The “permissible” radiation burden
The adjective doesn’t mean what many people think, and no one
knows precisely what it does mean; but here’s how it’s worked out
increases from man’s activities in the atomic age,
obviously must attempt to make such estimates as we
to find out where the risk gets serious, where intoler:
[Pree are many ways of measuring radiation. The
measure which weshall use is a measure of the energy
absorbed from radiation in a given amountoftissue:
the unit is called the rad (100 ergs absorbed per gram of
tissue).* This is not much energy; in terms of heat, 500
rads would serve to raise the temperature of the tissue only
1/500 of a degree Fahrenheit—imperceptible. But that same
500 rads delivered to a human being by penetrating radia-
THE TOLERANCE DOSE
short time has just about an even chance of surviving a
repair reduce to negligible proportions the chance of
ble amplification of cell damage. It is claimed that s
ation received by people in a lifetime (about 70 years) under ordinary conditions. The dose varies from place to
mal and even human exposures, although not at the
long-continued doses in which we are now intereste:
It is fairly well agreed, however, that genetic dai
tion, in a kind of three-dimensional sunburn, is deadly; a
man whoreceives that much over the whole body within a
month. The normalexternaldoseis, of course, very muchless.
It is interesting to tabulate some external doses of radi-
place. The high mountains have a thin layer of air above
them, and the cosmic rays from space, accordingly, are in
abundance there. Some places have sub-soil rather free
from radioactive elements, and the earth’s contribution is
small there. Here is a small table of typical results (taken
from an AEC source):
EXTERNAL RADIATION DOSES
rads per 70 years
AVERAGE OVER U.S.A.
about 7 (out-of-doors)
Other common sources of external radiation are: from
TV tubes and luminous watch dials, less than 1/5 rad per
70 years; from diagnostic X rays (in the U.S., at least—
see
CONSUMER REPORTS,
September
1958),
an
amount
In Kerala, at the southern tip of India, about 100,000
people live in fishing villages strung out along a hundred
miles of a geological curiosity, an ocean beach whose
black sands are radioactive; the people who live there
have always lived in a radiation field about ten times the
U.S. average. If adequate medical diagnosis andvital statistics existed for these people, their very long exposure might
furnish extensive information on the long-term effects of
greater-than-average radiation on human beings. Unfortunately,-we have no such statistics for them or for any
ather highly-exposed population, and no way of quickly
acquiring such data. Nevertheless, as radiation gradually
*A rad is roughly equivalent to a roentgen, which is an older, common unit for doses of X and gammarays.
me
k4a.ntl
lara
thing like a threshold, in fact, has been observed in
which in humans leads to embryonic deaths, still-t
and congenital defects, has no threshold. Every inc
in the dose which affects the genes of some person is
to appearstatistically in his progeny. Most of this da
is believed to be delayed in expressing itself; some r
generation maybethefirst to suffer from it. Such g
damageraises quite new problems of medicine, to say
to humans is not known with precision tends to
discussions of the problem.
In the words of the authoritative International Co
5.3
6.7
7.4
12
generally not exceeding 10 rads per 70 years.
terms of a “threshold” dose. Cross the threshold, and
ger begins; stay below, and the processes of recovery
ing of morals. And the fact that the extent of such d:
Place
TOLEDO, OHIO
PITTSBURGH, PA.
LITTLE ROCK, ARK.
COLORADO SPRINGS, COLO.
The view that there exists some degree of exposure b
which there is no appreciable harm often is expresse
sion on Radiological Protection (ICRP): “Since no
tion level higher than the natural background can
garded as absolutely ‘safe,’ the problem is to che
practical level that, in the light of present knowled:
volves a negligible risk.” The practice has been to ac
late information bearing on this problem by sur:
considerable numbers of individuals who work with r
or X rays, or in the atomic energy industry. On th
of such information, “permissible” levels (or “tole
doses}
for
exposure
to
external radiation
have
established.
But the history of this concept is not reassurir
cause today’s “permissible level” is not yesterday
level for such workers has been modified steadily, as
below:
1928-1936
1936-1947
1948-1957
1957-
100
35
15
5
rads
rads
rads
rads
per
per
per
per
year
year
year
year
The last figure correspands, ({dria $0-}ear working
some 40 times fhe! natutal U.S. background radia
about seven rads in the course of a lifetime.