Seven of the eight counties in and around Indianapolis are in EPA's highest radon zone. Here is what that means, how a radon test works during a home purchase, and what happens if a number comes back high.
I'm Wes Johnston, a Realtor with Trueblood Real Estate in Fishers. I'm not a radon tester, mitigator or health professional, and this page isn't medical or legal advice. Every fact below links to its source, mostly the EPA and the State of Indiana.
The short version: test every home, whatever the map says. The EPA says the zone map is for targeting resources, not for deciding whether a particular house needs a test. The number to know is 4.0 pCi/L, the EPA action level and the standard used in Indiana. At or above it, the EPA says to fix the home.
Radon is a radioactive gas that comes from the natural breakdown of uranium in soil and rock. You can't see or smell it, and it gets into homes through the foundation. The EPA calls it the leading cause of lung cancer among non-smokers and the second leading cause overall (EPA: Health Risk of Radon).
Indiana's environmental agency repeats the same warning (IDEM: Radon). The good news is that it's easy to measure and, in most houses, straightforward to fix.
The EPA's Map of Radon Zones sorts every county into three groups: Zone 1, where the predicted average indoor level is above 4 pCi/L (highest potential); Zone 2, between 2 and 4 pCi/L; and Zone 3, under 2 pCi/L (EPA: Map of Radon Zones).
Here are Marion County and every county that borders it, straight from the EPA's county spreadsheet:
| County | Includes | EPA zone |
|---|---|---|
| Marion County | Indianapolis, Lawrence, Beech Grove | Zone 1 |
| Hamilton County | Fishers, Carmel, Noblesville, Westfield | Zone 1 |
| Hancock County | Greenfield, McCordsville, Fortville | Zone 1 |
| Shelby County | Shelbyville | Zone 1 |
| Johnson County | Greenwood, Franklin, Bargersville | Zone 1 |
| Morgan County | Mooresville, Martinsville | Zone 2 |
| Hendricks County | Avon, Plainfield, Brownsburg | Zone 1 |
| Boone County | Zionsville, Lebanon | Zone 1 |
Statewide, 57 of Indiana's 92 counties are Zone 1 and the other 35 are Zone 2. No Indiana county is in Zone 3.
Source: EPA Map of Radon Zones county spreadsheet (EPA 402/A-16/001), read September 27, 2026. The map dates from 1993 and is built from indoor test results, geology and soil data. Homes with high radon have been found in every zone.
4.0 pCi/L (picocuries per liter) is the EPA action level and the standard used in Indiana. At or above it, the EPA says to fix the home. Between 2 and 4, the EPA says to consider fixing it (EPA: Health Risk of Radon).
For context, the EPA puts the average indoor level at about 1.3 pCi/L and outdoor air at about 0.4. Keep in mind that a zone is a county average; two houses on the same street can test very differently.
A radon test is one of the inspections you can order during the inspection period in your purchase agreement. For a real estate test, the EPA's buyer and seller guide calls for a short-term test of at least 48 hours, with the house closed up for at least 12 hours before and during the test (EPA: Home Buyer's and Seller's Guide to Radon).
The EPA accepts three setups: two test devices side by side, two tests back to back, or a continuous monitor. Because the test runs two days or more, schedule it early in the inspection period so the result is back before your deadline to respond.
In Indiana, anyone paid to test or fix radon in someone else's home must be certified by the Indiana Department of Health (IC 16-41-38-5; IDOH). For a sale, that means a certified Primary or Secondary Radon Tester. You can check a tester's certification with the state license lookup.
It's usually a negotiation, not a deal-breaker. Common outcomes are that the seller has a mitigation system installed before closing, the seller gives a credit so the buyer can install one, or the price is adjusted. Your purchase agreement's inspection terms set the options and the deadlines.
If the seller is installing the system, ask for a certified mitigator and a follow-up test after it's running. The EPA recommends retesting after mitigation, ideally by an independent tester (EPA guide).
The usual fix is sub-slab depressurization: a vent pipe and a small fan that pull soil gas from under the slab or basement floor and send it out above the roof line (EPA guide). The EPA says most homes can be fixed for about the cost of other common home repairs.
The American Lung Association estimates installation at roughly $1,000 to $2,500 or more (ALA: Radon, accessed September 2026). The house, the foundation and the installer all move the number, so get quotes from certified mitigators.
Building new? Ask the builder whether the home will have a passive radon vent pipe and whether a fan can be added later. It's a good question to raise before the drywall goes up.
It's worth thinking about, especially in a Zone 1 county where most buyers will ask. Testing before you list tells you where you stand and gives you time to get mitigation quotes on your own schedule instead of inside a buyer's inspection deadline.
If your house already has a mitigation system, find the install paperwork and any post-mitigation test results; buyers will ask for them. If you know about a high result, ask me how to handle it on the Indiana seller's disclosure.
The Indiana Radon Hotline, run with the American Lung Association, answers general radon questions at 1-800-272-9723 (IDOH). The EPA's Home Buyer's and Seller's Guide to Radon is the best single read if you want the details. To confirm a tester or mitigator is certified, use the Indiana license lookup.
If you're in the middle of a purchase or getting ready to sell and radon has come up, send me the details. I'll tell you how it usually gets handled in a deal and who to call.
I'm not a radon professional. For testing and mitigation, use a tester or mitigator certified by the Indiana Department of Health.
Fishers-based. Straight answers before you sign. Committed to your success.
Tell me the address and what came back. I'll walk you through the options.
Send a MessageGetting ready to sell? Start with a realistic price based on recent local sales.
Send the address and what's going on. I'll get back to you with how it usually gets handled.
General guidance only; use a certified radon tester or mitigator for the actual work.
This page is general information, not medical or legal advice. Radon zones are from the EPA Map of Radon Zones county spreadsheet, read September 2026. Testing and mitigation in Indiana must be done by professionals certified by the Indiana Department of Health.