~2-
.
skeletons of all but very young children were formed prior to the
introduction of Sr-90 to the soil.
Moreover, bone being formed at
the present time utilizes calcium which left the soil in months
gone by.
The fact that cattle may be fed on hay many months old
and the hold-up of human foods in the comercial distribution
system are but two of many factors which would lead one to expect
the human Sr-90 burden to lag in time behind the potential value
which might ultimately be expected from a given soil concentration.
The human skeleton cannot be expected to respond quickly to the
gradual accretion of Sr-90 by soil.
Equilibrium can be expected
to be achieved over a period of years but not over a period of
months.
In the United States, as in a number of other parts of the
world where the population derives much of its calcium from
dairy products, analyses of milk for Sr-90 provide a convenient
method of estimating the levels of himan absorption which may be
expected in the future.
During periods of actual fallout the concentration of Sr-90
in milk originates from two sources:
the Sr~-90 level may have been
metabolized from the soil by normal root uptake or it may have
short circuited the soil by having been deposited directly on the
leaf with which it is ingested by tne cow.
The presence of the
latter fraction is dependent on current fallout.
If all
detonations ceased, the fraction due to direct deposition on the
leaf surfaces would diminish with the reduction in the rate of
fallout.
With the cessation of fallout this fraction would be