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Radon Levels in Stephenson County, Illinois

EPA puts Stephenson County in Zone 1, its highest radon zone, and high results are common there: 57.2% of the 519 radon tests reported to CDC (2017) 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

    57.2%

    23.5 pts above IL

    Stephenson
    57.2%
    Illinois
    33.7%

    519 tests, 2017

  • Median result

    4.8pCi/L

    Action level reached

    Stephenson
    4.8 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2017

  • Tests reported

    519

    28th of 70 by tests

    Stephenson
    519
    Cook
    26,937

    Reported to CDC before any fix, 2017

  • EPA radon zone

    Zone 1

    Highest potential

    1. Zone 1
    2. 2
    3. 3

    1 highest, 3 lowest

    EPA predicts over 4 pCi/L indoors

  • Rank in Illinois

    8 of 70

    Top quarter

    Stephenson
    57.2%
    Stark
    69.2%

    By share, of 70 with 30+ tests

Read a result for Stephenson County

Starts at Stephenson County's median, 4.8 pCi/L

4.8pCi/L

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

EPA’s reading of 4.8 pCi/L

At or above 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 at or above 4 pCi/L. EPA recommends fixing a home when the radon level is 4 pCi/L or higher. Stephenson County's median reported result, 2017.

Interpret this readingWhat a fix costs

The data

Stephenson County, At a glance

Stephenson County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

EPA puts all of Stephenson County in and predicts an average indoor of greater than 4 pCi/L (greater than 150 Bq/m³) across the county. That is one forecast for the whole county, not a reading of any address in it; homes above EPA's turn up in all three zones.

USGS geologic potential: High across 100% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

Radon tests reported for Stephenson County, 2017

Share of reported tests at or above 4 pCi/L, by year, Stephenson County
Share of reported tests at or above 4 pCi/L, by year, Stephenson County
YearShare of reported tests at or above 4 pCi/L
201467.2%
201556.0%
201757.2%

2017: 57.2%. Across these years the share ran from 56.0% to 67.2%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

These are the 519 tests reported to CDC for Stephenson 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

Stephenson County, Compared

Stephenson County radon figures, compared

Calculated by Radon411

More than half: 57.2% of the 519 radon tests reported to CDC for Stephenson County in 2017 read at least 4 pCi/L. That places it 8th of 70 Illinois counties with 30 or more tests for the same period. That is 23.5 points above the 33.7% pooled from 93 Illinois counties.

The median result, 4.8 pCi/L, sits above EPA's 4 pCi/L action level, and the top reported result was 61.6 pCi/L. The rest split 20.8% under 2 pCi/L and 22.0% from 2 to 4 (Illinois: 37.7% and 28.6%). Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 57.2% share point the same way here.

1 of its 6 bordering counties with figures ran higher, led by Carroll County at 62.9%; Lafayette County, WI reported 31.3%. Statewide for the same period, Stark County reported the highest share (69.2%) and Williamson County the lowest (8.4%).

The busiest year on file was 2017, with 519 tests reported; the quietest, 2015, had 75.

Over 3 years of annual figures the share fell 10.0 points, from 67.2% in 2014 to 57.2% in 2017; its peak was 67.2% in 2014. Nationally, every county with a figure for the same period pools to 31.7%; Stephenson County sits 25.5 points above it.

Share of reported tests at or above 4 pCi/L by year, Stephenson County against Illinois2014 to 2017, years with 30 or more tests: Stephenson County 67.2% to 57.2%, the state's pooled share 33.7% in 2017.
Stephenson CountyIllinois, pooled
Share of reported tests at or above 4 pCi/L by year, Stephenson County against Illinois: data table
YearTestsStephenson County, at or above 4 pCi/LIllinois, pooled
201438467.2%37.8%
20157556.0%36.6%
201751957.2%33.7%
Stephenson County against Illinois, the United States and nearby counties, 2017
AreaTestsAt or above 4 pCi/LMedianDifference from Stephenson County
Stephenson County51957.2%4.8 pCi/LThis county
Illinois, pooled93 counties, pooled138,00633.7%Not pooled−23.5 points
United States, pooled937 counties, pooled528,06231.7%Not pooled−25.5 points
Stark Countyhighest in the state3969.2%5.8 pCi/L+12.0 points
Williamson Countylowest in the state2498.4%0.9 pCi/L−48.8 points
Carroll County, ILborders it21062.9%5.5 pCi/L+5.7 points
Green County, WIborders it7755.8%4.5 pCi/L−1.4 points
Ogle County, ILborders it63652.4%4.1 pCi/L−4.8 points
Jo Daviess County, ILborders it51345.0%3.5 pCi/L−12.2 points
Winnebago County, ILborders it2,70334.9%2.9 pCi/L−22.3 points
Lafayette County, WIborders it8031.3%2.6 pCi/L−25.9 points
Where Stephenson County's reported results fell, 2017, beside Illinois and the United States
ResultStephenson County testsStephenson County shareIllinois shareU.S. shareAgainst Illinois
Under 2 pCi/L10820.8%37.7%43.5%−16.9 points
2 to 4 pCi/L11422.0%28.6%24.8%−6.6 points
4 pCi/L and above29757.2%33.7%31.7%+23.5 points
Stephenson 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
Morgan County3 of 7038166.9%6.5 pCi/L
Fulton County4 of 7014464.6%5.8 pCi/L
Carroll County5 of 7021062.9%5.5 pCi/L
Livingston County6 of 7034561.7%5.3 pCi/L
Bureau County7 of 7033358.6%5.1 pCi/L
Stephenson County8 of 7051957.2%4.8 pCi/L
De Witt County9 of 7012657.1%5.2 pCi/L
Ford County9 of 704257.1%4.0 pCi/L
Knox County11 of 7024956.6%4.7 pCi/L
Henry County12 of 7042055.7%4.6 pCi/L
McDonough County12 of 7021055.7%4.3 pCi/L
Stephenson County radon tests by year, against Illinois' pooled share
YearTestsAt or above 4 pCi/LMedianHighestIllinois, pooledChange from the row before
201751957.2%4.8 pCi/L61.6 pCi/L33.7%+1.2 points
20157556.0%5.8 pCi/L23.3 pCi/L36.6%−11.2 points
201438467.2%5.8 pCi/L51.4 pCi/L37.8%First on file

