Radon

• A radioactive element that is one of the heaviest gases known. Its atomic number is 86. It is a daughter of radium.
  Source: U.S. Nuclear Regulatory Commission
• A colorless naturally occurring, radioactive, inert gas formed by radioactive decay of radium atoms in soil or rocks.
  Source: Terms of the Environment
• A radioactive gas that is released by uranium, a substance found in soil and rock. Breathing in too much radon can damage lung cells and lead to lung cancer.