Illinois Radon Levels: Why 40% of Homes Exceed EPA Limits
Why Illinois Homeowners Need to Take Radon Seriously
Illinois consistently ranks among the states with the highest residential radon levels in the nation. According to data from the Illinois Emergency Management Agency (IEMA) and the U.S. Environmental Protection Agency (EPA), more than 40% of homes tested in Illinois show radon concentrations at or above the EPA's action level of 4 picocuries per liter (pCi/L). That's nearly double the national average.
Radon is a colorless, odorless, radioactive gas that forms naturally from the decay of uranium in soil and rock. It seeps into homes through foundation cracks, sump pits, crawl spaces, and gaps around service pipes. Long-term exposure is the second leading cause of lung cancer in the United States, behind only smoking — and the only way to know your level is to test.
If you live in Illinois, the question isn't whether radon could be an issue in your home. It's whether you've measured it recently and acted on the results.
Understanding Illinois' EPA Radon Zone Classifications
The EPA divides every U.S. county into one of three radon zones based on predicted indoor radon levels:
- Zone 1 (Red): Predicted average indoor radon screening level greater than 4 pCi/L — highest potential
- Zone 2 (Orange): Predicted average between 2 and 4 pCi/L — moderate potential
- Zone 3 (Yellow): Predicted average less than 2 pCi/L — low potential
Here's the alarming reality for Illinois: the vast majority of Illinois counties — roughly 80 of 102 — fall into Zone 1, the highest-risk category. Almost all remaining counties are designated Zone 2, in the "consider-action zone" of 2-4 pCi/L. No Illinois county is classified as Zone 3.
That makes Illinois one of only a handful of states where essentially every county carries elevated radon risk. Geology, glacial history, and soil composition all conspire to push uranium-rich materials close to the surface where the resulting radon gas can readily migrate into basements and ground-level living spaces.
Why Illinois Geology Is So Problematic
During the last ice age, glaciers ground down bedrock and deposited thick layers of glacial till across most of the state. This till is loaded with uranium-bearing minerals. As that uranium decays, it produces radium, which in turn decays into radon gas. The permeable soils common throughout Illinois allow that gas to travel upward easily, often concentrating in tightly-sealed modern homes.
Limestone and dolomite bedrock — abundant in northern and western Illinois — further amplify radon production. Combine that geology with cold winters that drive homeowners to seal their houses tightly, and you have a recipe for elevated indoor radon concentrations.
The Highest-Risk Counties in Illinois
While radon can be a problem in any Illinois home, certain regions consistently report the highest average levels. Below are the counties and metro areas where testing data shows the most significant risk.
Northern Illinois: The Highest Average Levels in the State
Northern Illinois counties dominate the high-end of the state's radon map. According to IEMA testing data:
- Ogle County — Average indoor radon levels frequently exceed 8 pCi/L, with some homes testing well above 20 pCi/L. The county's combination of glacial till and proximity to uranium-bearing bedrock makes it one of the highest-risk areas in the entire Midwest.
- Stephenson County — Around Freeport and nearby communities, average levels often hover near 7-8 pCi/L.
- JoDaviess County — In the Galena area, the rugged "Driftless" topography exposes uranium-rich bedrock with very limited glacial cover, driving high readings.
- Winnebago County — Rockford and surrounding suburbs consistently report averages above 5 pCi/L, with mitigation systems now common in newer construction.
- Boone County — Belvidere and nearby areas show averages exceeding the EPA action level.
- DeKalb County — Home to Northern Illinois University, this county shows average readings well above 4 pCi/L.
Chicago Metro Area and Collar Counties
Many homeowners are surprised to learn that the densely populated Chicago suburbs carry significant radon risk. Despite the urban density, the soil and geology don't change at county lines.
- Kane County — Including Aurora, Elgin, and Geneva, average radon readings frequently exceed 4 pCi/L.
- DuPage County — Naperville, Wheaton, Downers Grove, and Oak Brook all sit in Zone 1.
- Lake County — From Waukegan to Highland Park, elevated levels are common, especially in homes with basements.
