Short-term kit
RA100 short-term liquid scintillation radon test kit
- Lab fee extra
Method: Liquid scintillation
In home: 48 to 96 hours
Locations and reports
Radon Levels in Williamson County, Illinois
EPA puts Williamson County in Zone 2, its middle radon zone, and most reported tests there come back low, though 8.4% of 249 (2017) reached 4 pCi/L. Homes with elevated levels occur in all three zones, so only a test measures yours.
Data checked By Radon411Methodology
Share at 4+ pCi/L
8.4%
25.3 pts below IL
- Williamson
- 8.4%
- Illinois
- 33.7%
249 tests, 2017
Median result
0.9pCi/L
Below action level
- Williamson
- 0.9 pCi/L
- EPA
- 4.0 pCi/L
Midpoint of the tests, 2017
Tests reported
249
38th of 70 by tests
- Williamson
- 249
- Cook
- 26,937
Reported to CDC before any fix, 2017
EPA radon zone
Zone 2
Moderate potential
- 1
- Zone 2
- 3
1 highest, 3 lowest
EPA predicts 2 to 4 pCi/L indoors
Rank in Illinois
70 of 70
Lowest quarter
- Williamson
- 8.4%
- Stark
- 69.2%
By share, of 70 with 30+ tests
Read a result for Williamson County
Starts at Williamson County's median, 0.9 pCi/L
EPA’s own thresholds; the figures are the area’s, never this home’s.
EPA’s reading of 0.9 pCi/L
Below 2 pCi/L
- Under 20 to 1.9 pCi/L
- 2 to 4consider a fix
- 4 and aboveEPA says fix it
This result is below 2 pCi/L, the level at which EPA says a fix should be considered. Williamson County's median reported result, 2017.
The data
Williamson County, At a glance
Williamson County at a glance
Predicted: EPA radon zone
EPA Zone 2: moderate predicted potential
Williamson County is a
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 Williamson County, 2017
| Year | Share of reported tests at or above 4 pCi/L |
|---|---|
| 2014 | 25.0% |
| 2015 | 10.5% |
| 2017 | 8.4% |
2017: 8.4%. Across these years the share ran from 8.4% to 25.0%. 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 Williamson 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).
Williamson County, Compared
Williamson County radon figures, compared
Calculated by Radon411Williamson County reports fewer high tests than Illinois overall. 8.4% of its 249 tests in 2017 were at or above 4 pCi/L. That is 25.3 points below the 33.7% pooled from 93 Illinois counties.
Ordered from highest share down, it comes 70th 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%; Williamson County sits 23.3 points below it.
Next door, the shares run from 10.0% in Franklin County to 25.0% in Jackson County. The reported tests do not follow EPA's Zone 2 label for Williamson County (predicted average between 2 and 4 pCi/L): 8.4% of them reached 4 pCi/L, fewer than that label suggests.
Half the tests came in under 0.9 pCi/L and half over it; the single highest reached 9.0 pCi/L. The rest split 81.9% under 2 pCi/L and 9.6% from 2 to 4 (Illinois: 37.7% and 28.6%). Year by year, 2014 opened at 25.0% and 2017 closed at 8.4% (down 16.6 points), with 2014 the high at 25.0%.
The busiest year on file was 2017, with 249 tests reported; the quietest, 2015, had 114.
Williamson CountyIllinois, pooled
8.4% % of tests33.7% % of tests
| Year | Tests | Williamson County, at or above 4 pCi/L | Illinois, pooled |
|---|---|---|---|
| 2014 | 120 | 25.0% | 37.8% |
| 2015 | 114 | 10.5% | 36.6% |
| 2017 | 249 | 8.4% | 33.7% |
Pooled shares: Radon411's sum of CDC's county counts for one period.
Williamson County, Geology
The ground under Williamson County
PredictedUSGS's geologic map puts all of Williamson County's mapped land in one class: moderate radon potential. The map puts 93.5% of Williamson County in the area it calls Southern Central Lowland Province and 6.5% in Shawnee Hills Section. USGS gives the first a radon index of 9 and the second 10, both in the moderate band (9 to 11).
| Area | The rock, in USGS's words | Potential | Radon index | Share of land |
|---|---|---|---|---|
| Southern Central Lowland Province | Illinoian glacial deposits, sandstone, and gray shale. | Moderate | 9 | 93.5% |
| Shawnee Hills Section | Unglaciated Ordovician to Pennsylvanian limestone, sandstone, and shale. | Moderate | 10 | 6.5% |
Names, descriptions and indexes: U.S. Geological Survey, Geologic Radon Potential Map (2025). Shares: Radon411, from the map cut to the boundary.
Williamson County beside the ground around it
Across Illinois 64.5% of the land is rated high; none of Williamson County's is. Of the 6 counties that border it, none has as large a share of moderate-potential ground; Franklin County comes nearest at 100%.
The map rates the ground, not the air in any home.
