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Radon Levels in Washington County, Illinois

EPA puts Washington County in Zone 2, its middle radon zone, and most reported tests there come back low, though 11.8% of 51 (2017) reached 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

    11.8%

    21.9 pts below IL

    Washington
    11.8%
    Illinois
    33.7%

    51 tests, 2017

  • Median result

    2.0pCi/L

    Below action level

    Washington
    2.0 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2017

  • Tests reported

    51

    60th of 70 by tests

    Washington
    51
    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

    68 of 70

    Lowest quarter

    Washington
    11.8%
    Stark
    69.2%

    By share, of 70 with 30+ tests

Read a result for Washington County

Starts at Washington County's median, 2.0 pCi/L

2.0pCi/L

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

EPA’s reading of 2.0 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. Washington County's median reported result, 2017.

Interpret this readingWhat a fix costs

The data

Washington County, At a glance

Washington County at a glance

Predicted: EPA radon zone

EPA Zone 2: moderate predicted potential

Washington 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: Moderate across 100% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

Radon tests reported for Washington County, 2017

Share of reported tests at or above 4 pCi/L, by year, Washington County
Share of reported tests at or above 4 pCi/L, by year, Washington County
YearShare of reported tests at or above 4 pCi/L
201433.3%
201520.0%
201711.8%

2017: 11.8%. Across these years the share ran from 11.8% to 33.3%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

These are the 51 tests reported to CDC for Washington 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

Washington County, Compared

Washington County radon figures, compared

Calculated by Radon411

Washington County reports fewer high tests than Illinois overall. 11.8% of its 51 tests in 2017 were at or above 4 pCi/L. That is 21.9 points below the 33.7% pooled from 93 Illinois counties.

Ordered from highest share down, it comes 68th of 70 Illinois counties with 30 or more tests for the same period. Nationally, every county with a figure for the same period pools to 31.7%; Washington County sits 19.9 points below it.

Next door, the shares run from 19.6% in Jefferson County to 37.6% in Clinton County. Zone 2 on EPA's map (predicted average between 2 and 4 pCi/L) and a 11.8% share point the same way here.

Half the tests came in under 2.0 pCi/L and half over it; the single highest reached 6.9 pCi/L. The rest split 47.1% under 2 pCi/L and 41.2% from 2 to 4 (Illinois: 37.7% and 28.6%). Year by year, 2014 opened at 33.3% and 2017 closed at 11.8% (down 21.5 points), with 2014 the high at 33.3%.

The busiest year on file was 2014, with 54 tests reported; the quietest, 2015, had 30.

Share of reported tests at or above 4 pCi/L by year, Washington County against Illinois2014 to 2017, years with 30 or more tests: Washington County 33.3% to 11.8%, the state's pooled share 33.7% in 2017.
Washington CountyIllinois, pooled
Share of reported tests at or above 4 pCi/L by year, Washington County against Illinois: data table
YearTestsWashington County, at or above 4 pCi/LIllinois, pooled
20145433.3%37.8%
20153020.0%36.6%
20175111.8%33.7%
Washington County against Illinois, the United States and nearby counties, 2017
AreaTestsAt or above 4 pCi/LMedianDifference from Washington County
Washington County5111.8%2.0 pCi/LThis county
Illinois, pooled93 counties, pooled138,00633.7%Not pooled+21.9 points
United States, pooled937 counties, pooled528,06231.7%Not pooled+19.9 points
Stark Countyhighest in the state3969.2%5.8 pCi/L+57.4 points
Williamson Countylowest in the state2498.4%0.9 pCi/L−3.4 points
Perry County, ILborders it1540.0%1.5 pCi/L+28.2 points
Clinton County, ILborders it58237.6%3.0 pCi/L+25.8 points
Randolph County, ILborders it6035.0%2.9 pCi/L+23.2 points
St. Clair County, ILborders it5,19333.7%2.6 pCi/L+21.9 points
Marion County, ILborders it4520.0%1.9 pCi/L+8.2 points
Jefferson County, ILborders it13819.6%1.9 pCi/L+7.8 points
Where Washington County's reported results fell, 2017, beside Illinois and the United States
ResultWashington County testsWashington County shareIllinois shareU.S. shareAgainst Illinois
Under 2 pCi/L2447.1%37.7%43.5%+9.4 points
2 to 4 pCi/L2141.2%28.6%24.8%+12.6 points
4 pCi/L and above611.8%33.7%31.7%−21.9 points
Washington 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
Fayette County60 of 705726.3%2.5 pCi/L
Macoupin County61 of 7025825.6%2.2 pCi/L
Jackson County62 of 709625.0%2.5 pCi/L
Iroquois County62 of 703625.0%1.9 pCi/L
Mason County64 of 704221.4%2.8 pCi/L
Marion County65 of 704520.0%1.9 pCi/L
Jefferson County66 of 7013819.6%1.9 pCi/L
Cook County67 of 7026,93719.4%1.9 pCi/L
Washington County68 of 705111.8%2.0 pCi/L
Franklin County69 of 703010.0%1.9 pCi/L
Williamson County70 of 702498.4%0.9 pCi/L
Washington County radon tests by year, against Illinois' pooled share
YearTestsAt or above 4 pCi/LMedianHighestIllinois, pooledChange from the row before
20175111.8%2.0 pCi/L6.9 pCi/L33.7%−8.2 points
20153020.0%2.5 pCi/L7.3 pCi/L36.6%−13.3 points
20145433.3%2.8 pCi/L35.0 pCi/L37.8%First on file

