Radon is a colorless, chemically unreactive inert gas, it is the densest gas know.
In homes and other buildings, radon produced by radioactive decay of uranium - 238 present in soil and rock can reach levels regarded as dangerous. Testing is the only way to know your home's radon levels. There are no immediate symptoms that will alert you to the presence of radon. It typically takes years of exposure before any problems surface and then its to late. The U.S. Environmental Protection Agency, Surgeon General, American Lung Association, American Medical Association and National Safety Council all recommend testing your home for Radon.