1) Natural sources : Man is exposed to natural radiation
from time immemorial.(c) Internal radiation : Man is also subjected to internal
radiation, i.e., from radioactive matter stored in the body
These radioactive materials
quantities of uranium, thorium, and related substances, and
i les on tain is thou in about 2 maraon
year on the body as a whole, but may be as high as 70 or
80.Areas exist (e.g., Kerala in India) where there are
rock formations containing uranium, it can be as high as
2,000 mrad a year, (ii) Atmospheric radiation: The external
radiation dose from the radioactive gases radon and thoron
in the atmosphere is rather small : about 2 mrad per year.(b) Environmental : (i) Terrestrial
radiation: Radioactive elements such as thorium, uranium,
radium and an isotope of potassium (K40) are present in
man's environment, e.g., soil, rocks, buildings.Natural background radiation arises
from three sources : (a) Cosmic rays : The cosmic rays which
originate in outer space are weakened as they pass through
the atmosphere.It has been calculated that a
commercial jet pilot receives about 300 mrad per year from
cosmic radiation (1).to
approximately 0.1 rad a year.