- McHenry County — Crystal Lake, Algonquin, and Woodstock area homes often test in the 5-7 pCi/L range.
- Will County — Joliet, Naperville (south portion), and Plainfield are all designated Zone 1.
- Cook County — Even within Chicago itself, basement-level radon concentrations can be substantial, particularly in older brick bungalows and split-level homes with finished lower levels.
Central Illinois Hot Spots
Central Illinois isn't immune. Some of the highest individual home readings in the state have come from this region.
- Peoria County — Average levels regularly test above the action level.
- Tazewell and Woodford Counties — Adjacent to Peoria, these counties show comparable risk profiles.
- McLean County — Bloomington-Normal homes frequently exceed 4 pCi/L.
- Champaign County — University students and faculty living in older rental housing should pay particular attention to basement and ground-floor units.
- Sangamon County — The Springfield area sits firmly in Zone 1.
Southern Illinois: Variable but Often High
Southern Illinois shows more geographic variation, but several counties carry serious radon risk.
- Madison and St. Clair Counties — The Metro East area across from St. Louis includes many homes with elevated levels.
- Macoupin County
- Effingham County
- Williamson and Jackson Counties — Including the Carbondale area, where the geology shifts but radon risk remains significant.
Why Even "Low-Risk" Illinois Counties Still Warrant Testing
A handful of southern Illinois counties technically fall into EPA Zone 2 (the 2-4 pCi/L consider-action category). But here's the catch: Zone 2 doesn't mean safe. The EPA explicitly recommends considering action in the 2-4 pCi/L range, and the World Health Organization sets its reference level even lower at 2.7 pCi/L.
More importantly, radon levels vary dramatically between neighboring homes — even between houses on the same block. Two homes with identical floor plans built side-by-side can show readings that differ by a factor of 10 or more, depending on:
- Foundation type and condition
- Soil permeability under the slab
- HVAC system design and operation
- Whether basements are finished or unfinished
- The presence of sump pits, drain tiles, or crawl spaces
- Construction quality and sealing details
In other words, your county's average tells you the risk category — but only a test in your specific home tells you your actual exposure.
How to Test Your Illinois Home for Radon
The EPA and IEMA both recommend that every home be tested at least once, regardless of zone. Two main testing approaches exist:
Short-Term Testing (2-7 Days)
Short-term tests are the fastest way to get a baseline reading. Common short-term test types include:
- Charcoal canister tests
- Electret ion chamber tests
- Continuous radon monitors (CRMs)
These work best when conducted under "closed-house conditions" — windows and exterior doors closed except for normal entry and exit — for at least 12 hours before and during the test.
Long-Term Testing (90+ Days)
Long-term tests provide a more accurate picture of year-round exposure because radon levels fluctuate seasonally. Levels are typically highest in winter when homes are sealed against cold weather. Alpha track detectors are the most common long-term testing technology.
DIY vs. Professional Testing
DIY test kits cost between $15 and $35 and are widely available at hardware stores. They're a reasonable starting point. However, for:
- Real estate transactions
- Post-mitigation verification
- Disputed results
- Homes near or above the action level
…you should hire a professional tester certified by the National Radon Proficiency Program (NRPP) or the National Radon Safety Board (NRSB). In Illinois, radon professionals must also be licensed by IEMA, which requires NRPP or NRSB credentials, ongoing continuing education, and adherence to strict testing protocols. A certified pro can deploy continuous monitors that detect anomalies — like windows being opened during a closed-house test — that DIY kits can't catch.
What to Do If Your Levels Are Elevated
If your test results show levels at or above 4 pCi/L, the EPA recommends taking action. For results between 2 and 4 pCi/L, mitigation should also be considered, especially if anyone in the home is a current or former smoker.
Active Soil Depressurization: The Gold Standard
The most widely used and effective mitigation method is active soil depressurization (ASD), sometimes called sub-slab depressurization. A certified mitigation contractor installs:
- A vertical PVC pipe that draws air from beneath the foundation slab
- A continuously-running inline fan that vents radon-laden air above the roofline
- Sealing of major foundation cracks and openings
- A manometer to verify ongoing system performance
A properly designed ASD system typically reduces indoor radon levels by 50-99%, often bringing readings below 1 pCi/L. Installation costs in Illinois generally range from $1,200 to $2,800, depending on the home's size, foundation type, and complexity.
