Radon exposure is the second leading cause of lung cancer, just behind smoking. Radon is an invisible and odorless natural radioactive gas—the result of decaying radioactive materials in rocks, soil, minerals, water, and natural gas. The biggest sources of it in any home are soil and building materials. It tends to be produced under homes or in basements and then seeps up through cracks.
The only way to know if you have radon is to test for it. Buy a test kit or contact your state radon agent to have your home tested.
All that is usually required to take care of radon is ventilation. So throw open your windows, especially in the basement.