EPA radon action level 2

https://48hourradontestdigest.lakeviewbrief.com/posts/understanding-the-48-hour-radon-test-for-st.-louis-home-transactions

The EPA radon action level is the point at which the agency recommends fixing your home—4 picocuries per liter of air. It's not a legal cutoff, but a health-based guideline. If your test shows that number or higher, the EPA suggests taking action to reduce radon, as long-term exposure can cause lung cancer. Many homes can be fixed for about the cost of other minor repairs.