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

Stephenson County, Geology

The ground under Stephenson County

Predicted

USGS's geologic map puts all of Stephenson County's mapped land in one class: high radon potential. All of it lies in the area the map calls Galesburg Plain, to which USGS gives a radon index of 13, in the high band (12 to 17).

USGS's geologic areas under Stephenson County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Galesburg PlainIllinoian glacial deposits and loess over Paleozoic sedimentary rocks.High13100%

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

Stephenson County beside the ground around it

Stephenson County has more high-potential ground than Illinois as a whole: 100% of its land against 64.5%. Of the 6 counties that border it, none has as large a share of high-potential ground; Jo Daviess County comes nearest at 100%.

Share of land by USGS radon potential: Stephenson County, the 6 counties bordering it and Illinois
AreaEPA zoneLow potentialModerate potentialHigh potential
Stephenson CountyZone 1NoneNone100%
Green County, WIZone 1NoneNone100%
Carroll CountyZone 1NoneNone100%
Winnebago CountyZone 1NoneNone100%
Jo Daviess CountyZone 1NoneNone100%
Ogle CountyZone 1NoneNone100%
Lafayette County, WIZone 1NoneNone100%
Illinois102 counties, pooled by land areaVaries0.9%34.5%64.5%

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

Local help

Stephenson County, Local resources

Radon testing and mitigation in Stephenson County

Listed radon businesses with an address in Stephenson County:

Listed radon professionals by town: Freeport, Lena

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

Stephenson County, Questions

Questions about radon in Stephenson County

Are radon levels high in Stephenson County?

In 2017, 57.2% of the radon tests reported to CDC for Stephenson County, Illinois were at or above 4 pCi/L, and the median reported pre-mitigation result was 4.8 pCi/L. Of 70 Illinois counties with 30 or more tests for the same period, 7 reported a higher share; pooled, the state comes to 33.7%.

Which counties border Stephenson County?

Stephenson County borders 6 counties: Green County, WI, Carroll County, IL, Winnebago County, IL, Jo Daviess County, IL, Ogle County, IL and Lafayette County, WI.

Products

Stephenson County, Products

Testing a home in Stephenson County

In Stephenson County, 57.2% of 519 tests reported to CDC in 2017 reached 4 pCi/L, against 33.7% pooled across Illinois. In Zone 1 the year-round level in Stephenson County is the one to know: a long-term test (more than 90 days) or a monitor, with a short-term kit for a first answer.

Test kits and monitors3

See every kit and monitor

Stephenson County is in EPA Zone 1, the highest predicted radon potential. Best radon monitors for Zone 1 counties like Stephenson County

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Stephenson County, Bordering counties

Counties bordering Stephenson County

Calculated by Radon411

Stephenson County, Cities and towns

Cities and towns in Stephenson County

About 43,220 people live in Stephenson County by the Census Bureau's estimate for July 1, 2025, 1,350 fewer than in 2020 (−3.0%). That works out to 77 people per square mile of land. Freeport alone is home to 23,202 of them, 53.7% of the total. Together its 11 incorporated places hold 69.8% of those residents; the other 30.2% live outside any of them. CDC's 519 reported tests for 2017 work out to 12.0 per 1,000 residents (2025 estimate), close to Illinois' 10.9.

From Freeport to Ridott, CDC publishes no figure by town: each reads Stephenson County's.

The cities and towns in Stephenson County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Freeport23,202−3.2%53.7%
Lena2,670−3.2%6.2%
Pearl City760−2.7%1.8%
Orangeville742−3.5%1.7%
Cedarville646−3.3%1.5%
Davis571−3.4%1.3%
Dakota480−3.6%1.1%
German Valley417−3.2%1.0%
Rock City285−3.4%0.7%
Winslow277−3.8%0.6%
Ridott123−3.1%0.3%

11 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).

Stephenson County, Homes

Homes in Stephenson County

Stephenson County is a place of houses on their own lots: 76% of its 21,375 homes are detached single-family houses, against 58% across Illinois. The homes are older than Illinois' as a whole: half were built before 1959 (1970 for Illinois), and 51% date from before 1960. 71% 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 Stephenson County, Illinois, 2019 to 2023
HomesStephenson CountyIllinois
Housing units21,3755,443,501
Detached houses76%58%
In buildings of 5 or more10%22%
Mobile homes3%2%
Built before 196051%38%
Built since 20009%16%
Middle year built19591970
Owner-occupied71%67%

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