1. Introduction
The earth's crust contains small amounts of the primordial
radionuclides
238
U and
232
Th which decay through a chain of
radioactive nuclides until they produce stable isotopes of lead.
Most of the decay products are isotopes of solid elements but two
are gases:
222
Rn (radon) from the decay of uranium and
220
Rn
(thoron) from thorium. They can migrate to the earth's surface by
transport of radon relative to the gas or liquid (molecular diffusion) and with the gas or liquid (convection or groundwaterflow)
(Cothern and Smith, 1987). The half-lives of radon (3.82 d) and
thoron (55 s) are different and this difference is important when
assessing their release from the ground and their distribution in
the open air above the ground as well as in the room air of
buildings. Radon and thoron enter the atmosphere mainly by
crossing the soil–air, building material–air or water–air interfaces.
In recent decades concern about public exposures due to natural
radiation sources has increased. The main contribution to natural
radiation comes from terrestrial sources contaminated with naturally