septième réunion technique seventh technical meeting ... - IUCN

accident we are concerned, therefore, with the effect of fission
products which have directly lodged on leaves. The proportion of
the deposited material which is thus retained on vegetation may vary
greatly, dependant both on the nature of the plants and the particle
size of the deposition; particles in excess of 40 microns in diameter
are, in general, little retained. Twenty-five per cent of a finely
divided deposition may, however, lodge on leaves of pasture of the
types common in Western Europe. This contaminated vegetation
may either be consumed by man or by cattle. The dairy cow occupies
a key place in the food chain for several reasons. When cattle are
allowed to graze freely they are able to ingest material deposited
on wide areas of land; under the conditions with which we are
familiar in Great Britain they can consume daily a quantity of fission
products equal to that deposited on 30 or 40 square metres. Both
iodine and strontium are relatively efficiently transferred to milk;
in the order of 5 % of the iodine in the diet of the cow and rather
less than 1 % of the strontium are likely to find their way into milk.
The contamination of milk could be a particularly serious matter,
because, especially in Western countries, it is the major diet of infants
who, because of their youth, are particularly susceptible to radiological
injury. The significance of the lactating cow in the food
chain is indicated by the fact that pastures could be contaminated to
an extent which would make milk hazardous to children by the standard
of the International Commission for Radiological Protection even
though the concentration of iodine 131 in the atmosphere was very
much less than that considered dangerous for breathing. Considerably
greater deposition would of course be necessary to give rise
to a comparable hazard from external radiation.
The areas of land from which man draws his vegetable foods are
trivial in relation to those which the cow grazes. Furthermore, man
discards a considerable amount of the other tissues of his vegetable
foods and neither iodine nor strontium can move freely from one plant
leaf to another. Thus immediately after the release of fission
products the levels of iodine 131 and strontium 89 and 90 in milk
are likely to provide the key to the ingestion hazard of populations
who derive a significant proportion of their diet from dairy produce.
The monitoring of milk will therefore provide a means for assessing
the immediate exposure.
In considering the design of control measures under these circumstances
it is important to have knowledge of the speed at which
radioactive iodine and strontium are transferred to milk. If cattle
ingest iodine 131 on a single occasion the highest level in milk will
occur after about 18 hours; for strontium 89 the corresponding time
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