Epidemiological studies have shown a clear link between breathing high concentrations of radon gas and developing lung cancer. According to the United States Environmental Protection Agency, radon is the second leading cause of lung cancer, causing 21,000 lung cancer deaths per year in the United States. Humans cannot feel radon, see radon, smell radon, or taste radon. Only radon tests can detect radon and measure its concentration.    


Due to numerous factors, buildings act like vacuum cleaners, sucking dangerous radon gas out of the ground and into the buildings. Because of radon's gaseous state, it is easily inhaled. In fact, radon is usually the single largest contributor to an individual's annual background ionizing radiation dose. Radon commonly accumulates to far higher than normal concentrations in buildings, especially in low areas such as basements and crawl spaces due to being heavier than the other air molecules (like nitrogen and oxygen). It can also be found dissolved in water (particularly private well water).


According to the city of Madison's website, 20% of homes tested in Dane County had levels of radon that are considered unsafe. While the national average indoor radon level is 1.3 pCi/L, the average indoor level in Dane County is a whopping 3.8 pCi/L.


You should test for radon. Testing is the only way to find out your home’s radon levels. The EPA and the Surgeon General recommend testing all homes below the third floor for radon.
— Home Buyer's and Seller's Guide to Radon