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.to
approximately 0.1 rad a year.