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Radon Levels in Winnebago County, Illinois

EPA puts Winnebago County in Zone 1, its highest radon zone, and 34.9% of its 2,703 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

    34.9%

    1.2 pts above IL

    Winnebago
    34.9%
    Illinois
    33.7%

    2,703 tests, 2017

  • Median result

    2.9pCi/L

    Below action level

    Winnebago
    2.9 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2017

  • Tests reported

    2,703

    15th of 70 by tests

    Winnebago
    2,703
    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

    51 of 70

    Middle half

    Winnebago
    34.9%
    Stark
    69.2%

    By share, of 70 with 30+ tests

Read a result for Winnebago County

Starts at Winnebago County's median, 2.9 pCi/L

2.9pCi/L

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

EPA’s reading of 2.9 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. Winnebago County's median reported result, 2017.

Interpret this readingWhat a fix costs

The data

Winnebago County, At a glance

Winnebago County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

EPA puts all of Winnebago 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 Winnebago County, 2017

Share of reported tests at or above 4 pCi/L, by year, Winnebago County
Share of reported tests at or above 4 pCi/L, by year, Winnebago County
YearShare of reported tests at or above 4 pCi/L
201439.0%
201540.6%
201618.5%
201734.9%

2017: 34.9%. Across these years the share ran from 18.5% to 40.6%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

These are the 2,703 tests reported to CDC for Winnebago 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

Winnebago County, Compared

Winnebago County radon figures, compared

Calculated by Radon411

In Winnebago County, about 3 in 10 reported tests in 2017 (942 of 2,703) were at or above EPA's action level. Against the national pool of 31.7%, the gap is 3.2 points higher. Across 93 Illinois counties pooled together, 33.7% of tests reached that level, so Winnebago County runs 1.2 points above its state.

Of 6 neighbours with a figure, all 6 reported a larger share at or above 4 pCi/L; the spread is 41.1% (DeKalb County) to 57.2% (Stephenson County). By share, Winnebago County is 51st of 70 Illinois counties with 30 or more tests for the same period.

Year by year, 2014 opened at 39.0% and 2017 closed at 34.9% (down 4.1 points), with 2015 the high at 40.6%. Half the tests came in under 2.9 pCi/L and half over it; the single highest reached 124.1 pCi/L. Below the action level, 894 tests (33.1%) came in under 2 pCi/L, against 37.7% statewide, and 867 tests (32.1%) fell between 2 and 4, against 28.6% statewide.

Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 34.9% share point the same way here. USGS geology rates 100% of the county's land high radon potential.

The busiest year on file was 2014, with 2,859 tests reported; the quietest, 2016, had 81.

Share of reported tests at or above 4 pCi/L by year, Winnebago County against Illinois2014 to 2017, years with 30 or more tests: Winnebago County 39.0% to 34.9%, the state's pooled share 33.7% in 2017.
Winnebago CountyIllinois, pooled
Share of reported tests at or above 4 pCi/L by year, Winnebago County against Illinois: data table
YearTestsWinnebago County, at or above 4 pCi/LIllinois, pooled
20142,85939.0%37.8%
20151,80340.6%36.6%
20168118.5%17.4%
20172,70334.9%33.7%
Winnebago County against Illinois, the United States and nearby counties, 2017
AreaTestsAt or above 4 pCi/LMedianDifference from Winnebago County
Winnebago County2,70334.9%2.9 pCi/LThis county
Illinois, pooled93 counties, pooled138,00633.7%Not pooled−1.2 points
United States, pooled937 counties, pooled528,06231.7%Not pooled−3.2 points
Stark Countyhighest in the state3969.2%5.8 pCi/L+34.3 points
Williamson Countylowest in the state2498.4%0.9 pCi/L−26.5 points
Stephenson County, ILborders it51957.2%4.8 pCi/L+22.3 points
Rock County, WIborders it36156.8%4.6 pCi/L+21.9 points
Green County, WIborders it7755.8%4.5 pCi/L+20.9 points
Ogle County, ILborders it63652.4%4.1 pCi/L+17.5 points
Boone County, ILborders it52546.9%3.4 pCi/L+12.0 points
DeKalb County, ILborders it1,70741.1%3.3 pCi/L+6.2 points
Where Winnebago County's reported results fell, 2017, beside Illinois and the United States
ResultWinnebago County testsWinnebago County shareIllinois shareU.S. shareAgainst Illinois
Under 2 pCi/L89433.1%37.7%43.5%−4.6 points
2 to 4 pCi/L86732.1%28.6%24.8%+3.5 points
4 pCi/L and above94234.9%33.7%31.7%+1.2 points
Winnebago 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
Clinton County46 of 7058237.6%3.0 pCi/L
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
Winnebago County radon tests by year, against Illinois' pooled share
YearTestsAt or above 4 pCi/LMedianHighestIllinois, pooledChange from the row before
20172,70334.9%2.9 pCi/L124.1 pCi/L33.7%+16.4 points
20168118.5%0.2 pCi/L6.8 pCi/L17.4%−22.1 points
20151,80340.6%3.3 pCi/L59.8 pCi/L36.6%+1.6 points
20142,85939.0%3.2 pCi/L38.2 pCi/L37.8%First on file

