Skip to content
RADON411

Locations and reports

Radon Levels in Johnson County, Illinois

EPA puts Johnson County in Zone 2, its middle radon zone, and only 12 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

How to test this homeFind a certified tester

  • Share at 4+ pCi/L

    12.2%

    31.5 pts below IL

    Johnson
    12.2%
    Illinois
    43.7%

    49 tests, 2007 to 2016

  • Median result

    1.8pCi/L

    Below action level

    Johnson
    1.8 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2007 to 2016

  • Tests reported

    49

    86th of 92 by tests

    Johnson
    49
    Cook
    13,699

    Reported to CDC before any fix, 2007 to 2016

  • 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

    92 of 92

    Lowest quarter

    Johnson
    12.2%
    Stark
    79.4%

    By share, of 92 with 30+ tests

Read a result for Johnson County

Starts at Johnson County's median, 1.8 pCi/L

1.8pCi/L

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

EPA’s reading of 1.8 pCi/L

Below 2 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 below 2 pCi/L, the level at which EPA says a fix should be considered. Johnson County's median reported result, 2007 to 2016.

Interpret this reading

The data

Johnson County, At a glance

Johnson County at a glance

Predicted: EPA radon zone

EPA Zone 2: moderate predicted potential

Johnson 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 Johnson County, 2017

Share of reported tests at or above 4 pCi/L, by year, Johnson County
Share of reported tests at or above 4 pCi/L, by year, Johnson County
YearShare of reported tests at or above 4 pCi/L
20140.0%
201725.0%

2017: 25.0%. Across these years the share ran from 0.0% to 25.0%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

Only 12 tests were reported to CDC for Johnson 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

Johnson County, Compared

Johnson County radon figures, compared

Calculated by Radon411

Johnson County reports fewer high tests than Illinois overall. 12.2% of its 49 tests over the 2007 to 2016 laboratory totals were at or above 4 pCi/L. That is 31.5 points below the 43.7% pooled from 102 Illinois counties.

The newest figure, in 2017, counts only 12 tests (25.0% at or above 4 pCi/L), too few to compare, so this reading uses the 49-test figure.

Ordered from highest share down, it comes 92nd of 92 Illinois counties with 30 or more tests for the same period. Nationally, every county with a figure for the same period pools to 35.8%; Johnson County sits 23.6 points below it.

Next door, the shares run from 13.6% in Williamson County to 22.7% in Union County. Zone 2 on EPA's map (predicted average between 2 and 4 pCi/L) and a 12.2% share point the same way here.

Half the tests came in under 1.8 pCi/L and half over it; the single highest reached 12.3 pCi/L. The rest split 63.3% under 2 pCi/L and 24.5% from 2 to 4 (Illinois: 31.2% and 25.1%).

The busiest year on file was 2014, with 18 tests reported; the quietest, 2017, had 12.

Johnson County against Illinois, the United States and nearby counties, 2007 to 2016, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Johnson County
Johnson County4912.2%1.8 pCi/LThis county
Illinois, pooled102 counties, pooled105,29343.7%Not pooled+31.5 points
United States, pooled2,666 counties, pooled1,939,02535.8%Not pooled+23.6 points
Stark Countyhighest in the state6879.4%7.8 pCi/L+67.2 points
Wayne Countylowest in the state8714.9%1.4 pCi/L+2.7 points
Union County, ILborders it11022.7%1.9 pCi/L+10.5 points
Saline County, ILborders it8318.1%1.7 pCi/L+5.9 points
Massac County, ILborders it3517.1%2.2 pCi/L+4.9 points
Williamson County, ILborders it60513.6%1.3 pCi/L+1.4 points
Pulaski County, ILborders it1612.5%1.3 pCi/L+0.3 points
Pope County, ILborders it80.0%1.8 pCi/L−12.2 points
Where Johnson County's reported results fell, 2007 to 2016, laboratory, beside Illinois and the United States
ResultJohnson County testsJohnson County shareIllinois shareU.S. shareAgainst Illinois
Under 2 pCi/L3163.3%31.2%39.6%+32.1 points
2 to 4 pCi/L1224.5%25.1%24.6%−0.6 points
4 pCi/L and above612.2%43.7%35.8%−31.5 points
Johnson County ranked among the 92 Illinois counties with 30 or more tests, by share at or above 4 pCi/L, 2007 to 2016, laboratory
CountyRankTestsAt or above 4 pCi/LMedian
Crawford County82 of 9217519.4%1.8 pCi/L
Richland County83 of 9214019.3%1.9 pCi/L
Franklin County84 of 9221019.0%1.8 pCi/L
Saline County85 of 928318.1%1.7 pCi/L
Clay County86 of 926917.4%1.1 pCi/L
Clark County87 of 927517.3%1.8 pCi/L
Massac County88 of 923517.1%2.2 pCi/L
Lawrence County89 of 924615.2%1.8 pCi/L
Wayne County90 of 928714.9%1.4 pCi/L
Williamson County91 of 9260513.6%1.3 pCi/L
Johnson County92 of 924912.2%1.8 pCi/L
Johnson County radon tests by year, against Illinois' pooled share
YearTestsAt or above 4 pCi/LMedianHighestIllinois, pooledChange from the row before
20171225.0%2.5 pCi/L4.0 pCi/L33.7%+25.0 points
2014180.0%1.5 pCi/L1.6 pCi/L37.8%First on file

