What is radon (Rn-222)?

 Radon-222 is a gaseous radioactive, short-lived (half-life is 3.82d), inert gas produced by the alpha decay of radium-226 in the naturally occurring uranium-238 series. Radon is ubiquitous. 

Uranium and thorium are natural radioactive elements present in the earth and rocks in very small amounts. Hence, building materials such as sand, bricks and cement also contain some amount of uranium, thorium, radium-226 or radium 224. Since they are present in the earth, the water-soluble elements present, such as uranium and radium in the decay series, get into water resources and land mass. The gaseous radionuclides radon and thoron seep out of soil and get mixed with the atmospheric air.  

Thus, we get exposed to these gaseous radionuclides in dwellings as well as outside the dwellings. They contribute maximum to the radiation dose received by the world population by inhalation of the air contaminated with these radionuclides. The concentration of these radionuclides varies from place to place, dwelling to dwelling depending on the ventilation/air supply available for dilution and dispersion, and the concentration uranium and thorium in the materials. In cold countries, the concentration of radon in dwelling is likely to be of concern, since the dwellings are closed most of the time, and recirculation of the indoor air facilitates build-up of radon and its daughter products, thus create potential inhalation hazard.