of leaves. Strontium-90 reaches the human body by
direct consumption of vegetation and of milk.from
animals that have fed on the vegetation.
Therelative importance of the various pathways
depends on many factors, for example, the composition of the soil (calcium level) and the nature
of plant cover. A heavy root mat will tend to trap
fall-out strontium and delay its reaching the soil
for dilution with soil calcium, while at the same
time permitting absorption into the plant from
the base of the stem. The agricultural management
of crops and livestock, which includes the plowing
depth, fertilizer practice, and type of feeding (barn
or pasture) employed, is another factor to be considered. Also of great importance are dietary habits
of the population and food technology. As anillus-
cium. In all steps of the food chain from vegetation to human bone, calcium is preferentially
utilized relative to strontium. Thus, it has beencal-
culated that at equilibrium the strontium-calcium
ratio in bones of young infants would be from 3 to
12% of that in vegetation, with values of 8 to 16%
in the bones of persons over 6 months of age.
Although milk is our primary source of calcium,
the discrimination against strontium relative to
calcium in passage from the feed of the cow to
its milk tends to reduce the importance of milk
as a source of strontium-90. While dairy products
furnish some 80% of our dietary calcium, they
may supply somewhat less than 40% of the total
strontium-90 intake when steady-state conditions
are established. Attention should also be given
Tasie 1.—Comparison of Radioisotopes in Environmental Contamination
collects in thyroid
Barium-Ho
13 days
like calcium, collects
in bone
=
pe — of — 2
slow
Samples for monitoring radicisotope
levels... cece eee ee eee tent eenee
Maxinum pertnissible radioisotope
coneentrations, zue per litert,......
Radioisotopes in milk, zuc per liter$..
Comparative radiation dose to
TUMTATIS, TAG... ce cece eens
Strontium-s89
Cesium-137
Strontium-90
il days
like ealeium, collects
in bone
28 years
like calcium, eolleets
in hone
slow
alow
co
~~,
/\
relatively fast
A
y
7
vy,
M-—— Man
vegetation, inilk,
amd thyroid
vegetiution and milk
3,000
121
200, (KK)
42
7,000
Ag
80
6.4
150,000
57
<0.04 to thyroid in
7935 and 1956
<10% of strontitm-.
approximately from
approximately 0.2 to
approximately 0.001
89 dose
soil, vegetation,
30 years
Uke potassium,
collects in muscle
\
sav
a
fast
o—
Removal rate from body.............
Main pathways in food echain*.......
5
Metabolle behavior.............0...+5
Todine-131
8 days
<A
NN
Physical half lHfe...........cecceercace
milk dairy products, bone, and
aquatic food
1 to 20% of stron-
tiim-90 dose
soil, yeretation,
nulk dairy prod-
ucts, bone, and
aqiatie food
(@.4 to skeleton
over 70-yr. period
from present
yeretution, milk,
dairy products,
meat, ind whole
body
per year from
present levels
tests$
*A4 = atmosphere V = vegetation M = milk S = soil (' = eattle MP = ineat products.
+ Reconunended maximum permissible concentrations for specific Tadioisotopes in drinking water as derived from recommendations of National
Committee on Radiation Proteetion and Meusurements. These concentrations and milk values are quoted by Public Health Service.
t Levels found in United States from July, 1957, to July, 1958.
§ This estimation inchides recent analytical data and concepts of nonuniform stratospheric inventories developed at May, 1959, congressional
subcommittee hearings.
tration, in the processing of frozen vegetables, washing them prior to freezing will remove some of the
surface contamination. If nuclear tests are stopped,
the soil reservoir will become increasingly important: therefore, the time pattern of contamination
has to be taken into account. Although food-chain
considerations are extremely complex, certain gen-
eralizations can be made. It is obvious that the
pathways that require considerable time in passage to man (for example, via the soil) are of no
significance for the short-lived radioisotopes,” iodine-131 and barium-140. It also appears that the
present contamination of diets originates mainly
from surface contamination rather than from the
soil reservoir.
The movement of strontium radioisotopes from
soil to man is interrelated and, to some extent,
governed by the simultaneous movement of cal-
to contaminated water supplies as a possible source
of strontium-90. In making estimations, the entire
strontium-90 and calcium intake of the population
must be considered.
The soil reservoir probably will not be an important factor for cesium-137 because of the fixation of this element in the soil and its consequent
unavailability to the plant. At the present time,
about 60% of the cesium-137 content of the average diet is derived from dairy products, 25% from
meat products, and the remainder from vegetables,
cereals, and fruits.
Radioisotope Levels
The present levels of these radioisotopes in the
biosphere (soil-plants-animal products—man) can
be determined with reasonable accuracy by radiochemical analysis. For purposesof illustration, table