How can radon affect people's health?

Almost all risk from radon comes from breathing air with radon and its decay products. Radon decay products cause lung cancer. 

There is no safe level of radon -- any exposure poses some risk of cancer. In two 1999 reports, the National Academy of Sciences (NAS) concluded after an exhaustive review that radon in indoor air is the second leading cause of lung cancer in the U.S. after cigarette smoking. The NAS estimated that 15,000-22,000 Americans die every year from radon-related lung cancer.