EPA radon action level 4

https://radontestcostscope.urbanvellum.com/posts/understanding-radon-test-cost-for-st.-louis-homes

The EPA radon action level is set at 4 picocuries per liter of air, meaning if your home tests at or above this amount, the agency strongly recommends taking steps to reduce radon levels. It's not a legal requirement but a health-based guideline, since prolonged exposure to high radon is linked to lung cancer. Many people test their basements or ground-floor living spaces, then install mitigation systems like sub-slab depressurization to bring levels down.