At what level should radon be mitigated?

http://business.sweetwaterreporter.com/sweetwaterreporter/news/read/39927116/What_are_the_Symptoms_of_Radon_Poisoning?

It's a natural byproduct of uranium located in all types of soils. The highest possible concentration is discovered in cellars and crawlspaces closest to the soil below your home. It's additionally high in residences