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Radon Levels in Knox County, Illinois

EPA puts Knox County in Zone 1, its highest radon zone, and high results are common there: 56.6% of the 249 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

How to test this homeFind a certified tester

  • Share at 4+ pCi/L

    56.6%

    22.9 pts above IL

    Knox
    56.6%
    Illinois
    33.7%

    249 tests, 2017

  • Median result

    4.7pCi/L

    Action level reached

    Knox
    4.7 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2017

  • Tests reported

    249

    37th of 70 by tests

    Knox
    249
    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

    11 of 70

    Top quarter

    Knox
    56.6%
    Stark
    69.2%

    By share, of 70 with 30+ tests

Read a result for Knox County

Starts at Knox County's median, 4.7 pCi/L

4.7pCi/L

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

EPA’s reading of 4.7 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. Knox County's median reported result, 2017.

Interpret this readingWhat a fix costs

The data

Knox County, At a glance

Knox County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

EPA puts all of Knox 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 Knox County, 2017

Share of reported tests at or above 4 pCi/L, by year, Knox County
Share of reported tests at or above 4 pCi/L, by year, Knox County
YearShare of reported tests at or above 4 pCi/L
201465.2%
201558.8%
201756.6%

2017: 56.6%. Across these years the share ran from 56.6% to 65.2%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

These are the 249 tests reported to CDC for Knox 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

Knox County, Compared

Knox County radon figures, compared

Calculated by Radon411

More than half: 56.6% of the 249 radon tests reported to CDC for Knox County in 2017 read at least 4 pCi/L. That places it 11th of 70 Illinois counties with 30 or more tests for the same period. That is 22.9 points above the 33.7% pooled from 93 Illinois counties.

The median result, 4.7 pCi/L, sits above EPA's 4 pCi/L action level, and the top reported result was 37.1 pCi/L. The rest split 18.1% under 2 pCi/L and 25.3% from 2 to 4 (Illinois: 37.7% and 28.6%). Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 56.6% share point the same way here.

3 of its 6 bordering counties with figures ran higher, led by Stark County at 69.2%; Peoria County reported 40.2%. 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 2014, with 396 tests reported; the quietest, 2015, had 102.

Over 3 years of annual figures the share fell 8.6 points, from 65.2% in 2014 to 56.6% in 2017; its peak was 65.2% in 2014. Nationally, every county with a figure for the same period pools to 31.7%; Knox County sits 24.9 points above it.

Share of reported tests at or above 4 pCi/L by year, Knox County against Illinois2014 to 2017, years with 30 or more tests: Knox County 65.2% to 56.6%, the state's pooled share 33.7% in 2017.
Knox CountyIllinois, pooled
Share of reported tests at or above 4 pCi/L by year, Knox County against Illinois: data table
YearTestsKnox County, at or above 4 pCi/LIllinois, pooled
201439665.2%37.8%
201510258.8%36.6%
201724956.6%33.7%
Knox County against Illinois, the United States and nearby counties, 2017
AreaTestsAt or above 4 pCi/LMedianDifference from Knox County
Knox County24956.6%4.7 pCi/LThis county
Illinois, pooled93 counties, pooled138,00633.7%Not pooled−22.9 points
United States, pooled937 counties, pooled528,06231.7%Not pooled−24.9 points
Morgan Countyhighest in the state38166.9%6.5 pCi/L+10.3 points
Williamson Countylowest in the state2498.4%0.9 pCi/L−48.2 points
Stark County, ILborders it3969.2%5.8 pCi/L+12.6 points
Mercer County, ILborders it6668.2%5.5 pCi/L+11.6 points
Fulton County, ILborders it14464.6%5.8 pCi/L+8.0 points
Henry County, ILborders it42055.7%4.6 pCi/L−0.9 points
Warren County, ILborders it4250.0%3.6 pCi/L−6.6 points
Peoria County, ILborders it3,81640.2%3.1 pCi/L−16.4 points
Where Knox County's reported results fell, 2017, beside Illinois and the United States
ResultKnox County testsKnox County shareIllinois shareU.S. shareAgainst Illinois
Under 2 pCi/L4518.1%37.7%43.5%−19.6 points
2 to 4 pCi/L6325.3%28.6%24.8%−3.3 points
4 pCi/L and above14156.6%33.7%31.7%+22.9 points
Knox 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
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
Piatt County14 of 7027954.8%4.3 pCi/L
Vermilion County15 of 703953.8%4.7 pCi/L
Putnam County16 of 704553.3%4.5 pCi/L
Knox County radon tests by year, against Illinois' pooled share
YearTestsAt or above 4 pCi/LMedianHighestIllinois, pooledChange from the row before
201724956.6%4.7 pCi/L37.1 pCi/L33.7%−2.2 points
201510258.8%5.3 pCi/L18.2 pCi/L36.6%−6.4 points
201439665.2%5.4 pCi/L33.0 pCi/L37.8%First on file

