From Radon Tests
Radon in Air (EPA recommended action level is 4 pCi/L) The migration of radon up from the soil contributes to the largest percent of radon found in the average home. Radon gas moves up through the ground to the air above and into your home through cracks and other holes in the foundation. Your home traps radon inside, where it can build up. Radon may be present in any home whether it is new or old, well sealed or drafty, or a home with or without a basement.