ABSTRACT
This chapter discusses the various sources contributing significantly to the radiation exposure, their relative importance to the effective absorbed dose, and their geographical variability. The radiation exposure is most conveniently expressed in terms of effective dose equivalent, which is obtained after modifying the absorbed radiation dose to account for differences in the relative biological effectiveness of radiations of different quality. The high-energy radiation entering the earth's atmosphere from outer space is known as primary cosmic rays, most of which originate outside of the solar system. The ultimate sources of the basic primordial radionuclides are the earth's crust and its underlying mantle. Potassium is an essential element to man and is under close homeostatic control in the body. Neither thorium nor uranium are compatible with the crystal structure of major silicate minerals. Moreover, both elements are normally present in too small concentrations to have a strong tendency to form specific thorium or uranium minerals.
