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Radon Levels in Marshall County, Illinois

EPA puts Marshall County in Zone 1, its highest radon zone, and high results are common there: 48.1% of the 81 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

    48.1%

    14.4 pts above IL

    Marshall
    48.1%
    Illinois
    33.7%

    81 tests, 2017

  • Median result

    3.8pCi/L

    Below action level

    Marshall
    3.8 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2017

  • Tests reported

    81

    53rd of 70 by tests

    Marshall
    81
    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

    23 of 70

    Middle half

    Marshall
    48.1%
    Stark
    69.2%

    By share, of 70 with 30+ tests

Read a result for Marshall County

Starts at Marshall County's median, 3.8 pCi/L

3.8pCi/L

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

EPA’s reading of 3.8 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. Marshall County's median reported result, 2017.

Interpret this readingWhat a fix costs

The data

Marshall County, At a glance

Marshall County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

EPA puts all of Marshall 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 Marshall County, 2017

Share of reported tests at or above 4 pCi/L, by year, Marshall County
Share of reported tests at or above 4 pCi/L, by year, Marshall County
YearShare of reported tests at or above 4 pCi/L
201476.9%
201561.5%
20160.0%
201748.1%

2017: 48.1%. Across these years the share ran from 0.0% to 76.9%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

These are the 81 tests reported to CDC for Marshall 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

Marshall County, Compared

Marshall County radon figures, compared

Calculated by Radon411

In Marshall County, about 5 in 10 reported tests in 2017 (39 of 81) were at or above EPA's action level. Against the national pool of 31.7%, the gap is 16.4 points higher. Across 93 Illinois counties pooled together, 33.7% of tests reached that level, so Marshall County runs 14.4 points above its state.

Of 6 neighbours with a figure, 4 reported a larger share at or above 4 pCi/L and 2 a smaller one; the spread is 40.2% (Peoria County) to 69.2% (Stark County). By share, Marshall County is 23rd of 70 Illinois counties with 30 or more tests for the same period.

Year by year, 2014 opened at 76.9% and 2017 closed at 48.1% (down 28.8 points), with 2014 the high at 76.9%. Half the tests came in under 3.8 pCi/L and half over it; the single highest reached 34.0 pCi/L. Below the action level, 15 tests (18.5%) came in under 2 pCi/L, against 37.7% statewide, and 27 tests (33.3%) fell between 2 and 4, against 28.6% statewide.

Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 48.1% share point the same way here. USGS geology rates 100% of the county's land high radon potential.

The busiest year on file was 2017, with 81 tests reported; the quietest, 2016, had 3.

Share of reported tests at or above 4 pCi/L by year, Marshall County against Illinois2014 to 2017, years with 30 or more tests: Marshall County 76.9% to 48.1%, the state's pooled share 33.7% in 2017.
Marshall CountyIllinois, pooled
Share of reported tests at or above 4 pCi/L by year, Marshall County against Illinois: data table
YearTestsMarshall County, at or above 4 pCi/LIllinois, pooled
20147876.9%37.8%
20153961.5%36.6%
20178148.1%33.7%
Marshall County against Illinois, the United States and nearby counties, 2017
AreaTestsAt or above 4 pCi/LMedianDifference from Marshall County
Marshall County8148.1%3.8 pCi/LThis county
Illinois, pooled93 counties, pooled138,00633.7%Not pooled−14.4 points
United States, pooled937 counties, pooled528,06231.7%Not pooled−16.4 points
Mercer Countyhighest in the state6668.2%5.5 pCi/L+20.1 points
Williamson Countylowest in the state2498.4%0.9 pCi/L−39.7 points
Stark County, ILborders it3969.2%5.8 pCi/L+21.1 points
Bureau County, ILborders it33358.6%5.1 pCi/L+10.5 points
Putnam County, ILborders it4553.3%4.5 pCi/L+5.2 points
Woodford County, ILborders it76552.9%4.2 pCi/L+4.8 points
LaSalle County, ILborders it1,00245.8%3.6 pCi/L−2.3 points
Peoria County, ILborders it3,81640.2%3.1 pCi/L−7.9 points
Where Marshall County's reported results fell, 2017, beside Illinois and the United States
ResultMarshall County testsMarshall County shareIllinois shareU.S. shareAgainst Illinois
Under 2 pCi/L1518.5%37.7%43.5%−19.2 points
2 to 4 pCi/L2733.3%28.6%24.8%+4.7 points
4 pCi/L and above3948.1%33.7%31.7%+14.4 points
Marshall 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
Ogle County18 of 7063652.4%4.1 pCi/L
Menard County19 of 7016851.8%4.3 pCi/L
Coles County20 of 7019851.5%4.2 pCi/L
Douglas County21 of 708450.0%3.8 pCi/L
Warren County21 of 704250.0%3.6 pCi/L
Marshall County23 of 708148.1%3.8 pCi/L
Boone County24 of 7052546.9%3.4 pCi/L
Moultrie County25 of 704546.7%3.3 pCi/L
Rock Island County26 of 702,28346.5%3.7 pCi/L
Jersey County27 of 7012946.5%3.6 pCi/L
Tazewell County28 of 702,72446.4%3.6 pCi/L
Marshall County radon tests by year, against Illinois' pooled share
YearTestsAt or above 4 pCi/LMedianHighestIllinois, pooledChange from the row before
20178148.1%3.8 pCi/L34.0 pCi/L33.7%+48.1 points
201630.0%2.7 pCi/L2.7 pCi/L17.4%−61.5 points
20153961.5%4.7 pCi/L10.7 pCi/L36.6%−15.4 points
20147876.9%10.7 pCi/L31.4 pCi/L37.8%First on file

