Radon is an invisible, tasteless, odorless, and cancer-inducing radioactive gas that occurs naturally in the environment - Naturally Occurring Radioactive Material (NORM) - as a decay product of uranium and thorium. Discovered by the English physicist Ernest Rutherford and German physicist Friedrich Ernst Dorn at the turn of the 20th century, radon is most often found in rock, soil, and well water, and easily penetrates paper, leather, low-density plastics, paints, building materials, concrete, mortar, tar paper, wood paneling, insulation, and more.
A noble gas, there are only two isotopic forms of radon that occur in high enough concentrations to be concerned about, namely Radon-220 and Radon-222. A decay product of Thorium-232, Radon-222 occurs most prevalently in the natural environment, with decay products forming as a result of its atmospheric release. These decay products are radioisotopes of heavy metals and readily attach to other airborne substances, such as dust, that are easily inhaled.
Humans are exposed to radon primarily through inhaling and/or ingesting it. Radon makes its way into buildings, structures, homes, etc. through rocks, soil, groundwater, building materials, and more, where it subsequently disintegrates into its decay products and can end up being inhaled or ingested. Constant exposure to high concentrations of radon poses a elevated risk of lung cancer, while smoking vastly exacerbates this risk.
Fortunately, the health risks from radon are entirely manageable, as long as the proper testing, mitigation, and monitoring measures are taken.