Radon (atomic number 86) is a noble gas like helium, neon and argon that is relatively unreactive because of its electron configuration, where the octet rule is fulfilled (the atom has eight valence electrons). Because of its stability, radon is reluctant to undergo reactions with other molecules and therefore is difficult to contain. The most abundant isotope of radon is radon-222, which undergoes decay over time into what are called "radon daughters." These daughters are also radioactive, however they are atoms of heavy metals and will adhere to whatever they contact, including the respiratory epithelium. Health problems come from the inhalation of radon daughters or dust particles with radon daughters affixed. You can click here to locate radon in the Periodic Table.

The mechanism for the release of radon from the earth's crust is a complex geological problem. The place to start is with uranium and thorium, the "parents" of all radon. A trace element in the earth's crust, uranium sometimes becomes enriched to form deposits of ore. If in the presence of water, uranium reacts with dissolved oxygen as an ion with a charge of +6. U6+ can complex with the carbonate present in most surface waters to form the uranium carbonate complex commonly found in nature. Thorium occurs as an ion with a charge of +4, and is four times as prevalent as uranium in the minerals and rocks of the earth's crust. Because of its value in mining, the location of uranium in the earth's crust has been the subject of a number of studies by agencies of the federal government. After W.W. II, the Atomic Energy Commission (AEC) established a program to develop alternative sources of uranium other than the deposits of the Colorado Plateau in the Western U.S. The AEC (now known as the Department of Energy) established that uranium in phosphate rocks is most promising, and the U.S. Geological Survey found the highest concentrations of these rocks in Florida and the rocky mountain states. Most of these phosphate deposits occur as calcium fluorapatite [Ca10(PO4)6F2] matrix, and accounts for 80 percent of domestic phosphate production. Most radon in the earth's crust is immobilized in minerals, however a small percentage does escape the crystal. The melting of old, deep rocks that crystallize into new igneous rocks releases all the radon that has accumulated, as does the prograde metamorphism and the weathering of rocks below or at the earth's surface.

Radon escapes into the air from land, water, and plants. It has been estimated that some 2.4 billion Ci/year are released from continental land surfaces and some 23 million Ci/year from ocean surfaces. Moreover, it is estimated that crops and other vegetation and groundwaters may add an amount equal to 20% of the land release value (480 million pCi/yr). Insoluble in water, radon is released when water is warmed (in showers, washers etc.) and contributes to indoor radon in some areas. Outdoors, people are exposed to approximately 0.1-0.15 pCi/L of radon. When released from the ground, radon is mixed vertically during the day three times greater than at night, and radon concentration is also affected by winds (outdoors) and ventilation (indoors).