Radon in Williamson County, Illinois
Williamson County, Illinois, is in EPA radon Zone 2: predicted average indoor radon screening level from 2 to 4 pCi/L (moderate potential). Radon laboratories reported 450 building tests here for 2008 to 2017: median 1.3 pCi/L, with 13.3% at or above EPA's 4 pCi/L action level (Illinois 43.2%, US 36.5%).
Radon test results in Williamson County
| Test results | Period | Tests | Typical result | 4 pCi/L or more |
|---|---|---|---|---|
| Radon testing laboratories | 2008-2017 | 450 | median 1.3 pCi/L | 13.3% |
| Illinois, all counties (labs) | 2008-2017 | 85,532 | n/a | 43.2% |
| United States (labs) | 2008-2017 | 1,408,802 | n/a | 36.5% |
Test results describe buildings whose owners chose to test and whose results reached the lab or agency, not a random sample; small counts swing widely. State and US shares are computed from county counts and percents. The laboratory data comes from national radon testing laboratories and supplements EPA's zone map.
Radon in nearby counties
| County | EPA zone | Lab tests | 4 pCi/L or more |
|---|---|---|---|
| Franklin County, IL | Zone 2 | 151 | 19.6% |
| Johnson County, IL | Zone 2 | 41 | 14.6% |
| Saline County, IL | Zone 2 | 71 | 20.7% |
| Union County, IL | Zone 2 | 64 | 25.3% |
| Jackson County, IL | Zone 2 | 361 | 28.5% |
| Pope County, IL | Zone 2 | 7 | 0% |
The six counties with the nearest centers.
What to do about radon in Williamson County
Test the building. EPA recommends fixing a home at 4 pCi/L or more and considering a fix between 2 and 4 pCi/L, and says there is no known safe level of exposure. Illinois's state radon program can point you to testers and test kits.
Buying here? The Environmental Records Report lists this county's radon zone and test results with the flood zone, mapped faults, power lines and federal contamination records near the address.
Radon in Williamson County: questions
What is the radon level in Williamson County?
The median of the 450 laboratory tests reported for Williamson County (2008 to 2017) was 1.3 pCi/L, and 13.3% of them were at or above 4 pCi/L. These are results of tested buildings; a single home can be far above or below them.
Is Williamson County in radon Zone 1?
No. EPA's 1993 map puts Williamson County in Zone 2 (predicted average indoor radon screening level from 2 to 4 pCi/L (moderate potential)). EPA says homes in every zone can have high radon and should be tested.
How does radon in Williamson County compare with Illinois?
13.3% of laboratory tests in Williamson County were at or above 4 pCi/L for 2008 to 2017, against 43.2% across Illinois (85,532 tests) and 36.5% nationally. 56 of Illinois's 102 counties are in Zone 1.
Should I test for radon in Williamson County?
Yes. EPA recommends testing every home whatever its zone: "This map should not be used to determine if individual homes need to be tested." Fix the home at 4 pCi/L or more and consider a fix between 2 and 4 pCi/L; EPA says there is no known safe level of radon exposure.
More for Williamson County
- Who owns this LLC?The properties an LLC bought or sold in Illinois since 2014 (PTAX-203 transfer declarations) and, where an agent signed, who signed for it.
- Williamson loan limitsThis year's FHFA conforming, FHA and VA county loan limits for 1 to 4 unit homes, from FHFA's and HUD's official files.
- Williamson fair market rentHUD Fair Market Rents for studios to 4 bedrooms, the voucher payment standard range and the change from last year.
- Williamson property tax rateEffective property tax rate from Census survey data: median taxes paid against median home value, ranked in the state.
- Environmental records reportSuperfund, hazardous waste, leaking tank and brownfield sites near any Williamson address, with distances and a site map. A records search, not a Phase I.
- Superfund sites in IllinoisEvery EPA Superfund site in Illinois with its status, county and a map, refreshed nightly from federal records.
- USDA eligibility map: IllinoisHow much of each Illinois county is in a USDA-eligible rural area, the towns mapped ineligible, and a free address check.
- Opportunity zones in IllinoisEvery designated Opportunity Zone tract in Illinois by county, from the IRS list, and the tracts eligible for the 2027 round.
- Census tract lookupThe census tract of any Williamson address with its FFIEC income level and HUD Qualified Census Tract and DDA status.
Other free hazard lookups
- Fault lines near an addressUSGS-mapped earthquake faults within 5 miles, their age and slip rate, and the USGS seismic hazard value.
- Power lines near an addressHigh-voltage transmission lines within 1 mile from federal maps, with voltage and distance.
- Flood zone lookupThe FEMA flood zone mapped at any US address. Not an official flood zone determination.
- Superfund sites near an addressThe nearest EPA Superfund (NPL) sites and their status.
All of these, plus wetlands, FEMA hazard ratings and the federal contamination lists, are in the Environmental Records Report for any US address ($29).
Radon in every Illinois county · Radon map of every state
Data compiled 2026-10-20. County FIPS 17199. County-level screening information, not a radon measurement of any building.