First of all there are really no safe radon levels since it's a radioactive gas that causes lung cancer. With that said, we need to recognize that we all live life with some unavoidable risk.
For instance, the average outdoor level is 0.4pCi/L. Thus it's impossible to completely eliminate our exposure to radon gas. But we do have options if our indoor radon results are elevated. So what's a high radon reading?
The EPA set 4pCi/L as the action level for the United States back in 1988. This is not a "health based level" as they are quick to add.
Homes with readings between 2pCi/L and 4pCi/L should consider mitigation also. As of 2009, the World Health Organization set their threshold to mitigate at 2.7pCi/L.
Radon readings will vary throughout the year. Highest levels are typically recorded in mid-winter (when you house is sealed up) with levels decreasing to there lowest concentrations in summer (when you might have your windows open allowing the radon to escape).
So 4pCi/L in summer could be considerably higher in December. Thus, an average of well above the EPA action level can be assumed.
Whereas the opposite may be true of a level of 4pCi/L in winter. Thus the only way to really know for sure is conduct a long term test to account for the fluctuations through out the seasons.
Also important to consider is the time spent in the lowest level of the building. If your level is 2pCi/L in a basement used only for storage, then it's likely that your exposure will be low. But if you have a forced air furnace and central AC system this could create the same level on all floors of the home when in operation.
To be sure of your exposure a long term test kit will give the best picture of overall exposure to radon in your home.