Understanding Your Radon Test Results

Your radon test results are in. The report shows a number. What does it mean, and what should you do next? Here's a plain-language guide.

What Is pCi/L?

Radon levels are measured in picocuries per liter (pCi/L), a unit that describes the concentration of radon gas in the air. One picocurie is a tiny amount of radioactivity, and the "per liter" refers to the volume of air tested.

The average indoor radon level in U.S. homes is about 1.3 pCi/L. Outdoor air typically measures around 0.4 pCi/L.

The EPA Action Level: 4.0 pCi/L

The U.S. Environmental Protection Agency recommends taking action to reduce radon in your home if your level is 4.0 pCi/L or higher. Because EPA says there is no known safe level of radon exposure, it also recommends considering mitigation between 2.0 and 4.0 pCi/L.

Illinois Regulations

Illinois regulates professional radon services under 32 Ill. Adm. Code Part 422. Section 422.130 covers measurement protocol, Section 422.140 covers device protocol, and Section 422.150 sets the mitigation standard for existing housing. Anyone providing radon measurement or mitigation services in Illinois must hold the license that applies to that work.

What To Do If Your Level Is Elevated

The testing context matters. For an initial homeowner test outside a sale, Illinois Appendix A provides a follow-up testing path after an elevated short-term result. For a valid real-estate test, Appendix B says to fix the home when the result is 4.0 pCi/L or higher. If test conditions were not followed or the results conflict, consult an Illinois-licensed measurement professional before treating the number as settled.

When mitigation is the next step, an Illinois-licensed mitigation professional evaluates the foundation, layout, and test information and designs an appropriate system. Active soil depressurization is common, but the design may also need to address drain tile, crawlspace, or block-wall conditions.

Illinois requires post-mitigation testing after the permanent system has operated for at least 24 hours and no later than 30 days after completion and activation. The system runs normally and continuously during that test. The resident may perform the test, or an independent Illinois-licensed measurement professional may do it. Illinois also recommends retesting at least every two years and after significant changes to the building.