There is no such thing as a safe level of radon. It does not exist. The EPA has set a action level of 4.0 pC/iL. This is true.... but at the same time you want the level as low as you can get it. The 4.0 is really just a base line for real estate transaction. Nothing more. You have to understand also that radon runs on dienare cycles - temp, rain, day and night etc. When a test is done during a real estate transaction the EPA guidelines is 48 hours min. That is because in the sale of a home....time is very short and the 48 hours is because of that. Think of it this way. two days is only 0.05% of a year....and radon is higher in the evenings when it is cooler and it is higher during the winter because of the cold. Radon gas is heavier then air...so it hugs the ground when it is cold or cooler.
If buying a home and it is tested and it is 4.0 or higher the seller pays for the system. If it is 3.9 or lower then no system is required to be installed at the time of the sale. BUT DO NOT ! think that that is the over all reading all the time ! The best thing to do is run a long term test (91 days or more) after you buy the house....try to catch two seasons....middle of sept, oct, nov and dec. It will give you a better overall reading of what is really there. And if it is high install a system but of course at that point since you own the house you would have to pay for the system.
Hope this helps !
Radon Systems 4U LLC