Local help
Williamson County, Local resources
Radon testing and mitigation in Williamson County
Listed radon businesses with an address in Williamson County:
- IEMA-OHS Radon ProgramIEMA-OHS Radon Program, run by Illinois Emergency Management Agency and Office of Homeland Security, licenses or certifies radon professionals and states the rule for disclosing radon when a home is sold, and answers questions about testing and mitigation in Illinois.Visit the program’s page(opens in a new tab)
- Free and low-cost kitsAsk the program whether it has free or discounted kits, or compare kits you can buy.
- Compare kits
- Professionals in Illinois339 professionals are listed in Illinois.
Listed radon professionals by town: Carbondale, Herrin, Carterville
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
Williamson County, Questions
Questions about radon in Williamson County
Are radon levels high in Williamson County?
In 2017, 8.4% of the radon tests reported to CDC for Williamson County, Illinois were at or above 4 pCi/L, and the median reported pre-mitigation result was 0.9 pCi/L. Of 70 Illinois counties with 30 or more tests for the same period, 69 reported a higher share; pooled, the state comes to 33.7%.
Which counties border Williamson County?
Williamson County borders 6 counties: Johnson County, Franklin County, Saline County, Union County, Jackson County and Pope County.
Products
Williamson County, Products
Testing a home in Williamson County
In Williamson County, 8.4% of 249 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 Williamson County; a long-term test tells the year-round average.
Test kits and monitors3
See every kit and monitor
Long-term kit
RL116 long-term radon test kit
- Lab fee extra
Method: Alpha track
In home: 90 days to 12 months
Monitor
Wave Plus battery radon and air quality monitor
- C-NRPP 2024/25: Not included
- Battery
- Bluetooth
- App only
Method: Alpha spectrometry
- See every kit and monitor Test kits and monitors
Test kits and monitors: 1 of 3
Explore
Williamson County, Bordering counties
Counties bordering Williamson County
Calculated by Radon411- Radon levels in Johnson CountyEPA zone 2
25.0%at or above 4 pCi/L
Median 2.5 pCi/L · 12 tests · 16.6 points above Williamson County
- Radon levels in Jackson CountyEPA zone 2
25.0%at or above 4 pCi/L
Median 2.5 pCi/L · 96 tests · 16.6 points above Williamson County
- Radon levels in Franklin CountyEPA zone 2
10.0%at or above 4 pCi/L
Median 1.9 pCi/L · 30 tests · 1.6 points above Williamson County
- Radon levels in Saline CountyEPA zone 2
0.0%at or above 4 pCi/L
Median 1.6 pCi/L · 12 tests · 8.4 points below Williamson County
- Radon levels in Union CountyEPA zone 2
0.0%at or above 4 pCi/L
Median 2.3 pCi/L · 21 tests · 8.4 points below Williamson County
- Radon levels in Pope CountyEPA zone 2
0.0%at or above 4 pCi/L
Median 0.4 pCi/L · 3 tests · 8.4 points below Williamson County
Williamson County, Cities and towns
Cities and towns in Williamson County
Williamson County's population has barely moved since 2020: 67,225 residents on July 1, 2025 against 67,127 then, by the Census Bureau's estimates. That works out to 160 people per square mile of land. Carbondale, the largest place on its list (21,811 residents in all), extends beyond Williamson County, so no share of the total is worked out for it. Reported testing is thin here: 249 tests over 2017, or 3.7 for every 1,000 residents (2025 estimate), where Illinois as a whole comes to 10.9.
From Carbondale to Colp, CDC publishes no figure by town: each reads Williamson County's.
| Place | Residents | Since 2020 | Share of residents |
|---|---|---|---|
| Carbondaleextends beyond Williamson County | 21,811 | −2.1% | Counted whole |
| Marion | 17,148 | +1.9% | 25.5% |
| Herrin | 12,268 | −0.5% | 18.2% |
| Carterville | 5,839 | −0.5% | 8.7% |
| Johnston City | 3,313 | −2.0% | 4.9% |
| Cambria | 1,796 | +16.5% | 2.7% |
| Crainville | 1,507 | +4.5% | 2.2% |
| Energy | 960 | −1.3% | 1.4% |
| Hurst | 751 | −1.1% | 1.1% |
| Pittsburg | 561 | −1.2% | 0.8% |
| Creal Springs | 491 | −2.0% | 0.7% |
| Freeman Spurextends beyond Williamson County | 272 | −2.9% | Counted whole |
- Bush
- Stonefort (part)
- Spillertown
- Colp
16 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 Williamson County is counted whole, so it has no share.
Williamson County, Homes
Homes in Williamson County
Of Williamson County's 31,957 homes, 74% are detached houses, against 58% across Illinois; 7% are mobile homes. Much of Williamson County is recent building: 27% of its homes have gone up since 2000 and the middle year built is 1981, against 1970 for Illinois. 71% 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 | Williamson County | Illinois |
|---|---|---|
| Housing units | 31,957 | 5,443,501 |
| Detached houses | 74% | 58% |
| In buildings of 5 or more | 5% | 22% |
| Mobile homes | 7% | 2% |
| Built before 1960 | 26% | 38% |
| Built since 2000 | 27% | 16% |
| Middle year built | 1981 | 1970 |
| Owner-occupied | 71% | 67% |
Census Bureau, American Community Survey, 2019 to 2023: estimates from a sample of homes, looser for a small place. Shares by Radon411.