-~19-
thus be estimated to be 45 millicuries per square mile, about
1.8 times the level in mid-1956.
To define the potential risk from a given distribution of
Sr-90 on the surface of the earth requires that the distribution
be quantitatively related to the skeletal burden of Sr-90 of
a human population in dietary equilibrium with the soil from
which its nourishment is derived.
This equilibrium is
already established for a variety of trace elements normally
present in the earth's crust.
Some of these, like potassium
and radium, are radioactive, and this is reflected by the
presence of these substances in the human body,
For
example, the upper foot of soil in the United States
contains, on the average, about 1000 millicuries of radiwm
per square mile.
The averege adult skeleton in this
country contains about 10-4 microcuries of radium, which
is derived from assimilation of this trace element fran
foods end water.
Thus, the value of 1074 microcuries of
radium represents the amoimt deposited in the skeletons
of the populations whose mineral metabolism is in
equilibrium with the soil minerals.
The freshly deposited Sr-90 takes a relatively long
tine to complete the biological route to bone.
At the present
time the
EAR
Mt