EPA radon action level

https://foxtrot-wiki.win/index.php/York_PA_Radon_Contractors_See_Increased_Demand_as_Home_Testing_Rates_Rise

The EPA radon action level is set at 4 picocuries per liter of air—the point where homeowners should seriously consider fixing a radon problem. It's not a mandatory safety limit but rather a guideline based on risk reduction, since radon is a leading cause of lung cancer after smoking. Think of it as a nudge to take action if your home tests above this benchmark.