Radon is a radioactive gas that forms naturally in soil when uranium in the soil decays.  Radon is colorless, odorless, and completely ignorable, yet radon is the second-leading cause of lung cancer in the United States.  Only smoking causes more lung cancer. Radon is the leading cause of lung cancer for non-smokers.

The various decay products of uranium emit alpha particles.  But alpha particles are only harmful when they strike vulnerable lung tissue.  Clothing and even the outer layers of our skin cells block the particles.  While all the radioactive decay products emit alpha particles, radon is a gas.  It becomes trapped in the air in a home.   This is how it enters your lungs.