There is no "safe" radon level. There is believed to be some risk to be associated with any exposure, and as a general rule, the higher the radon level and the longer the exposure, the greater the risk.
Congress has a set a long-term goal of reducing indoor radon levels so that they are no greater than exposure to ambient (outdoor) air. The average outdoor level is between 0.3 picocuries per liter (pCi/l) and 0.7 pCi/l, and while that level is not yet technologically achievable, many homes can be brought down to levels below 2 pCi/l. In the meantime, we are using a guideline of 4 pCi/l. This guideline was selected because the rule of thumb is to keep exposure to radiation as low as reasonably achievable (ALARA), and 4 pCi/l is a reasonably achievable radon level. Whether a home has 100 pCi/l, or 50 pCi/l, or 20 or 10 pCi/l, the current technology is able to bring the level down to below 4 pCi/l for a reasonable amount of money, with a reasonable amount of effort, over a reasonable period of time.