EPA radon action level

https://radongastestcentral.capitaljays.com/posts/why-every-st.-louis-homeowner-should-consider-a-radon-gas-test

If you're testing your home for radon, the EPA's action level of 4 picocuries per liter is the threshold where they strongly recommend fixing the problem. It's not a safe zone—anything below that still carries risk—but it's the point where mitigation systems typically become a priority. Think of it less like a clean bill of health and more like a red light telling you to take action.