Average Radon Levels in Illinois: EPA Zone Data and What It Means
Illinois Is Entirely EPA Radon Zone 1
The EPA divides the country into three radon zones. Zone 1 counties have a predicted average indoor radon level above 4 pCi/L, the EPA action level. Zone 2 runs from 2 to 4 pCi/L. Zone 3 is below 2 pCi/L.
Every county in Illinois is Zone 1. The Illinois Emergency Management Agency (IEMA) estimates that 30 to 40 percent of Illinois homes test above 4 pCi/L, putting hundreds of thousands of households above the level at which the EPA recommends mitigation. Some regions of Illinois, particularly central Illinois, have average radon readings among the highest measured anywhere in the United States.
Zone 1 classification sets a statistical baseline, not a guarantee about any individual home. The only way to know what is in your house is to test it.
Why Illinois Has High Radon: Glacial Geology
The single largest driver of Illinois radon levels is glaciation. Wisconsin-era glaciers covered nearly the entire state, depositing a thick layer of glacial drift, a mixture of clay, sand, gravel, and rock fragments scraped from the Canadian Shield and redistributed across the landscape. Canadian Shield rock is uranium-bearing, and that uranium decays through radium into radon gas.
When uranium-rich glacial drift sits beneath a home, radon works upward through the soil and enters the structure through foundation cracks, slab joints, floor drains, and gaps around utility penetrations. Illinois soils are ideal for radon transport in many areas: the till is permeable enough for gas movement but dense enough to hold radon near the surface rather than letting it escape to the atmosphere.
Southern Illinois was largely spared by glaciation. The Shawnee Hills region (the Illinois Ozarks) has different geology: sandstone, limestone, and shale. Radon is still present in some southern Illinois homes, but levels are lower and more variable than in the glaciated north and center of the state.
Radon Levels by Region
| Region | Key Counties/Cities | Radon Risk Level | Primary Driver |
|---|---|---|---|
| Chicago Metro (suburban) | Cook, DuPage, Lake, Kane, McHenry, Will | High | Glacial lake sediments, Lake Michigan basin till |
| Chicago City | Cook (urban core) | Low to moderate | High-rise building type; low soil contact |
| Central Illinois Till Plains | Champaign, McLean, Sangamon, Peoria | Very High | Deep glacial outwash, Wisconsin till plain |
| Rock River Valley | Winnebago (Rockford), Whiteside, Rock Island | High | Glacial outwash valleys |
| Illinois-Iowa Border | Hancock, Adams, Pike, Jo Daviess | Very High | Loess deposits over glacial till |
| Southern Illinois | Jackson, Union, Johnson, Pope | Lower but present | Shawnee Hills sandstone and limestone |
Central Illinois: The Highest-Testing Region
Central Illinois sits on the Till Plains physiographic province, an exceptionally flat landscape formed by repeated glacial advances that deposited deep, fine-grained outwash. This is the most consistently high-radon region in the state.
Champaign County is one of the highest-testing counties in Illinois. Testing programs run through the University of Illinois and local health departments have found a large share of Champaign-Urbana homes above 4 pCi/L, with many well above 10 pCi/L. Awareness in this community is high and radon testing is routine.
McLean County (Bloomington-Normal), Sangamon County (Springfield), and Peoria County are similarly elevated. The deep glacial outwash underlying these communities acts as a radon reservoir, and the tight construction of newer suburban homes can trap soil gas more efficiently than older housing stock with less insulation and more natural air leakage.
Chicago Metro: Suburbs vs. City
Suburban Cook, DuPage, Lake, Kane, McHenry, and Will Counties
The six-county collar region around Chicago has consistently high radon in single-family homes. The Chicago metro was shaped by the Lake Michigan lobe of the Wisconsin glacier, which deposited thick till across the entire suburban landscape. Glacial lake sediments from ancient Lake Chicago, the precursor to Lake Michigan, also cover parts of the near-north suburbs.
Radon testing is essentially universal in suburban Chicago real estate transactions. Real estate agents, home inspectors, and buyers all expect it. The market has normalized mitigation: a home in Naperville or Schaumburg with a mitigation system and a clean post-test result is a standard disclosure, not a stigma.
Chicago City: A Different Risk Profile
Chicago's building stock is fundamentally different from its suburbs. High-rise apartment towers and elevator buildings account for a large share of the city's housing units. These buildings have high natural air exchange rates, multiple floors of living space with no soil contact, and concrete construction that limits radon infiltration. Average radon levels in these buildings are genuinely low.
Chicago's bungalow belt, the dense band of single-story homes on the northwest and southwest sides, is a different matter. These homes have basements with direct soil contact and less air movement than a high-rise. Owners and buyers of Chicago bungalows, two-flats, and coach houses on grade should test.
