The Earth’s crust contains among others, for the radiation exposure of humans less important natural radionuclides uranium-238, uranium-235, thorium-232 and potassium-40. The radioactive noble gas radon-222 (Rn-222, half-life: 3.8 days) is part of the uranium-238 decay chain and is built up by radium-226.
Radon is colourless, odourless and tasteless. Radon and its short-lived decay products contribute most to the radiation exposure of the population from natural radiation sources.
Information contributed by BfS (http://www.bfs.de/en/ion/radon)