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Radon Levels in Clark County, Illinois

EPA puts Clark County in Zone 2, its middle radon zone, and only 9 radon tests from it are on file with CDC (2017). 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

    22.1%

    21.0 pts below IL

    Clark
    22.1%
    Illinois
    43.1%

    77 tests, 2008 to 2017

  • Median result

    2.0pCi/L

    Below action level

    Clark
    2.0 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    77

    78th of 93 by tests

    Clark
    77
    Cook
    14,199

    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 Illinois

    80 of 93

    Lowest quarter

    Clark
    22.1%
    Stark
    80.5%

    By share, of 93 with 30+ tests

Read a result for Clark County

Starts at Clark 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. Clark County's median reported result, 2008 to 2017.

Interpret this readingWhat a fix costs

The data

Clark County, At a glance

Clark County at a glance

Predicted: EPA radon zone

EPA Zone 2: moderate predicted potential

Clark 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 97% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

Radon tests reported for Clark County, 2017

Share of reported tests at or above 4 pCi/L, by year, Clark County
Share of reported tests at or above 4 pCi/L, by year, Clark County
YearShare of reported tests at or above 4 pCi/L
20140.0%
201533.3%
201733.3%

2017: 33.3%. Across these years the share ran from 0.0% to 33.3%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

Only 9 tests were reported to CDC for Clark County over 2017, 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: 2017. Measured data period: 2005 to 2017 (every CDC figure on file).

How to read these figures

Clark County, Compared

Clark County radon figures, compared

Calculated by Radon411

Clark County's 77 reported tests over the 2008 to 2017 laboratory totals had a middle value of 2.0 pCi/L and a maximum of 12.6 pCi/L, and 22.1% reached 4 pCi/L. Illinois as a whole (102 Illinois counties, pooled) comes to 43.1%: 21.0 points higher than here.

The newest figure, in 2017, counts only 9 tests (33.3% at or above 4 pCi/L), too few to compare, so this reading uses the 77-test figure.

Among its state's counties it ranks 80th of 93 Illinois counties with 30 or more tests for the same period. Statewide for the same period, Stark County reported the highest share (80.5%) and Williamson County the lowest (13.3%).

Below the action level, 38 tests (49.4%) came in under 2 pCi/L, against 31.6% statewide, and 22 tests (28.6%) fell between 2 and 4, against 25.3% statewide. The busiest year on file was 2015, with 9 tests reported; the quietest, 2014, had 6.

EPA's 1993 zone map puts Clark County in Zone 2, a predicted county average between 2 and 4 pCi/L. USGS geology rates 97% of the county's land moderate radon potential.

Next door, the shares run from 19.3% in Crawford County to 44.0% in Vigo County, IN, with Cumberland County and Crawford County below Clark County.

Clark County against Illinois, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Clark County
Clark County7722.1%2.0 pCi/LThis county
Illinois, pooled102 counties, pooled108,90943.1%Not pooled+21.0 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled+13.6 points
Stark Countyhighest in the state8780.5%7.4 pCi/L+58.4 points
Williamson Countylowest in the state60113.3%1.3 pCi/L−8.8 points
Sullivan County, INborders it1346.2%3.1 pCi/L+24.1 points
Vigo County, INborders it45544.0%3.6 pCi/L+21.9 points
Coles County, ILborders it29538.6%3.1 pCi/L+16.5 points
Edgar County, ILborders it11637.9%3.2 pCi/L+15.8 points
Jasper County, ILborders it7623.7%1.9 pCi/L+1.6 points
Cumberland County, ILborders it5521.8%1.8 pCi/L−0.3 points
Crawford County, ILborders it18119.3%1.8 pCi/L−2.8 points
Where Clark County's reported results fell, 2008 to 2017, laboratory, beside Illinois and the United States
ResultClark County testsClark County shareIllinois shareU.S. shareAgainst Illinois
Under 2 pCi/L3849.4%31.6%39.6%+17.8 points
2 to 4 pCi/L2228.6%25.3%24.7%+3.3 points
4 pCi/L and above1722.1%43.1%35.7%−21.0 points
Clark County ranked among the 93 Illinois counties with 30 or more tests, by share at or above 4 pCi/L, 2008 to 2017, laboratory
CountyRankTestsAt or above 4 pCi/LMedian
Washington County75 of 9313226.5%2.6 pCi/L
Union County76 of 938725.3%1.9 pCi/L
Jasper County77 of 937623.7%1.9 pCi/L
Hamilton County78 of 933023.3%1.9 pCi/L
Jefferson County79 of 9332022.2%1.9 pCi/L
Clark County80 of 937722.1%2.0 pCi/L
Cumberland County81 of 935521.8%1.8 pCi/L
Saline County82 of 938220.7%1.7 pCi/L
Marion County83 of 9336420.6%1.7 pCi/L
Cook County84 of 9314,19920.2%1.8 pCi/L
Franklin County85 of 9319919.6%1.8 pCi/L
Clark County radon tests by year, against Illinois' pooled share
YearTestsAt or above 4 pCi/LMedianHighestIllinois, pooledChange from the row before
2017933.3%3.1 pCi/L6.5 pCi/L33.7%Level
2015933.3%2.0 pCi/L6.1 pCi/L36.6%+33.3 points
201460.0%1.6 pCi/L1.6 pCi/L37.8%First on file

