Executive Summary : | Radon, a naturally occurring radionuclide, can be found in both natural and anthropogenic sources. It is primarily produced in soil, rocks, ore bodies in the earth's crust and building materials like bricks, cement, and tiles. Radon atoms in solid grains are unlikely to be released into the atmosphere due to their low diffusion coefficients. However, if they are located in the interstitial/pore space between grains, they may diffuse to the surface. Radian releases from a radium-bearing matrix to the atmosphere occur through two processes: Emanation and Transport. Emanation occurs when radon atoms escape from the grains due to recoil, while Transport causes the movement of the emanated radon atoms through the soil matrix to the ground surface. Exhalation occurs when radon atoms are transported to the ground surface and emitted into the atmosphere. After emanation, radon can be transported in porous media like soil and rock through diffusion or advective transport. Advection is not limited to air exchange between open spaces but can also occur in porous materials due to pressure differences in the pore air. Darcian flow-induced advective transport is generally neglected in radon exhalation studies in open atmospheres. Adective transport is considered the most important mechanism for radon/thoron entry into buildings from the soil, especially in regions with cold climates and saturated soil. |