é
3
Table 2. Contribution of pasture to iodinc* 131 burden of cattle.
Millimicrocuries
.
of I" per gram
Animal number
of fissue
(fresh weight)
3
5
13
100
101
102
Barn-fed cattle
0.0092
0.012
0.012
0.013
Thyroid Doses
_ It is of interest to estimate the radia-
0.013
0.08
tion dose received by the thyroid glands
of the humanandcattle populations during the period of this study. The level of
iodine-131 in an individual at any given
time represents a balance between the
Avg: 0.012
Pasture-fed cattle
16
0.042
18
Ce
‘0,039
,
fall-out is deposited on the forage and
the time the dairy product is consumed.
This may be an important factor and
should be looked at more carefully under
situations where milk from grazing animals is consumed within a matter of
days after secretion.
intake, perhaps from several tests, and
Avg. 0.041
the biological and physical removalrates,
_This makes it difficult to calculate inSpat Ae
having thethyroid levels shown in Fig. 1
should have been about 0.2 millimicrocurie per 100 milliliters at the 1955 peak
and averaged about 0.02 for the whole’
period. These values are higher than
those found, probably because dairy cows
usually consume older feed than do the
cattle reported in this article. The data
in this article do not permit estimations
of the contribution of iodine-13! carried
in milk, although the low milk levels
found and the fact that the levels in
younger age groups werenotsignificantly
higher would indicate this route to have
been of minor importance. The contribution of milk is, of course, decreased by
the physical decay between the time the
Table 3. Examples of spread in iodine-131
levels in human and cattle thyroids. All
continental samples except from Nevadasouthern Utah area.
May
1955
Human
No. of samples
Percentage of samples
with
Less than 0.0008
mpc/g
Betweeen 0.0008 and
0.01 mpc/g
More than 0.01 muc/g
Cattle
No. of samples
* Percentage of samples
with
Less than 0.001 muc/g
Between 0.001 and
0.1 mpc/g
More than 0.1 myc/g
August
1956
101
60
29
62
60
11
35
3
20
°
.
22
10
0
40
86
50
14
.
finity dosages in the usual way (8). By
integration of the area under the curves
in Fig. | and estimation that 1 millimicrocurie of iodine-131 per gram of tissue
delivers about 0.012 rep per day, it is
calculated that the tetal dose from the
iodine-131 beta rays, delivered over the
23-month period to the thyroid gland,
was about 3 rep for cattle and 0.01 rep
for man. In considering thyroid dosages,
it is necessary to take into.account the
contribution of short-lived isotopes of
iodine. This situation has been analyzed
by Dunning (8), who shows that, up to
10 hours after detonation, the short-lived
radioiodines may contribute’ about 4
times the iodine-131 dose and by 2 days
about equally, but that after 10 days the
contribution of the short-lived activities
becomes negligible. It is not possible to
correct the over-all dose of iodine-131
for the contribution of the short-lived activities, but in any event the total dose
could not have been greater than 4 times
the average dose from iodine-131 (4x
0.01 rep) for man and was probably
muchless.
There seemsto be little question that
the levels of radioiodine introduced into
the biological cycles by weapons tests
during 1955 and 1956 are far below those
that are expected to produce any observable effects. This can best be demonstrated by comparison of levels found
with the official maximum permissible
values and with findings on the lowest
levels of radiation or of iodine-131 that
could possibly have produced detectable
changes. Such a comparison follows.
1) Average peak level observed in
man, 0.005 millimicrocurie per gram.
2) Maximum permissible level in man
for continuous exposure (9), 15 milli-
microcurie per gram.
3) Estimated peak level in milk, less
than 0.01 millimicrocurie per 100 milliliters,
4) Maximum permissible concentration in water for continuous exposure
(9), 3 millimicrocuries per 100 milliliters.
5) Estimated average dose to human
thyroid from radioiodine during the
period January 1955 to December 1956,
less than 0.04 rep.
6) External dose to neck area in infants and children that has been suggested as cause of later thyroid malignancy (/0), 200 to 725 roentgens.
7) Dose to thyroids of sheep on daily
intake of iodine-131 at which no damage
was observed (11), 936 roentgens (3
rocntgens per week for 6 years).
8) Dose to thyroid of sheep on daily
intake of iodine-131 at which minor
physiological change occurred (11),
3000 to 5000 roentgens (over 242 years).
References and Notes
1,
L. Van Middlesworth, Nucleonics 12, No. 9,
wo
a
*
R. L. Gunther and H. B. Jones, U.S. Atomic
56 (1954).
Energy Commission Document UCRL-2689
and addendum (1954).
3. M. R. White and E. L. Dobson, U.S. Atomic
Energy Commission Document UCRL-3355
(1956).
a
.
L. Van Middlesworth, Science 123, 982 (1956).
This work was performed at the Medical Di-
vision, Oak Ridge Institute of Nuclear Studies,
Qak Ridge, Tenn. under contract with the
U.S. Atomic Energy Commission.
6. Cattle thyroids and milk sampies were supplied through R. Miller, Omaha, Nebr., and
with the cooperation of the Army Veterinary
Corps, Office of the Surgeon General, and the
Department of Defense. Human thyroids were
received from Nicholas J. Chetta, Orleans Par-
ish, New Orleans, La.; John H. Childers,
University of Texas; T. H. Cochran, University of Utah; James R. Dawson, Jr., University of Minnesota; David Freiman, Beth Isract
Hospital, Boston, Mass.; Nathan B. Friedman,
Cedars of Lebanon Hospigal, Los Angeles,
Calif.; Marvin Kuschner, Bellevue Hospital
Center, New York, N.Y.; Kenneth P. McConnell, Veterans Administration Hospital,
Louisville, Ky.; Hans Popper, Cook County
Hospital, Chicago, Il.; Leopold Reiner, Beth
Israel Hospital, Boston, Mass.; Vinton D.
Sneeden, Emanuel Hospital, Portland, Ore.;
and Walker B. Sorrell, Tulane University. The
cooperation of these persons is gratefully acknowledged—it is emphasized that the succes:
of this program depended largely on the procurement and prompt handling of uncontaminated samples.
7. C. L. Comar and R. H. Wasserman, Progr.
in Nuclear Energy Ser. VI, Biol. Sei. 1, 153
(1956) .
8 G. M. Dunning, Nucleonics 14, No. 2, 38
(1956).
‘
9.
U.S. National Committee on Radiation Protection, “Maximum permissible amounts of
radioisotopes in the human body and maximum permissible concentrations in air and
water.” Natl. Bur. Standards (U.S.) Handbook 52 (Government Printing Office, Washington, D.C., 1953).
10. D. E. Clark, J. Am. Med. Assoc. 159, 1007
(1955).
\l. L. K. Bustad e¢ al., U.S. Atomic Energy Commission Document HW-38757 (1955).