RADIATION STANDARDS, INCLUDING FALLOUT
83
through food chains, and is deposited within the body in a small
gland—the thyroid. Because of its 8-day half-life, I*** injected into
the stratosphere disappears by radioactive decay almost entirely before it can be returned to earth. It is likewise true that the D**
reaching the soil will disappear by radioactive decay before it can be
taken up through the roots of growing plants and thereby be transmitted to the food of man.
Chart I showsthe route of I** through the food chain. There are
two primary pathways. The radioiodine is deposited from the atmosphere on the surface of vegetation which is grazed by dairy
animals, and the ingested radionuclide secreted into milk.
Manalso inhales I*** that is present in the air. Exposure of man
to I* could also occur by consumption of fresh fruits and vegetables;
however, such consumption is minor since it normally represents
very small surface area of plant comparedto that grazed by an animal,
and most often the surface contamination of fresh fruits and vege-
tables is removed by washing or skinning before consumption.
As supported by recent work of Eisenbud and coworkers, there seems
little question but that fresh milk is the main contributor of I’** to the
human diet. As indicated on the chart, for example, individuals in
New York City in October 1961, who had from 300 to 700 picocuries
(micromicrocuries) of I** in the thyroid are estimated to have received only about 20 picocuries by inhalation.
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If the need should everarise, there are several factors that simplify
the reduction of exposure to radioiodine: (1) only one item of the
diet, fresh milk, is an important source of contamination; (2) the
half-life is short enoughso that substitution of processed milk or dairy
products for fresh milk effectively reduces the level of contamination
in the diet; (3) measurementsare relatively simple and rapid because
of the radiation characteristics; (4) the time periods during which
I'*1 could be of importance can be predicted from knowledge of the
detonation of nuclear devices.
Strontium 90: It is generally accepted that the movement of
strontium through the food chainis to a large extent interrelated and
governed by the simultaneous movement of calcium.
Stable strontium is normally present. in the food chain and an understanding of the behavior of radioactive strontium added by man to
the food chain can most easily be gained by consideration of the behaviorof stable strontium and calcium.
As will be pointed out later, Sr® will not exactly parallel the behavior of stable strontium until steady state conditons leading to comparable physical distribution are attained in the future.
As time passes after the cessation of testing or even after constant
testing at about the same rate, there will be an approach to identical
behavior.
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Chart 2 shows some actual data primarily from extensive British
surveys giving typical daily intakes and body contents of calcium and
stable strontium expressed in milligrams.
I have put the numbers down here to give some feel for the actual
amounts mvolved.
These are now in the same units.
The sources of calcium are broken down into milk, plant foods,
and “other? which includes fish, eggs, meat, and mineral sources.
Values for the United States would be similar except that the “other”
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