pCi/L37.8%First on file

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

St. Clair County, Geology

The ground under St. Clair County

Predicted

USGS's geologic map puts all of St. Clair County's mapped land in one class: moderate radon potential. 98.8% of it lies in the area the map calls Southern Central Lowland Province, to which USGS gives a radon index of 9, in the moderate band (9 to 11).

USGS's geologic areas under St. Clair County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Southern Central Lowland ProvinceIllinoian glacial deposits, sandstone, and gray shale.Moderate998.8%
Salem PlateauOrdovician dolomite, sandstone, limestone, and shale.Moderate111.2%

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

St. Clair County beside the ground around it

Across Illinois 64.5% of the land is rated high; none of St. Clair County's is. Of the 7 counties that border it, 1 has more high-potential ground, Madison County the most at 8.3%.

Share of land by USGS radon potential: St. Clair County, the 7 counties bordering it and Illinois
AreaEPA zoneLow potentialModerate potentialHigh potential
St. Clair CountyZone 2None100%None
Monroe CountyZone 2None100%None
St. Louis city, MOZone 2None100%None
Madison CountyZone 2None91.7%8.3%
Clinton CountyZone 2None100%None
Washington CountyZone 2None100%None
St. Louis County, MOZone 2None100%None
Randolph CountyZone 2None100%None
Illinois102 counties, pooled by land areaVaries0.9%34.5%64.5%

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

Local help

St. Clair County, Local resources

Radon testing and mitigation in St. Clair County

Listed radon businesses with an address in St. Clair County:

Listed radon professionals by town: Belleville, Collinsville, Shiloh, Swansea, Freeburg, Millstadt

What Illinois's program says
Phone
(800) 325-1245
Licensing of radon professionals
The Agency also licenses and regulates the individuals who provide radon measurement and mitigation to the public.
Disclosure when a home is sold or rented
Illinois Radon Awareness Act - This is the legislation that requires the seller to provide to the buyer of any interest in residential real property the IEMA-OHS pamphlet entitled "Radon Testing Guidelines for Real Estate Transactions" and the Illinois Disclosure of Information on Radon Hazards.

What it means

St. Clair County, Questions

Questions about radon in St. Clair County

Are radon levels high in St. Clair County?

In 2017, 33.7% of the radon tests reported to CDC for St. Clair County, Illinois were at or above 4 pCi/L, and the median reported pre-mitigation result was 2.6 pCi/L. Of 70 Illinois counties with 30 or more tests for the same period, 51 reported a higher share; pooled, the state comes to 33.7%.

Which counties border St. Clair County?

St. Clair County borders 7 counties: Monroe County, IL, St. Louis city, MO, Madison County, IL, Clinton County, IL, Washington County, IL, St. Louis County, MO and Randolph County, IL.

Products

St. Clair County, Products

Testing a home in St. Clair County

In St. Clair County, 33.7% of 5,193 tests reported to CDC in 2017 reached 4 pCi/L, against 33.7% pooled across Illinois. In Zone 2 a short-term kit (2 to 90 days) is the quick first check in St. Clair County; a long-term test tells the year-round average.

Test kits and monitors3

See every kit and monitor

Best radon monitors for St. Clair County homes, compared

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St. Clair County, Bordering counties

Counties bordering St. Clair County

Calculated by Radon411

St. Clair County, Cities and towns

Cities and towns in St. Clair County

About 250,708 people live in St. Clair County by the Census Bureau's estimate for July 1, 2025, 6,349 fewer than in 2020 (−2.5%). That works out to 381 people per square mile of land. No town dominates: Belleville, the largest, has 40,237 residents, 16.0% of the total. Testing is reported here more often than across the state: CDC's 5,193 reported tests for 2017 come to 20.7 for every 1,000 residents (2025 estimate), against 10.9 for Illinois.

From Belleville to Centreville, CDC publishes no figure by town: each reads St. Clair County's.

The 12 largest cities and towns in St. Clair County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Belleville40,237−4.8%16.0%
O'Fallon32,656+0.8%13.0%
Collinsvilleextends beyond St. Clair County23,717−2.7%Counted whole
East St. Louis17,682−4.1%7.1%
Fairview Heights16,175−3.1%6.5%
Shiloh14,604+3.6%5.8%
Swansea14,356+0.1%5.7%
Columbiaextends beyond St. Clair County10,865−1.6%Counted whole
Mascoutah9,239+5.2%3.7%
Lebanon4,884+2.6%1.9%
Freeburg4,507−2.0%1.8%
Caseyville4,402+0.3%1.8%
  • Millstadt
  • Smithton
  • Dupo (part)
  • New Baden (part)
  • Madison (part)
  • Washington Park
  • Fairmont City (part)
  • New Athens
  • Marissa
  • Brooklyn
  • St. Libory
  • Lenzburg
  • Hecker (part)
  • East Carondelet
  • Summerfield
  • Fayetteville
  • Sauget
  • Alorton
  • Cahokia
  • Centreville

32 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. Clair County is counted whole, so it has no share.

St. Clair County, Homes

Homes in St. Clair County

Of St. Clair County's 115,348 homes, 71% are detached houses, against 58% across Illinois; 11% are in buildings of five or more homes. Half of its homes were built before 1975, close to Illinois' 1970: 33% date from before 1960 and 21% from 2000 or later. 68% of occupied homes are lived in by their owner (Illinois: 67%); in the rest a tenant can run a test, but a fix is the landlord's to make.

Homes in St. Clair County, Illinois, 2019 to 2023
HomesSt. Clair CountyIllinois
Housing units115,3485,443,501
Detached houses71%58%
In buildings of 5 or more11%22%
Mobile homes5%2%
Built before 196033%38%
Built since 200021%16%
Middle year built19751970
Owner-occupied68%67%

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