Radon is a radioactive gas that comes from the soil. Starts out as Uranium, transforms into Radium and then into Radon. 21,000 people a year die from Radon induced lung cancer every year. A simple, cheap test will tell you if you are at risk. Test kits can be obtained at you local homestore or online. Also, only 10% of people who test their houses actually follow up and mitigate thier problem. If you have a result that comes back higher that 2.5, you may want to do another short term test or a long term test. Please, if you find high Radon content in your house, don't be the 90% who do nothing and are potentially dying a slow death.
The Minnesota Department of health has some good information if you need to study up on this dangerous problem.