Radon and finished basements
Radon is a gas that can collect in the lowest level of any home. A test is simple, and the best time to act on a result is before the walls are closed.
Radon comes from the ground and enters a house through the slab, cracks and sumps. You cannot see or smell it. Illinois public health officials say central and northern Illinois have higher radon levels in soil, and that lower floors such as basements tend to test higher.
What the official sources say
- The U.S. EPA shows 4 picocuries per liter as the level at which to fix your home, and says to consider fixing between 2 and 4.
- The Illinois Radon Awareness Act requires a seller to give the buyer the state radon pamphlet and disclosure before the buyer is bound, and the form states that the state emergency management agency strongly recommends every homebuyer have an indoor radon test performed and mitigated if levels are elevated.
- The Illinois Emergency Management Agency licenses and regulates the individuals who provide radon measurement and mitigation.
Why a basement project is the time to test
The Awareness Act is a sale and lease disclosure law. We found nothing in it that requires a remodeler to test or fix radon. That makes it your choice, and the best moment to make it is before the work. A system to draw radon from beneath the slab to a vent above the roof runs a pipe through the house. Running it before the walls and ceilings are finished is simpler, and you can do it with the finished plan in front of you.
What to do
- Test. Use a kit or a licensed professional. Follow the instructions for how long to run the test, closed-house conditions and location in the lowest lived-in level.
- Read the result against the EPA guidance. At 4 or above, fix it. Between 2 and 4, consider fixing.
- Use a licensed professional for any fix. Illinois licenses radon measurement and mitigation professionals through IEMA.
- Tell your remodeler. The vent pipe route and the fan location affect framing, ceilings and the electrical plan.
Questions
Who tests and fixes radon?
Testing and mitigation are done by measurement and mitigation professionals licensed through the State of Illinois. The building work is planned around the result.
If I test low now, am I done?
A result describes one place and one period. Retesting after major changes to the house, such as new walls, a new heating system or sealing the basement, is a reasonable question to ask your tester.
Homes where this comes up
Keep reading
Sources checked
Checked November 5, 2026. Codes and programs change. Check the current rule with your city or village before you rely on it.
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