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Radon Levels in McHenry County, Illinois

EPA puts McHenry County in Zone 2, its middle radon zone, and 35.1% of its 6,474 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

    35.1%

    1.4 pts above IL

    McHenry
    35.1%
    Illinois
    33.7%

    6,474 tests, 2017

  • Median result

    2.9pCi/L

    Below action level

    McHenry
    2.9 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2017

  • Tests reported

    6,474

    6th of 70 by tests

    McHenry
    6,474
    Cook
    26,937

    Reported to CDC before any fix, 2017

  • 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

    48 of 70

    Middle half

    McHenry
    35.1%
    Stark
    69.2%

    By share, of 70 with 30+ tests

Read a result for McHenry County

Starts at McHenry 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. McHenry County's median reported result, 2017.

Interpret this readingWhat a fix costs

The data

McHenry County, At a glance

McHenry County at a glance

Predicted: EPA radon zone

EPA Zone 2: moderate predicted potential

McHenry 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: High across 100% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

Radon tests reported for McHenry County, 2017

Share of reported tests at or above 4 pCi/L, by year, McHenry County
Share of reported tests at or above 4 pCi/L, by year, McHenry County
YearShare of reported tests at or above 4 pCi/L
201441.6%
201541.3%
201633.3%
201735.1%

2017: 35.1%. Across these years the share ran from 33.3% to 41.6%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

These are the 6,474 tests reported to CDC for McHenry 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

McHenry County, Compared

McHenry County radon figures, compared

Calculated by Radon411

In McHenry County, about 4 in 10 reported tests in 2017 (2,274 of 6,474) were at or above EPA's action level. Against the national pool of 31.7%, the gap is 3.4 points higher. Across 93 Illinois counties pooled together, 33.7% of tests reached that level, so McHenry County runs 1.4 points above its state.

Of 7 neighbours with a figure, 5 reported a larger share at or above 4 pCi/L and 2 a smaller one; the spread is 19.4% (Cook County) to 56.0% (Walworth County, WI). By share, McHenry County is 48th of 70 Illinois counties with 30 or more tests for the same period.

Year by year, 2014 opened at 41.6% and 2017 closed at 35.1% (down 6.5 points), with 2014 the high at 41.6%. Half the tests came in under 2.9 pCi/L and half over it; the single highest reached 33.7 pCi/L. Below the action level, 2,181 tests (33.7%) came in under 2 pCi/L, against 37.7% statewide, and 2,019 tests (31.2%) fell between 2 and 4, against 28.6% statewide.

Zone 2 on EPA's map (predicted average between 2 and 4 pCi/L) and a 35.1% share point the same way here. USGS geology rates 100% of the county's land high radon potential.

The busiest year on file was 2014, with 6,717 tests reported; the quietest, 2016, had 90.

Share of reported tests at or above 4 pCi/L by year, McHenry County against Illinois2014 to 2017, years with 30 or more tests: McHenry County 41.6% to 35.1%, the state's pooled share 33.7% in 2017.
McHenry CountyIllinois, pooled
Share of reported tests at or above 4 pCi/L by year, McHenry County against Illinois: data table
YearTestsMcHenry County, at or above 4 pCi/LIllinois, pooled
20146,71741.6%37.8%
20153,82841.3%36.6%
20169033.3%17.4%
20176,47435.1%33.7%
McHenry County against Illinois, the United States and nearby counties, 2017
AreaTestsAt or above 4 pCi/LMedianDifference from McHenry County
McHenry County6,47435.1%2.9 pCi/LThis county
Illinois, pooled93 counties, pooled138,00633.7%Not pooled−1.4 points
United States, pooled937 counties, pooled528,06231.7%Not pooled−3.4 points
Stark Countyhighest in the state3969.2%5.8 pCi/L+34.1 points
Williamson Countylowest in the state2498.4%0.9 pCi/L−26.7 points
Walworth County, WIborders it23456.0%4.5 pCi/L+20.9 points
Boone County, ILborders it52546.9%3.4 pCi/L+11.8 points
Kenosha County, WIborders it21041.9%3.4 pCi/L+6.8 points
DeKalb County, ILborders it1,70741.1%3.3 pCi/L+6.0 points
Kane County, ILborders it8,68236.6%2.8 pCi/L+1.5 points
Lake County, ILborders it13,05027.4%2.4 pCi/L−7.7 points
Cook County, ILborders it26,93719.4%1.9 pCi/L−15.7 points
Where McHenry County's reported results fell, 2017, beside Illinois and the United States
ResultMcHenry County testsMcHenry County shareIllinois shareU.S. shareAgainst Illinois
Under 2 pCi/L2,18133.7%37.7%43.5%−4.0 points
2 to 4 pCi/L2,01931.2%28.6%24.8%+2.6 points
4 pCi/L and above2,27435.1%33.7%31.7%+1.4 points
McHenry 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
Monroe County43 of 7071439.9%3.0 pCi/L
Macon County44 of 701,25138.8%3.1 pCi/L
Effingham County45 of 7014138.3%2.7 pCi/L
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
McHenry County radon tests by year, against Illinois' pooled share
YearTestsAt or above 4 pCi/LMedianHighestIllinois, pooledChange from the row before
20176,47435.1%2.9 pCi/L33.7 pCi/L33.7%+1.8 points
20169033.3%2.6 pCi/L13.6 pCi/L17.4%−8.0 points
20153,82841.3%3.3 pCi/L110.0 pCi/L36.6%−0.3 points
20146,71741.6%3.3 pCi/L38.1 pCi/L37.8%First on file

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

McHenry County, Geology

The ground under McHenry County

Predicted

USGS's geologic map puts all of McHenry County's mapped land in one class: high radon potential. The map puts 85.3% of McHenry County in the area it calls Bloomington Ridged Plain and 14.7% in Galesburg Plain. USGS gives both a radon index of 13, in the high band (12 to 17).

