Radon is a radioactive gas that comes from the natural radioactive decay of radium, which is a natural decay product of uranium. Scientifically, “radon” is known to be radon-222, the most abundant isotope of the element radon. As a noble gas, radon is colorless, odorless and chemically inert and cannot be detected by human senses. Also, since radon is not chemically reactive with most materials, it will move freely as a gas. Radon has a radiological half-life of 3.8 days, and can move substantial distances from its point of origin.
As radon-222 decays, the first four radioactive isotopes formed are polonium-218, lead-214, bismuth-214 and polonium-214. They are commonly referred to as “radon daughters” or “radon progeny.” These short-lived isotopes are not gases but are chemically active solids. They are present in any environment where radon is found and, like radon, cannot be detected by human senses.
Uranium is a natural part of the earth’s crust; therefore, radium and radon are also naturally present. Since uranium and radium concentrations vary throughout the earth’s crust, radon concentrations will also vary. The amount of radon gas that escapes into the atmosphere is dependent on the depth at which it is formed and the permeability of the surrounding earth. Radon formed in the top 10 meters of soil and rocks provide the largest component of radon entering the atmosphere.
The second most important contributor to radon is emanation from groundwater. Underground radon is readily carried in groundwater and when this groundwater surfaces, most of the radon is released to the atmosphere.
Other sources of radon are very small contributors and are largely due to human activities. For example, there are radium-rich industrial by-products spread upon the earth, and sometimes construction materials are produced from raw materials that contain uranium or radium.