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

Washington County, Geology

The ground under Washington County

Predicted

USGS's geologic map puts all of Washington County's mapped land in one class: moderate radon potential. All of it lies in the area the map calls Southern Central Lowland Province, to which USGS gives a radon index of 9, in the moderate band (9 to 11).

USGS's geologic areas under Washington County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Southern Central Lowland ProvinceIllinoian glacial deposits, sandstone, and gray shale.Moderate9100%

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

Washington County beside the ground around it

Across Illinois 64.5% of the land is rated high; none of Washington County's is. Of the 6 counties that border it, none has as large a share of moderate-potential ground; Clinton County comes nearest at 100%.

Share of land by USGS radon potential: Washington County, the 6 counties bordering it and Illinois
AreaEPA zoneLow potentialModerate potentialHigh potential
Washington CountyZone 2None100%None
Perry CountyZone 2None100%None
Clinton CountyZone 2None100%None
Jefferson CountyZone 2None100%None
St. Clair CountyZone 2None100%None
Randolph CountyZone 2None100%None
Marion CountyZone 2None100%None
Illinois102 counties, pooled by land areaVaries0.9%34.5%64.5%

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

Local help

Washington County, Local resources

Radon testing and mitigation in Washington 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

Washington County, Questions

Questions about radon in Washington County

Are radon levels high in Washington County?

In 2017, 11.8% of the radon tests reported to CDC for Washington County, Illinois were at or above 4 pCi/L, and the median reported pre-mitigation result was 2.0 pCi/L. Of 70 Illinois counties with 30 or more tests for the same period, 67 reported a higher share; pooled, the state comes to 33.7%.

Which counties border Washington County?

Washington County borders 6 counties: Perry County, Clinton County, Jefferson County, St. Clair County, Randolph County and Marion County.

Products

Washington County, Products

Testing a home in Washington County

In Washington County, 11.8% of 51 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 Washington County; a long-term test tells the year-round average.

Test kits and monitors3

See every kit and monitor

Best radon monitors for Washington County homes, compared

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Washington County, Bordering counties

Counties bordering Washington County

Calculated by Radon411

Washington County, Cities and towns

Cities and towns in Washington County

About 13,564 people live in Washington County by the Census Bureau's estimate for July 1, 2025, 165 fewer than in 2020 (−1.2%). That works out to 24 people per square mile of land. Centralia, the largest place on its list (11,863 residents in all), extends beyond Washington County, so no share of the total is worked out for it. Reported testing is thin here: 51 tests over 2017, or 3.8 for every 1,000 residents (2025 estimate), where Illinois as a whole comes to 10.9.

From Centralia to Venedy, CDC publishes no figure by town: each reads Washington County's.

The 12 largest cities and towns in Washington County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Centraliaextends beyond Washington County11,863−2.1%Counted whole
Nashville3,011−2.7%22.2%
Okawville1,343−1.7%9.9%
Wamacextends beyond Washington County962−2.2%Counted whole
Irvington566−2.9%4.2%
Hoyleton502−3.3%3.7%
Ashley449−2.6%3.3%
Addieville256−0.4%1.9%
Richview232−1.7%1.7%
Oakdale197Level1.5%
Radom183Level1.3%
Du Bois180−0.6%1.3%
  • New Minden
  • Venedy

14 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 Washington County is counted whole, so it has no share.

Washington County, Homes

Homes in Washington County

Washington County is a place of houses on their own lots: 87% of its 6,402 homes are detached single-family houses, against 58% across Illinois. Half of its homes were built before 1970, close to Illinois' 1970: 38% date from before 1960 and 15% from 2000 or later. Owners live in 81% of the occupied homes (Illinois: 67%), so for most households a test and a fix are their own to order.

Homes in Washington County, Illinois, 2019 to 2023
HomesWashington CountyIllinois
Housing units6,4025,443,501
Detached houses87%58%
In buildings of 5 or more2%22%
Mobile homes9%2%
Built before 196038%38%
Built since 200015%16%
Middle year built19701970
Owner-occupied81%67%

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