Other Mitigation Approaches
For homes with crawl spaces, sub-membrane depressurization is often used — a heavy-duty vapor barrier is installed over the crawl space floor and connected to a suction system. For homes with significant block-wall foundations, block wall ventilation may be needed. Some homes benefit from HRV (heat recovery ventilator) systems, though these are typically supplementary rather than primary solutions.
Post-Mitigation Testing
After any mitigation system is installed, retest within 30 days to verify performance. The EPA also recommends re-testing every two years, after major renovations, or after any HVAC changes.
Special Considerations for Illinois Home Buyers and Sellers
Illinois law has specific requirements that homeowners and buyers should understand.
The Illinois Radon Awareness Act
Under the Illinois Radon Awareness Act, sellers of residential property must provide buyers with:
- The Illinois Disclosure of Information on Radon Hazards form
- The IEMA Radon Testing Guidelines for Real Estate Transactions pamphlet
- Any known records of prior radon test results
This disclosure must happen before a buyer is bound by a purchase contract. Failure to disclose known elevated levels can create significant legal liability.
What Buyers Should Do
If you're purchasing a home in Illinois — particularly in any Zone 1 county — radon testing should be part of your inspection contingency. Best practices include:
- Hire an independent NRPP- or NRSB-certified tester (not the seller's contractor)
- Insist on continuous radon monitor testing rather than charcoal-canister-only testing
- Verify that windows and doors remained closed during the test period
- If results exceed 4 pCi/L, negotiate either price reduction or seller-paid mitigation
- Retest after mitigation is complete, before closing
What Sellers Should Do
If you're listing your Illinois home:
- Consider pre-listing radon testing to avoid surprises during inspection
- If results are elevated, mitigate proactively — many buyers prefer a home with a functioning mitigation system to a home with an unknown radon history
- Keep all testing records and mitigation documentation for the new owner
- Disclose any prior knowledge of elevated readings, as required by law
How Often Should You Re-Test?
Radon levels can change over time due to:
- Seasonal fluctuations (winter highs, summer lows)
- Foundation settlement or new cracks
- HVAC system modifications
- Home additions or basement finishing projects
- Changes in soil moisture and groundwater
- Aging of existing mitigation systems
The EPA recommends re-testing every two years at minimum, after any significant home modifications, and after any HVAC or foundation work. Homes with active mitigation systems should be tested annually to confirm continued performance, and the system fan should be inspected for proper operation.
Common Radon Myths Illinois Homeowners Should Ignore
"My neighbor's house tested fine, so mine is fine too." False. Levels vary house to house, even on the same block.
"My home is new, so it doesn't have radon." Newer, tighter homes often have higher radon readings because they're better sealed against air exchange.
"I don't have a basement, so I'm safe." Slab-on-grade and crawl space homes can have significant radon levels in living areas.
"Opening windows fixes the problem." Temporary ventilation reduces levels briefly but is not a real solution — and isn't practical during Illinois winters.
"My home has a mitigation system, so I never need to think about it again." Fans fail, seals degrade, and home modifications can disrupt system performance. Test regularly.
Take Action: Find a Certified Radon Professional Near You
If you live in Illinois — and especially if you live in one of the dozens of Zone 1 counties — testing your home is one of the highest-leverage health decisions you can make for your family. A short-term test costs less than dinner out. A long-term test costs less than a tank of gas. And mitigation, when needed, is a one-time investment that protects your household for decades.
The most important step is working with a qualified, certified professional who knows Illinois building styles, local geology, and IEMA requirements. NRPP and NRSB certified testers carry the credentials, training, and quality controls necessary to give you reliable results — and they're required by law to be licensed in Illinois.
Ready to find out where your home stands? Search FindRadonTesters.com to connect with certified radon testing professionals in your Illinois county today. Whether you're in Rockford, Naperville, Peoria, Springfield, or anywhere in between, the right expert is just a few clicks away — and peace of mind starts with a test.