The term 'Atomic Number 86' refers to the element RADON, which is a naturally occurring radioactive gas. It has an atomic number of 86, meaning it has 86 protons in its nucleus. RADON is colorless, odorless, and tasteless, and is found in soil and rocks. It is also found in some building materials, and can be released into the air when these materials are disturbed. RADON is a known carcinogen, and can cause lung cancer if inhaled in large amounts.