272
RUSSELL
nite 3
ond Report? of the United Nations Scientific Committee on the Effects of
Atomic Radiation (2) contains the most comprehensive estimate which has
yet been made of the average dose experienced by the worid population.
Their findings are summarised in Table I, column1.
External radiation is received both from cosmic rays and from terrestrial
sources, which consist of both the decay products of the uranium and
thorium series and potassium 40 in the earth’s crust. It is of interest that
the dose from cosmic rays estimated in the U.N. Report (2) is appreciably
higher than that which was assumed until recently; this is because the sig-
nificance of the neutron component of cosmic rays has but lately become
apparent. Whereasall tissues are exposed to a relatively constant radiation
dose from external sources, that from ingested materials may vary widely
dependent on their distribution between organs. Thus, the major dose from
the uranium and thorium series (due mainly to radium and its daughter
products) is received by the bone.
The average dose from all natural sources to different tissues is esti-
mated as between 120 and 130 mrems per year.* Considerable variations in
exposure, however, occur between different localities on the earth’s surface.
The dose from cosmic rays, for example, rises both with increasing altitude
and, at the same altitude, with distance from the equator. The dose from
radioactive substances in the earth’s crust is subject to even greater variations. In granitic areas, exposure may be several times above the average
and appreciable differences can be caused by the choice of building materials (3). In exceptional areas of extremely high background, values of
several 1000 mrems per year are known, The internal dose from the radium
and thorium series varies also.
:
Man made sources—The sources of extra radiation which man has
created are, for present purposes, conveniently divided into those resulting
from the contamination of the environment with the radioactive substances
and those due to other causes. The latter group will be considered first;
diagnostic radiology is the predominant source. Estimates of the mean dose
to the populations in different countries where modern medical procedures
are employed range from 10 mremsper year in Norway to 58 in France (2).
Much smalier exposure comes from such sources as luminous watches, shoefitting equipment and television sets.
? This Report is an exhaustive source of information on all aspects of man’s exposure to radiation. It contains extensive annexes and bibliographies covering not
only medical questions but also natural radioactivity and the behaviour of radioactive
substances in soils, plants and animals. The existence of this document makes it unnecessary here to give a comprehensive coverage of the literature, except on topics
which have been selected for special consideration.
* The relative biological effective dose of radiation is usually measured in rems.
Doses in rems are calculated by multiplying the absorbed dose in rads (1 rad is 100
ergs absorbed per gm) by the RBE(relative biological effectiveness) for the type of
radiation in question. The RBE for beta and gammaradiation is 1, that for alpha and
neutron radiation is taken as 10 (2).
- *
:
-
gr ed eg
Ey
,
,
|
=
;