Radon is a gas produced by the radioactive decay of the element radium and has a half-life of 3.8 days. Because it is a gas, Radon can more easily leave the rocks and soils by escaping into fractures and openings in rocks and into the pore spaces between grains of soil. The soil's permeability controls the amount and rate at which Radon is able to enter a structure. The three factors that determine a soil’s permeability are: the amount of water present in the pore space, the percentage of pore space in the soil and the interconnectedness of those pore spaces. Coarse sand and gravel therefore allow for a greater amount of Radon to move through it as opposed to more permeable soils like clay. The more readily Radon can pass through the soil, the greater opportunity it has to collect inside a building.
In the United States, Radon is second only to cigarette smoking in causing lung cancer. Radon is odorless and colorless and therefore unrecognizable when present.