Radon has been 'known' of since the about the mid 1970's.  It was first discovered in Pennsylvania.  It was discovered when a worker at an atomic plant tested higher for radioactivity coming into work than he did when he left work.  This both confused and concerned the management as you can well imagine.

After investigating the worker and the cause of this 'curiosity' it was determined to be the gas we call radon.

Since that time testing has been ongoing in an effort to determine the effects of radon gas on the human body.

When the radon molecule is inhaled it presents the risk of damaging the soft tissue of the lungs and thus lend to the increased risk of lung cancer.

It has been determined from these tests that radon is the second leading cause of lung cancer in the United States.