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

Clark County, Geology

The ground under Clark County

Predicted

Most of the ground under Clark County is one class on USGS's geologic map: 96.5% of its land is moderate radon potential, with 3.5% high. 96.3% 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 Clark County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Southern Central Lowland ProvinceIllinoian glacial deposits, sandstone, and gray shale.Moderate996.3%
Bloomington Ridged PlainWisconsinan glacial deposits and loess over Paleozoic sedimentary rocks.High133.5%

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

Clark County beside the ground around it

Illinois as a whole has more high-potential ground than Clark County: 64.5% of its land, against 3.5% here. Of the 7 counties that border it, 4 have more high-potential ground, Coles County the most at 98.1%.

Share of land by USGS radon potential: Clark County, the 7 counties bordering it and Illinois
AreaEPA zoneLow potentialModerate potentialHigh potential
Clark CountyZone 2None96.5%3.5%
Crawford CountyZone 2None100%None
Edgar CountyZone 1None5.1%94.9%
Vigo County, INZone 2None86.2%13.8%
Cumberland CountyZone 2None94.7%5.3%
Sullivan County, INZone 2None100%None
Jasper CountyZone 2None100%None
Coles CountyZone 1None1.9%98.1%
Illinois102 counties, pooled by land areaVaries0.9%34.5%64.5%

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

Local help

Clark County, Local resources

Radon testing and mitigation in Clark County

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

Clark County, Questions

Questions about radon in Clark County

Are radon levels high in Clark County?

In 2017, 33.3% of the radon tests reported to CDC for Clark County, Illinois were at or above 4 pCi/L, and the median reported pre-mitigation result was 3.1 pCi/L. Of 93 Illinois counties with 30 or more tests for the same period, 79 reported a higher share; pooled, the state comes to 43.1%.

Which counties border Clark County?

Clark County borders 7 counties: Crawford County, IL, Edgar County, IL, Vigo County, IN, Cumberland County, IL, Sullivan County, IN, Jasper County, IL and Coles County, IL.

Products

Clark County, Products

Testing a home in Clark County

In Clark County, 22.1% of 77 tests reported to CDC over the 2008 to 2017 laboratory totals reached 4 pCi/L, against 43.1% pooled across Illinois. In Zone 2 a short-term kit (2 to 90 days) is the quick first check in Clark County; a long-term test tells the year-round average.

Test kits and monitors3

See every kit and monitor

Best radon monitors for Clark County homes, compared

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Clark County, Bordering counties

Counties bordering Clark County

Calculated by Radon411

Clark County, Cities and towns

Cities and towns in Clark County

About 15,114 people live in Clark County by the Census Bureau's estimate for July 1, 2025, 342 fewer than in 2020 (−2.2%). That works out to 30 people per square mile of land. Its largest town, Marshall, has 3,880 residents, 25.7% of the total. Reported testing is thin here: 77 tests over 2008 to 2017, or 5.1 for every 1,000 residents (2025 estimate), where Illinois as a whole comes to 8.6.

From Marshall to Westfield, CDC publishes no figure by town: each reads Clark County's.

The cities and towns in Clark County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Marshall3,880−1.8%25.7%
Caseyextends beyond Clark County2,374−2.3%Counted whole
Martinsville1,083−2.7%7.2%
Westfield518−2.8%3.4%

4 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). A place that extends beyond Clark County is counted whole, so it has no share.

Clark County, Homes

Homes in Clark County

Clark County is a place of houses on their own lots: 81% of its 7,333 homes are detached single-family houses, against 58% across Illinois. Half of its homes were built before 1969, close to Illinois' 1970: 38% date from before 1960 and 15% from 2000 or later. Owners live in 82% of the occupied homes (Illinois: 67%), so for most households a test and a fix are their own to order.

Homes in Clark County, Illinois, 2019 to 2023
HomesClark CountyIllinois
Housing units7,3335,443,501
Detached houses81%58%
In buildings of 5 or more6%22%
Mobile homes7%2%
Built before 196038%38%
Built since 200015%16%
Middle year built19691970
Owner-occupied82%67%

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