Breathing radon in indoor air is the second leading cause of lung cancer in the United States. Between 7,000 and 30,000 lung cancer deaths each year are due to radon. Radon in drinking water is less significant, causing an estimated 168 cancer deaths a year. In New England it's predicted that one in four homes would have such levels. The only way to know if there is radon in a building to test for it.
Testing can be easy and inexpensive.
If high levels of radon are found, there are ways to reduce such levels. The web site also has valuable information.