A safe level is no radon. Any value more than zero might be dangerous for your health. The US EPA has put it plainly, stating, “Any radon exposure has some risk of causing lung cancer. The lower the radon level in your home, the lower your family’s risk of lung cancer.”
There are acceptable levels though. By acceptable it means that the risk is low (although is not zero risk). Radon Act 51 passed by Congress set the natural outdoor level of radon gas (0.4 pCi/L) as the target radon level for indoor radon levels. Unfortunately two-thirds of all homes exceed this level.
The US EPA has set an action level of 4 pCi/L. At or above this level of radon, the EPA recommends you take measures to reduce your exposure to radon gas. This does not imply that a level below 4.0 pCi/L is considered acceptable.