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

Knox County, Geology

The ground under Knox County

Predicted

USGS's geologic map puts all of Knox 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 Knox 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.

Knox County beside the ground around it

Knox 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; Fulton County comes nearest at 100%.

Share of land by USGS radon potential: Knox County, the 6 counties bordering it and Illinois
AreaEPA zoneLow potentialModerate potentialHigh potential
Knox CountyZone 1NoneNone100%
Warren CountyZone 1NoneNone100%
Peoria CountyZone 1NoneNone100%
Fulton CountyZone 1NoneNone100%
Stark CountyZone 1NoneNone100%
Mercer CountyZone 1NoneNone100%
Henry 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

Knox County, Local resources

Radon testing and mitigation in Knox 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

Knox County, Questions

Questions about radon in Knox County

Are radon levels high in Knox County?

In 2017, 56.6% of the radon tests reported to CDC for Knox County, Illinois were at or above 4 pCi/L, and the median reported pre-mitigation result was 4.7 pCi/L. Of 70 Illinois counties with 30 or more tests for the same period, 10 reported a higher share; pooled, the state comes to 33.7%.

Which counties border Knox County?

Knox County borders 6 counties: Warren County, Peoria County, Fulton County, Stark County, Mercer County and Henry County.

Products

Knox County, Products

Testing a home in Knox County

In Knox County, 56.6% of 249 tests reported to CDC in 2017 reached 4 pCi/L, against 33.7% pooled across Illinois. In Zone 1 the year-round level in Knox 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

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

Explore

Knox County, Bordering counties

Counties bordering Knox County

Calculated by Radon411

Knox County, Cities and towns

Cities and towns in Knox County

About 47,767 people live in Knox County by the Census Bureau's estimate for July 1, 2025, 2,045 fewer than in 2020 (−4.1%). That works out to 67 people per square mile of land. Galesburg alone is home to 28,581 of them, 59.8% of the total. Reported testing is thin here: 249 tests over 2017, or 5.2 for every 1,000 residents (2025 estimate), where Illinois as a whole comes to 10.9.

From Galesburg to St. Augustine, CDC publishes no figure by town: each reads Knox County's.

The 12 largest cities and towns in Knox County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Galesburg28,581−4.6%59.8%
Abingdon2,832−3.9%5.9%
Knoxville2,810−3.0%5.9%
East Galesburg734−3.2%1.5%
Wataga712−3.9%1.5%
Oneida677−3.3%1.4%
Yates City629−3.7%1.3%
Williamsfield550−3.3%1.2%
Altona449−3.4%0.9%
London Millsextends beyond Knox County342−2.3%Counted whole
Henderson254−3.1%0.5%
Victoria252−3.8%0.5%
  • Maquon
  • Rio
  • St. Augustine

15 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 Knox County is counted whole, so it has no share.

Knox County, Homes

Homes in Knox County

Knox County is a place of houses on their own lots: 77% of its 23,788 homes are detached single-family houses, against 58% across Illinois. The homes are older than Illinois' as a whole: half were built before 1956 (1970 for Illinois), and 56% date from before 1960. 70% 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 Knox County, Illinois, 2019 to 2023
HomesKnox CountyIllinois
Housing units23,7885,443,501
Detached houses77%58%
In buildings of 5 or more10%22%
Mobile homes4%2%
Built before 196056%38%
Built since 20007%16%
Middle year built19561970
Owner-occupied70%67%

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