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

Johnson County, Geology

The ground under Johnson County

Predicted

Most of the ground under Johnson County is one class on USGS's geologic map: 97.3% of its land is moderate radon potential, with 2.7% low. The map puts 93.9% of Johnson County in the area it calls Shawnee Hills Section and 3.4% in Southern Central Lowland Province, with one smaller area beside them. USGS gives the first a radon index of 10 and the second 9, both in the moderate band (9 to 11).

USGS's geologic areas under Johnson County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Shawnee Hills SectionUnglaciated Ordovician to Pennsylvanian limestone, sandstone, and shale.Moderate1093.9%
Southern Central Lowland ProvinceIllinoian glacial deposits, sandstone, and gray shale.Moderate93.4%
Coastal Plain ProvinceCretaceous marine sediments and Quaternary alluvium.Low82.7%

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

Johnson County beside the ground around it

Across Illinois 64.5% of the land is rated high; none of Johnson County's is. Of the 6 counties that border it, 3 have more moderate-potential ground, Saline County the most at 100%.

Share of land by USGS radon potential: Johnson County, the 6 counties bordering it and Illinois
AreaEPA zoneLow potentialModerate potentialHigh potential
Johnson CountyZone 22.7%97.3%None
Williamson CountyZone 2None100%None
Pope CountyZone 22.8%97.2%None
Massac CountyZone 397.9%2.1%None
Union CountyZone 2None100%None
Pulaski CountyZone 378.9%21.1%None
Saline 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

Johnson County, Local resources

Radon testing and mitigation in Johnson 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

Johnson County, Questions

Questions about radon in Johnson County

Are radon levels high in Johnson County?

In 2017, 25.0% of the radon tests reported to CDC for Johnson County, Illinois were at or above 4 pCi/L, and the median reported pre-mitigation result was 2.5 pCi/L. Of 92 Illinois counties with 30 or more tests for the same period, 91 reported a higher share; pooled, the state comes to 43.7%.

Which counties border Johnson County?

Johnson County borders 6 counties: Williamson County, Pope County, Massac County, Union County, Pulaski County and Saline County.

Products

Johnson County, Products

Testing a home in Johnson County

In Johnson County, 12.2% of 49 tests reported to CDC over the 2007 to 2016 laboratory totals reached 4 pCi/L, against 43.7% pooled across Illinois. In Zone 2 a short-term kit (2 to 90 days) is the quick first check in Johnson County; a long-term test tells the year-round average.

Test kits and monitors3

See every kit and monitor

Best radon monitors for Johnson County homes, compared

Explore

Johnson County, Bordering counties

Counties bordering Johnson County

Calculated by Radon411

Johnson County, Cities and towns

Cities and towns in Johnson County

Johnson County's population has barely moved since 2020: 13,363 residents on July 1, 2025 against 13,317 then, by the Census Bureau's estimates. That works out to 39 people per square mile of land. No town dominates: Vienna, the largest, has 1,352 residents, 10.1% of the total. Together its 7 incorporated places hold 23.3% of those residents; the other 76.7% live outside any of them. Reported testing is thin here: 49 tests over 2007 to 2016, or 3.7 for every 1,000 residents (2025 estimate), where Illinois as a whole comes to 8.3.

From Vienna to Simpson, CDC publishes no figure by town: each reads Johnson County's.

The cities and towns in Johnson County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Vienna1,352−0.1%10.1%
Goreville1,055−0.7%7.9%
Cypress217Level1.6%
Buncombe207+1.0%1.5%
New Burnside153Level1.1%
Belknap86Level0.6%
Simpson45Level0.3%

7 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).

Johnson County, Homes

Homes in Johnson County

Of Johnson County's 5,538 homes, 70% are detached houses, against 58% across Illinois; 23% are mobile homes. Much of Johnson County is recent building: 29% of its homes have gone up since 2000 and the middle year built is 1985, against 1970 for Illinois. Owners live in 85% of the occupied homes (Illinois: 67%), so for most households a test and a fix are their own to order.

Homes in Johnson County, Illinois, 2019 to 2023
HomesJohnson CountyIllinois
Housing units5,5385,443,501
Detached houses70%58%
In buildings of 5 or more3%22%
Mobile homes23%2%
Built before 196016%38%
Built since 200029%16%
Middle year built19851970
Owner-occupied85%67%

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