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

Marshall County, Geology

The ground under Marshall County

Predicted

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

USGS's geologic areas under Marshall County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Bloomington Ridged PlainWisconsinan 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.

Marshall County beside the ground around it

Marshall 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; LaSalle County comes nearest at 100%.

Share of land by USGS radon potential: Marshall County, the 6 counties bordering it and Illinois
AreaEPA zoneLow potentialModerate potentialHigh potential
Marshall CountyZone 1NoneNone100%
Putnam CountyZone 1NoneNone100%
Woodford CountyZone 1NoneNone100%
Stark CountyZone 1NoneNone100%
LaSalle CountyZone 1NoneNone100%
Bureau CountyZone 1NoneNone100%
Peoria 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

Marshall County, Local resources

Radon testing and mitigation in Marshall 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

Marshall County, Questions

Questions about radon in Marshall County

Are radon levels high in Marshall County?

In 2017, 48.1% of the radon tests reported to CDC for Marshall County, Illinois were at or above 4 pCi/L, and the median reported pre-mitigation result was 3.8 pCi/L. Of 70 Illinois counties with 30 or more tests for the same period, 22 reported a higher share; pooled, the state comes to 33.7%.

Which counties border Marshall County?

Marshall County borders 6 counties: Putnam County, Woodford County, Stark County, LaSalle County, Bureau County and Peoria County.

Products

Marshall County, Products

Testing a home in Marshall County

In Marshall County, 48.1% of 81 tests reported to CDC in 2017 reached 4 pCi/L, against 33.7% pooled across Illinois. In Zone 1 the year-round level in Marshall 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

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

Explore

Marshall County, Bordering counties

Counties bordering Marshall County

Calculated by Radon411

Marshall County, Cities and towns

Cities and towns in Marshall County

About 11,473 people live in Marshall County by the Census Bureau's estimate for July 1, 2025, 240 fewer than in 2020 (−2.0%). That works out to 30 people per square mile of land. No town dominates: Henry, the largest, has 2,280 residents, 19.9% of the total. Reported testing is thin here: 81 tests over 2017, or 7.1 for every 1,000 residents (2025 estimate), where Illinois as a whole comes to 10.9.

From Henry to La Rose, CDC publishes no figure by town: each reads Marshall County's.

The cities and towns in Marshall County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Henry2,280−2.6%19.9%
Lacon1,822−2.6%15.9%
Toluca1,310−2.3%11.4%
Washburnextends beyond Marshall County1,008−2.4%Counted whole
Wenonaextends beyond Marshall County950−2.4%Counted whole
Hopewell411−1.7%3.6%
Varna365−1.6%3.2%
Sparland353−1.1%3.1%
La Rose103−1.9%0.9%

9 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 Marshall County is counted whole, so it has no share.

Marshall County, Homes

Homes in Marshall County

Marshall County is a place of houses on their own lots: 90% of its 5,824 homes are detached single-family houses, against 58% across Illinois. The homes are older than Illinois' as a whole: half were built before 1962 (1970 for Illinois), and 48% date from before 1960. Owners live in 80% of the occupied homes (Illinois: 67%), so for most households a test and a fix are their own to order.

Homes in Marshall County, Illinois, 2019 to 2023
HomesMarshall CountyIllinois
Housing units5,8245,443,501
Detached houses90%58%
In buildings of 5 or more2%22%
Mobile homes1%2%
Built before 196048%38%
Built since 200011%16%
Middle year built19621970
Owner-occupied80%67%

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