Radon is a naturally occurring, colorless, odorless, water-soluble gas produced by the radioactive decay of radium. Radium, which emits radon gas, is a radioactive metallic chemical element found in pitchblende and other uranium minerals. It is common in the earth's crust. The rate of radon's radioactive decay is defined by its half-life, which is the time required for one half of any amount of the element to break down. The half-life of radon is 3.8 days.
All rocks contain some trace uranium concentrations.
The highest concentrations are in plutonic rocks, such as granite. Because the source of radon is the radioactive decay of uranium, higher radon amounts are commonly detected in areas underlain by granites, dark shales, light-colored volcanic rocks, sedimentary rocks containing phosphate, and metamorphic rocks. High levels of radon have been found in all 50 states. Fortunately, radon in air and in ground water, once identified, can be eliminated. Exposure to radon has been recognized as a health risk.