Average Radon Levels in Indiana: EPA Zone Data and What It Means
Indiana Is Entirely EPA Zone 1
Every county in Indiana falls in EPA Radon Zone 1, the highest-risk category on the federal radon zone map. Zone 1 means the predicted average indoor radon screening level exceeds 4 pCi/L, the EPA's recommended action level. Indiana is one of a smaller group of states where every county carries this designation, with no Zone 2 or Zone 3 counties anywhere in the state.
The Indiana State Department of Health (ISDH) estimates that 30 to 40 percent of Indiana homes test above 4 pCi/L. That is a high percentage by national standards. For context, the national average indoor radon level is about 1.3 pCi/L. Indiana's statewide geology creates conditions where radon accumulates in homes at rates well above that baseline.
Why Indiana Has High Radon Levels
Indiana's radon problem comes from its geology. Two distinct geological formations drive elevated radon across the state, each with its own characteristics.
Glacial Till: Northern and Central Indiana
The Wisconsin glaciation, which retreated roughly 10,000 to 12,000 years ago, deposited thick layers of glacial till across the northern two-thirds of Indiana. Glacial till is a mixed sediment of clay, sand, gravel, and rock fragments ground up and deposited by the glacier. This material is rich in uranium-bearing minerals, particularly in the coarser fractions. As uranium decays through radium to radon gas, the till acts as a radon source. The permeability of glacial materials also allows radon to migrate upward toward the surface and into building foundations.
The Indianapolis basin sits on deep glacial outwash and lake plain sediments, a thick accumulation of water-sorted glacial material that creates particularly high radon potential. The outwash plain that underlies much of central Indiana holds both the source material for radon generation and the permeable pathways that allow it to reach homes.
Limestone Karst: Southern Indiana
Southern Indiana was not covered by the last glaciation. Instead, this region is underlain by Mississippian-age limestone, which has been dissolved over millennia into karst topography: sinkholes, caves, losing streams, and fractured rock. The karst belt runs through Monroe, Lawrence, Orange, Crawford, and adjacent counties.
Karst produces radon through a different mechanism than glacial till. Fractures and solution channels in the limestone allow radon from deeper uranium-bearing rock to migrate toward the surface more directly. The pathways are less predictable than glacial deposits. A house built directly over a fracture zone can show very high readings while a house three lots away tests low. The Mitchell Plateau and Crawford Upland areas in the karst belt have documented elevated radon, but the variability within small areas is higher than in the till-dominated north.
Radon Levels by Region
Northern Indiana: South Bend, Fort Wayne, Elkhart
Northern Indiana counties sit on some of the state's thickest glacial till deposits. St. Joseph County (South Bend), Allen County (Fort Wayne), Elkhart County, and LaPorte County are among the highest-risk areas in the state. The dense till plain in this region, combined with the cold winters that keep homes sealed for months at a time, creates conditions for high radon accumulation. ISDH data consistently shows northern Indiana counties with elevated average test results. Homes in South Bend, Fort Wayne, Elkhart, Mishawaka, and surrounding communities should treat radon testing as a baseline home safety step, not an optional one.
Central Indiana: Indianapolis Metro, Muncie, Anderson
The Indianapolis metro sits at the center of the glacial outwash plain. Marion County and the ring counties, including Hamilton, Hendricks, Johnson, Hancock, and Boone, show consistently high radon rates. The deep sediment profile under the Indianapolis basin means uranium-bearing material is present at depth and that radon has permeable pathways to travel upward. Muncie (Delaware County) and Anderson (Madison County) to the northeast of Indianapolis are in the same glacial deposit zone with similar risk profiles.
Carmel, Fishers, and Noblesville are among the most frequently tested communities in the state because of their high volume of residential real estate transactions. Local real estate professionals treat radon as a standard inspection item in these markets.
Eastern Indiana: Richmond and the Glacial Till Plain
Eastern Indiana counties including Wayne County (Richmond), Randolph County, and Henry County sit on the eastern glacial till plain. The geology is similar to central Indiana: Wisconsin-age till, permeable sediments, and uranium-bearing parent material. Radon levels here track closely with the rest of the till-dominated state, with a significant fraction of homes testing above 4 pCi/L.
Western Indiana: Terre Haute, Lafayette, and the Wabash Valley
The Wabash River valley cuts through western Indiana, creating a corridor of alluvial and glacial outwash sediments. Vigo County (Terre Haute) and Tippecanoe County (Lafayette, West Lafayette) have elevated radon consistent with the statewide Zone 1 designation. The Wabash alluvial plain and the surrounding till uplands both contribute radon source material. Purdue University has conducted radon research in the region, and local awareness of the issue is relatively high.
Southern Indiana: Bloomington, Bedford, Columbus
Southern Indiana presents a distinct risk picture. Monroe County (Bloomington), Lawrence County (Bedford), Bartholomew County (Columbus), and the surrounding karst belt sit outside the glaciated region. The limestone karst geology produces radon through fracture migration rather than till permeability. The result is high variability: some areas test consistently elevated, while others test low even within the same county.
Bedford, in Lawrence County, sits in one of the most active karst zones in Indiana. The limestone quarry industry in this area has exposed fresh rock faces, and the broader geology creates elevated radon potential. Columbus (Bartholomew County) straddles the boundary between glaciated and unglaciated terrain, with correspondingly mixed radon behavior.