Rock River Valley: Rockford and Western Illinois
Winnebago County (Rockford) and the Rock River Valley corridor, including Whiteside and Rock Island counties, have high radon driven by glacial outwash along the river valley. Outwash deposits are coarser and more permeable than till, which means radon can move through the soil quickly and enter homes efficiently. Rock Island County sits at the confluence of the Rock and Mississippi rivers, an area with layered glacial and loess deposits that produce consistently elevated readings.
Illinois-Iowa Border: Loess and Till
The counties along the Mississippi River from the Iowa border southward, including Hancock, Adams, and Pike, have some of the highest radon potential in western Illinois. Wind-deposited loess (fine glacial silt) sits over glacial till in this region, and the combination of uranium-bearing parent material and permeable loess creates favorable conditions for radon entry. Jo Daviess County in the far northwest corner, with its different bedrock geology (dolomite, galena limestone), also sees elevated radon in homes built on unweathered rock.
Southern Illinois: Unglaciated Shawnee Hills
The Shawnee Hills region in the southernmost counties of Illinois was not covered by the Wisconsin glacier. The underlying rock is sandstone, limestone, and shale rather than uranium-rich Canadian Shield material. Average radon levels in this region are lower than in central or northern Illinois, but radon is not absent. Shale formations can contain uranium and off-gas radon, and some Shawnee Hills homes test above 4 pCi/L.
The key point for southern Illinois homeowners is that lower average levels do not mean zero risk. A home built on fractured shale in a hollow in Union or Johnson County may test higher than a home in suburban Champaign built on permeable outwash. Testing is the only reliable answer.
Seasonal Variation in Illinois
Radon levels in Illinois homes follow a seasonal pattern, rising in winter and falling in summer. The drivers are the same as in other cold-weather states:
- Homes are sealed against the cold, cutting natural ventilation
- Frozen ground traps soil gas and redirects it toward structures
- Furnaces and forced-air heating systems create negative indoor pressure, drawing in soil gas through foundation gaps
Short-term tests (48 to 96 hours) taken in January may read 20 to 30 percent higher than the same home tested in July. For real estate transactions, the EPA protocol calls for short-term tests under closed-house conditions, which controls for some but not all seasonal variation. Long-term tests (90 days to one year) give a more accurate picture of annual average exposure but are impractical for most transactions.
The 4 pCi/L Action Level and What It Means
The EPA recommends mitigation when a home tests at or above 4 pCi/L. IEMA follows the same threshold. At 4 pCi/L, the estimated lifetime lung cancer risk for a non-smoker is roughly 7 in 1,000. For smokers, the risk is substantially higher because radon and tobacco smoke act synergistically to damage lung tissue.
The World Health Organization recommends action at 2.7 pCi/L. At that threshold, a large majority of Illinois homes in the central Till Plains would qualify for mitigation. Most Illinois contractors and IEMA guidance use 4 pCi/L as the benchmark, but many homeowners choose to mitigate at lower levels, particularly 2 to 3 pCi/L, as a precaution.
Sub-slab depressurization, the standard mitigation technique, typically brings Illinois homes to below 2 pCi/L. There is no level that carries zero risk, but a post-mitigation result of 1 to 1.5 pCi/L represents a very large reduction in exposure.
Find a Certified Radon Tester in Illinois
Illinois requires radon testing professionals to be licensed by IEMA. A licensed tester uses calibrated equipment, follows proper protocol, and produces a result you can rely on for real estate negotiations or mitigation decisions. Find certified radon testers in Illinois licensed through IEMA and certified through NRPP or NRSB.
Frequently Asked Questions
What EPA radon zone is Illinois?
Illinois is entirely EPA Radon Zone 1, the highest category. Every county in the state has a predicted average indoor radon level above 4 pCi/L. IEMA estimates that 30 to 40 percent of Illinois homes test above the EPA action level.
Which part of Illinois has the highest radon levels?
Central Illinois, particularly Champaign County, McLean County (Bloomington-Normal), Sangamon County (Springfield), and Peoria County, consistently records some of the highest radon levels in the state. These areas sit on deep glacial outwash deposits from Wisconsin-era glaciation.
Why do Chicago high-rises have lower radon than suburban homes?
High-rise and elevator buildings have very high natural air exchange rates, multiple floors with no soil contact, and concrete construction that limits radon infiltration. Single-family suburban homes in Cook, DuPage, Lake, Kane, McHenry, and Will counties sit directly on high-radon glacial till and are the primary concern.
Does southern Illinois have lower radon than northern Illinois?
Generally yes. Southern Illinois was not covered by the Wisconsin glacier, so it lacks the uranium-bearing glacial till that drives high radon in central and northern Illinois. But the Shawnee Hills region has sandstone and shale that can still produce elevated radon, and individual homes should be tested.
Will radon levels be higher in winter in Illinois?
Yes. Illinois homes typically read 20 to 30 percent higher in winter than summer. Sealed homes, frozen ground, and heating systems drawing negative pressure all increase radon accumulation. Long-term tests (90 days or more) average out this seasonal variation and give a more accurate picture of annual exposure.