From Healthy House Institute website:
Radon is the number one cause of lung cancer among non-smokers, according to EPA estimates. Overall, radon is the second leading cause of lung cancer. Radon is responsible for about 21,000 lung cancer deaths every year. About 2,900 of these deaths occur among people who have never smoked.
Radon comes from the natural breakdown of uranium in soil, rock and water. It is harmless when dispersed into outdoor air. But when it becomes trapped inside buildings, it can be harmful at elevated levels. It typically moves up through the ground to the air in your home through cracks and other holes in the foundation. This means any home can have a radon problem, whether it is new or old, well-sealed or drafty, with or without basements.
Conducting a radon test is the only way to know if you and your family are at risk of radon because you can’t see it, smell it, or even taste it. Fall and winter are the best times to conduct a test because doors and windows are sealed tight against the cold, which ensures a more accurate radon reading.