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The whole-body dose from external radiation in 1968 was essentinily due
to natural background radiation.
’
An estimate of this dose is given in the
table below.
The variability with geographic Location should be within a
factor of 2.
The higher doses occur in mountain areas where man is subjected
to both higher levels of cosmic ridiation and to higher levels of terrestrial
radiation because of the rocky nature of the environment.
vee
Dose Rates of External Irradiation from Natural Sources
Source
Whole Body Dose Rate
Cosmic Rays
Ionizing Component
28 millirads/yr
Neutrons
0.7
Terrestrial Radiation
(including air)
Total
50
79
.
=
The whole-body dose rates from fallout in the northern hemisphere ranged
from 1 to 2 mrad per year in the period 1965-1967.
Measurements in the United
States in 1968 yielded estimates of one-half to one mrad per year.
The highest dose rates to any part of the body from natural sources
come from inhalation of the short-lived daughter products of radon.
Current
estimates give local dose rates of several hundred millirads per year to the
bronchi, with other portions of the lung receiving smaller doses by factors
of 10 (vronchioles) to 100 (alveoli).
No other natural or artificial radio-
nucl$e produces any significant exposure to lung tissue.
It should be
noted thet the whole-body dose from inhalation is negligible, since the
weight of irradiated tissue is very small.
There are no continuing measurements of exposure of the gastrointestinal
tract by material passing through.
‘An indication of the magnitude of the
dose can be obtained from the following quotations from the 1962 UNSCEY : report.