I'm fresh from a Continuing Education class on Radon and brushed up my awarenes on this issue quite a bit. The short story is Radon is a radioactive gas that is naturally generated from the ground and can occur almost anywhere. It is the second leading cause of lung cancer, second only to smoking, causing an estimated 21,000 deaths per year in the US.
The good news is there's an accurate test for the presence and concentration of Radon in your home and standard protocols for mitigating if it's present at unacceptable levels.
A Citizen's Guide to Radon from the EPA is here.
Today's class was conducted by Portland's Environmental Works. Their web site also has some interesting information here
And my friend Dr. Orna Izakson of the Celilo Natural Health Center has posted a thoughtful blog entry and additional resources here