Radon is the #1 cause of lung cancer among non-smokers. Overall, it is second only to smoking in causing lung cancer, killing over 20,000 people per year — over 2,900 who have never smoked.
To repeat — there is no known safe level of exposure to radon. The Environmental Protection Agency strongly recommends that you address the issue if testing in your home shows levels over 3.9 pCi/L.