For southern Indiana homeowners, the key takeaway is that you cannot estimate your home's radon level from a neighbor's test result. The karst fracture network is irregular enough that house-to-house variation is high. Test your specific home.
Highest-Risk Counties in Indiana
Based on ISDH data and geology, the counties with the highest documented radon rates include:
- Allen County (Fort Wayne): Dense glacial till, consistently high average test results
- St. Joseph County (South Bend): Thick till deposits, cold winters compound accumulation
- Elkhart County: High till density, elevated average indoor levels
- Hamilton County (Carmel, Noblesville, Fishers): Deep glacial outwash, high testing volume confirms elevated rates
- Marion County (Indianapolis): Indianapolis basin sediments, widespread testing shows significant percentage above 4 pCi/L
- Monroe County (Bloomington): Karst geology, high variability but elevated in many areas
- Lawrence County (Bedford): Active karst zone, elevated radon documented in stone-belt communities
- Delaware County (Muncie): Glacial till plain, elevated rates consistent with eastern Indiana profile
Understanding the Numbers: A Reference Table
| Radon Level | EPA Classification | Equivalent Risk Comparison |
|---|---|---|
| Below 2 pCi/L | Below action level | Near national average indoor level |
| 2 to 4 pCi/L | Elevated, consider mitigation | Between EPA guidance thresholds |
| 4 pCi/L | EPA action level | Roughly equivalent to smoking 8 cigarettes per day for lifetime lung cancer risk |
| 8 pCi/L | High, mitigate promptly | Roughly equivalent to smoking a half pack per day |
| 20 pCi/L or above | Very high, mitigate as soon as possible | Comparable to smoking 1 to 2 packs per day |
Radon is the leading cause of lung cancer among non-smokers in the United States and accounts for approximately 21,000 lung cancer deaths per year nationally. At Indiana's statewide testing rates, a large number of state residents are living with elevated exposure without knowing it.
House-to-House Variation: Why Zone Data Is Not Enough
Indiana's Zone 1 designation tells you the state has high radon potential. It does not tell you what level is in your home. Even within a single neighborhood in Indianapolis or Fort Wayne, measured levels can vary from 2 pCi/L at one address to 15 pCi/L or more next door. Factors that drive this variation include:
- Foundation type: Slab-on-grade homes and homes with basements or crawl spaces accumulate radon differently. Basements typically show higher levels than upper floors because radon enters from the soil below.
- Foundation integrity: Cracks, penetrations, and construction gaps are entry points. Newer homes with modern construction practices sometimes test lower than older homes with more foundation openings, but this is not a reliable predictor.
- Soil permeability beneath the foundation: A house sitting over a gravel pocket in the till will typically show higher levels than one over tighter clay.
- Ventilation habits: How tightly a home is sealed affects how much radon accumulates. This is why Indiana winters matter.
When to Test: Indiana's Seasonal Timing
Radon testing should be done during closed-house conditions, when windows and doors are kept closed for at least 12 hours before and during the test. In Indiana, this means the October through April testing window aligns with normal heating-season behavior. During these months, homes are naturally sealed against the cold, which is when radon accumulates to its highest levels.
Testing in June through August with windows open will produce artificially low readings that do not represent typical exposure. If your test was done during summer with active ventilation, consider retesting during the heating season for a more accurate picture. For real estate transactions, most Indiana protocols require closed-house conditions regardless of season.
Long-term tests (90 days or more) are more accurate than short-term tests because they average out daily and weekly fluctuations. If you are not facing a real estate deadline, a long-term test run through the heating season gives you the most reliable data.
Test Your Home
The only way to know your home's radon level is to test. Professional testing in Indiana costs $100 to $250 using licensed ISDH-certified professionals. Find certified radon testers in Indiana through our directory, organized by location across the state.
Frequently Asked Questions
What EPA radon zone is Indiana?
Indiana is entirely EPA Zone 1, the highest-risk category. Every county in the state carries this designation, meaning the predicted average indoor radon screening level exceeds 4 pCi/L across the entire state.
What percentage of Indiana homes have elevated radon?
The Indiana State Department of Health estimates that 30 to 40 percent of Indiana homes test above the EPA action level of 4 pCi/L. This is significantly higher than the national average, reflecting Indiana's Zone 1 geology throughout the state.
Which Indiana counties have the highest radon levels?
Allen (Fort Wayne), St. Joseph (South Bend), Elkhart, Hamilton (Carmel, Fishers), and Marion (Indianapolis) counties consistently show high rates based on ISDH data. Monroe (Bloomington) and Lawrence (Bedford) counties in the karst belt also have elevated radon, though with more variability from house to house.
Why does southern Indiana have different radon risk than northern Indiana?
Northern and central Indiana sit on glacial till deposited during the Wisconsin glaciation, which creates radon through uranium decay in uranium-bearing sediments. Southern Indiana was not glaciated and instead has limestone karst geology with sinkholes and fractures. Radon in karst areas migrates through rock fractures, creating more variability between neighboring properties than in the till-dominated regions further north.
When is the best time to test for radon in Indiana?
October through April, during the heating season when homes are sealed against the cold. Closed-house conditions are required for accurate radon testing. Testing during summer with windows open will produce artificially low results. If a real estate transaction is involved, most Indiana testing protocols require closed-house conditions regardless of the season.