cause this serves as the basis for estimating the number of radiation-induced
chromosome mutations in the population
as a whole. Byreferring to the tables, the
average dose may be taken to be in the
order of 10 millirads for the 4-year period
covered bythis report. This is small when
compared with the gammaradiation re-
ceived from natural sources by populations throughout the world. Libby (6)
estimates that the average external dose
from natural sources, both terrestrial and
cosmic, is of the order of 75 millirads per
year, or 300 millirads in four years. On
this basis, the gammadose delivered from
fallout is about 3 percent of the average
gamma dose from natural sources. Thus,
even the maximum theoretical dose from
fallout to date is a small fraction added
to the gammadose received from natural
radioactivity, and this slight incrementis
considerably smaller in magnitude than
tium-90 in debris which falls out relatively close to the site of detonation and
a corresponding enrichment from debris
Rural populations are less shielded by
buildings and do not have the advantage
of large paved areas to encourage runoff
during storms. Nevertheless, the true situ-
which falls out at greater distances. This
may explain the low ratios of measured-
ation is never the infinite smooth plane
on whichthese calculations are based, and
are relatively close to the Nevada site.
and other factors.
enriched in strontium-90 by as much as a
Significance of Findings
higher than the predicted values. Based
In interpreting the significance of these
data, one needs to consider only the esti-
factor of 2, which explains why the measured values at the other stations were
on this soil study, one might be justified
in doubling the strontium-90 values given,
but this factor was not used in preparing
the tables.
The estimates of gammadose, like the
estimates of strontium-90 were derived
by calculations from estimates of the fall-
out of mixed fission products. It should
ties in the terrain, the plowing of fields,
00
L
of
T
Similarly, fallout analyzed at great distances from detonations is known to be
some reduction is afforded by irregulari-
oO
to-predicted values for Albuquerque, Salt
Lake City, and Grand Junction, which
e7
mates of gamma dose anddistribution of
strontium-90. The total accumulations of
mixed fission products are important only
insofar as they serve as the basisforesti-
mating the gammadose delivered and the
amount of strontium-90 present.
Geneticists are concerned with the
average gamma dose to populations be-
°4
0/4
a
tium-90 in any given particle of dust is
variable—there is a depletion of stron-
e/
oO
fireball. The relative abundanceof stron-
0/6
Ze
ri
is formed relatively late in the life of the
24
™~
of 33 seconds, some of the strontium-90
cal properties. In the upper { foot of the
earth’s crust, radium is present in amounts
approximating 1000 millicuries per
square mile. This radium,like other trace
elements, is absorbedintoall living things.
Adult North Americans contain about
PREDICTED~Sp an?
OS SS
os wT &
T
T
TT
Hunter and Ballou curvesof relative iso-
topic abundance in conjunction with the
daily measurement of mixedfission products. Because strontium-90 is derived following fission from its precursor krypton-90, an inert gas which has a half-life
be noted that the assumptions underlying
the calculation of gamma dose tend to |
produce values which are much higher
than the doses to which populations are
actually exposed. The reported values
do not allow for weathering or shielding.
The effects of weathering and shielding
cannot be treated quantitatively. For
populationsin cities, the true dose would
be very much reduced by the fact that
fallout to the surface is soon washed into
gutters and storm sewers. For these reasons, it is likely that the actual dose to
urban populations does not exceed 10
percent of the values reported here.
n
of measured to predicted strontium-90.
These findings are consistent with present knowledge of the way strontium-90 is
formed and distributed. The estimates of
strontium-90 were.obtained by using the
the tables can be understood in relationship to the occurrence of natural radium,
with which strontium shares many chemi-
AH
Fig. 1. Cumulative deposition of mixed fission products in millicuries per square mile at
the stations shown. These data are for fallout from October 1952 to September {955 and
are extrapolated to 1 January 1956.
the normal variations in natural dose
which occur from place to place throughout the world.
The significance of the deposition of
strontium-90 in the quantities shown in
4
6
8
#
Zz
MEASURED Sr? "me?
4A
&
Fig. 2. Plot of measured versus predicted
strontium-90 for soils from 17 locations
in the United States: 1, Albuquerque; 2,
Atlanta; 3, Binghamton; 4, Boise; 5, Des
Moines; 6, Detroit; 7, Grand Junction;
8, Jacksonville ; 9, Los Angeles; 10, Memphis; 11, New Orleans; 12, New York:
13, Philadelphia; /¢4, Rapid City; /5,
Rochester; 16, Salt Lake City; /7, Seattle.