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

Winnebago County, Geology

The ground under Winnebago County

Predicted

USGS's geologic map puts all of Winnebago 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 Winnebago 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.

Winnebago County beside the ground around it

Winnebago 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; Boone County comes nearest at 100%.

Share of land by USGS radon potential: Winnebago County, the 6 counties bordering it and Illinois
AreaEPA zoneLow potentialModerate potentialHigh potential
Winnebago CountyZone 1NoneNone100%
Boone CountyZone 1NoneNone100%
Rock County, WIZone 1NoneNone100%
Stephenson CountyZone 1NoneNone100%
Ogle CountyZone 1NoneNone100%
Green County, WIZone 1NoneNone100%
DeKalb CountyZone 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

Winnebago County, Local resources

Radon testing and mitigation in Winnebago County

Listed radon businesses with an address in Winnebago County:

Listed radon professionals by town: Rockford, Machesney Park

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

Winnebago County, Questions

Questions about radon in Winnebago County

Are radon levels high in Winnebago County?

In 2017, 34.9% of the radon tests reported to CDC for Winnebago County, Illinois were at or above 4 pCi/L, and the median reported pre-mitigation result was 2.9 pCi/L. Of 70 Illinois counties with 30 or more tests for the same period, 50 reported a higher share; pooled, the state comes to 33.7%.

Which counties border Winnebago County?

Winnebago County borders 6 counties: Boone County, IL, Rock County, WI, Stephenson County, IL, Ogle County, IL, Green County, WI and DeKalb County, IL.

Products

Winnebago County, Products

Testing a home in Winnebago County

In Winnebago County, 34.9% of 2,703 tests reported to CDC in 2017 reached 4 pCi/L, against 33.7% pooled across Illinois. In Zone 1 the year-round level in Winnebago 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

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

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Winnebago County, Bordering counties

Counties bordering Winnebago County

Calculated by Radon411

Winnebago County, Cities and towns

Cities and towns in Winnebago County

Winnebago County's population has barely moved since 2020: 283,674 residents on July 1, 2025 against 284,939 then, by the Census Bureau's estimates. That works out to 553 people per square mile of land. Rockford, the largest place on its list (147,384 residents in all), extends beyond Winnebago County, so no share of the total is worked out for it. CDC's 2,703 reported tests for 2017 work out to 9.5 per 1,000 residents (2025 estimate), close to Illinois' 10.9.

From Rockford to New Milford, CDC publishes no figure by town: each reads Winnebago County's.

The cities and towns in Winnebago County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Rockfordextends beyond Winnebago County147,384−0.9%Counted whole
Loves Parkextends beyond Winnebago County23,712+1.2%Counted whole
Machesney Park22,805−0.6%8.0%
Roscoe11,034+0.3%3.9%
South Beloit7,996+0.3%2.8%
Rockton7,870+0.1%2.8%
Winnebago2,903−0.8%1.0%
Cherry Valleyextends beyond Winnebago County2,887+0.2%Counted whole
Pecatonica2,052−1.3%0.7%
Durand1,380−0.1%0.5%
New Milford808+0.9%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). A place that extends beyond Winnebago County is counted whole, so it has no share.

Winnebago County, Homes

Homes in Winnebago County

Of Winnebago County's 124,959 homes, 70% are detached houses, against 58% across Illinois; 12% are in buildings of five or more homes. Half of its homes were built before 1971, close to Illinois' 1970: 36% date from before 1960 and 13% from 2000 or later. 65% 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 Winnebago County, Illinois, 2019 to 2023
HomesWinnebago CountyIllinois
Housing units124,9595,443,501
Detached houses70%58%
In buildings of 5 or more12%22%
Mobile homes1%2%
Built before 196036%38%
Built since 200013%16%
Middle year built19711970
Owner-occupied65%67%

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