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of calcium in man’s diet would in general also be the. main sources of
strontium 90, water being relatively unimportant; thus, for example, in
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European countries where milk is the main dietary source of calcium, it
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is the main source of strontium 90 also (2). The importance of studying
the soil-plant-animal food chain was thus recognised. The amount of attention which has been given to strontium 90 from this viewpoint cannot, however, be explained solely in terms of the magnitude of the radiation dose
it delivers, relative to that from other sources. The food chains whereby it
enters man’s diet merit greater study than those for many other nuclides
because they are more complex and variable. There are two main reasons
for this. Firstly, strontium is considerably more readily absorbed from the
soil so that its effects can be assessed adequately only on the basis of its
behaviour over very long periods. Secondly, like calcium it moves only
basipetally throughout plants (9, 10, 11); thus strontium 90 which is absorbed into an individual leaf or other above ground tissue is not appreciably
transferred to other organs. Accordingly, the levels of strontium 90 in
tissues which man consumes cannot be inferred from the total quantity
which the plant absorbs; very different situations may occur, depending on
whether entry occurs into roots or through the aerial tissues. This question
is of much less practical importance with nuclides such as caesium 13/7,
which are freely redistributed.
Relationships between strontium and calciwm—The currently accepted
terminology for defining relationships between strontium and calcium was
originated by Comar et al. (12) who introduced the term “Observed Ratio”
of strontium to calcium (OR) as defined by the equation:
Ratio: Sr/Ca in sample
OR (sample/precursor) = Ratio: Sr/Ca in precursor
In practice the transfer of the two ions through any biological process which
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can be studied experimentally, i.e., from diet to bone, or from the soil to the
ear of a cereal plant, involves a number of successive steps; the term “Discrimination Factor” was introduced to define the relative behaviour of the
two ions in each such step, the “OR” of the entire process being the product
of the relevant discrimination factors.
Although widely and profitably used, difficulties have sometimes arisen
in the use of this terminology. One cause has been disregard of the precise
1
definition of the OR. Comar ct al. (12) laid emphasis on the fact that the
4
available for entry into the biological system was known, as well as the ratio
within it. It is dificult to make precise estimates of the relative availability
OR could be validly estimated only if the ratio in which the two ions were
of ions in the soil and misleading conclusions as to the magnitude of the
OR “plant/external medium” have on occasions been reached through the
assumption that the ratio of added tracer strontium to “exchangeable”
calcium in the soil validly reflects the ratio in which the two ions are
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