USGS's geologic areas under McHenry County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Bloomington Ridged PlainWisconsinan glacial deposits and loess over Paleozoic sedimentary rocks.High1385.3%
Galesburg PlainIllinoian glacial deposits and loess over Paleozoic sedimentary rocks.High1314.7%

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

McHenry County beside the ground around it

McHenry County has more high-potential ground than Illinois as a whole: 100% of its land against 64.5%. Of the 7 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: McHenry County, the 7 counties bordering it and Illinois
AreaEPA zoneLow potentialModerate potentialHigh potential
McHenry CountyZone 2NoneNone100%
Boone CountyZone 1NoneNone100%
Lake CountyZone 2None85.8%14.1%
Walworth County, WIZone 1NoneNone100%
Kenosha County, WIZone 2None67.3%32.6%
Kane CountyZone 1NoneNone100%
DeKalb CountyZone 1NoneNone100%
Cook CountyZone 2None96.8%3.2%
Illinois102 counties, pooled by land areaVaries0.9%34.5%64.5%

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

Local help

McHenry County, Local resources

Radon testing and mitigation in McHenry County

Listed radon businesses with an address in McHenry County:

Listed radon professionals by town: Algonquin, McHenry, Huntley, Lake in the Hills, Woodstock, Cary, Fox Lake, Harvard, Johnsburg, Wonder Lake, Richmond, Oakwood Hills

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

McHenry County, Questions

Questions about radon in McHenry County

Are radon levels high in McHenry County?

In 2017, 35.1% of the radon tests reported to CDC for McHenry 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, 47 reported a higher share; pooled, the state comes to 33.7%.

Which counties border McHenry County?

McHenry County borders 7 counties: Boone County, IL, Lake County, IL, Walworth County, WI, Kenosha County, WI, Kane County, IL, DeKalb County, IL and Cook County, IL.

Products

McHenry County, Products

Testing a home in McHenry County

In McHenry County, 35.1% of 6,474 tests reported to CDC in 2017 reached 4 pCi/L, against 33.7% pooled across Illinois. In Zone 2 a short-term kit (2 to 90 days) is the quick first check in McHenry County; a long-term test tells the year-round average.

Test kits and monitors3

See every kit and monitor

Best radon monitors for McHenry County homes, compared

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McHenry County, Bordering counties

Counties bordering McHenry County

Calculated by Radon411

McHenry County, Cities and towns

Cities and towns in McHenry County

The Census Bureau estimates 317,751 residents in McHenry County for July 1, 2025, 7,774 more than in 2020 (+2.5%). That works out to 527 people per square mile of land. No town dominates: Crystal Lake, the largest, has 42,288 residents, 13.3% of the total. Testing is reported here more often than across the state: CDC's 6,474 reported tests for 2017 come to 20.4 for every 1,000 residents (2025 estimate), against 10.9 for Illinois.

From Crystal Lake to Greenwood, CDC publishes no figure by town: each reads McHenry County's.

The 12 largest cities and towns in McHenry County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Crystal Lake42,288+5.1%13.3%
Algonquinextends beyond McHenry County30,824+3.9%Counted whole
McHenry29,217+6.7%9.2%
Huntleyextends beyond McHenry County29,132+4.9%Counted whole
Lake in the Hills28,590−1.2%9.0%
Woodstock25,740+0.4%8.1%
Cary17,932+0.7%5.6%
Fox Lakeextends beyond McHenry County10,849−1.0%Counted whole
Harvard9,504+0.5%3.0%
Island Lakeextends beyond McHenry County8,047+0.1%Counted whole
Marengo7,737+2.4%2.4%
Lakemoorextends beyond McHenry County6,449+4.5%Counted whole
  • Johnsburg
  • Wonder Lake
  • Spring Grove
  • Fox River Grove (part)
  • Lakewood
  • Barrington Hills (part)
  • Richmond
  • Oakwood Hills
  • Prairie Grove
  • Port Barrington (part)
  • Hebron
  • Bull Valley
  • McCullom Lake
  • Ringwood
  • Union
  • Holiday Hills
  • Trout Valley
  • Greenwood

30 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 McHenry County is counted whole, so it has no share.

McHenry County, Homes

Homes in McHenry County

McHenry County is a place of houses on their own lots: 77% of its 120,605 homes are detached single-family houses, against 58% across Illinois. Much of McHenry County is recent building: 25% of its homes have gone up since 2000 and the middle year built is 1989, against 1970 for Illinois. Owners live in 83% of the occupied homes (Illinois: 67%), so for most households a test and a fix are their own to order.

Homes in McHenry County, Illinois, 2019 to 2023
HomesMcHenry CountyIllinois
Housing units120,6055,443,501
Detached houses77%58%
In buildings of 5 or more8%22%
Mobile homes1%2%
Built before 196018%38%
Built since 200025%16%
Middle year built19891970
Owner-occupied83%67%

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