Rn is the chemical symbol for radon. It is a radioactive gas that  occurs naturally and can be found in soils, rock, and water throughout the world. It has numerous different isotopes, but radon-220, and -222 are the most common. Radon is a gas that is created in the soils where uranium and radium are found. Since these elements can be found everywhere in the world any building has the potential for elevated levels of radon. The more uranium found in the soil, the higher the potential for elevated radon levels within that building.

Because radon is an inert (or noble) gas, meaning that it does not react or combine chemically with other substances except under certain special conditions, it can move up through the soil into the atmosphere, where it is easily diluted and presents little concern. However, when radon enters a building, it can build up and become a health concern

You cannot see or smell radon, and there is no way that your body can sense the presence of radon; however, it can have a detrimental effect on the inhabitants by increasing their likelihood of developing lung cancer.