~10At the present time monthly samples at four stratospheric altitudes
are being teken at Minneapolis, Houston, Panama Canal Zone, end at
one station in the southern hemisphere.
This pilot program is
designed only to determine the degree of wmiformity and thus the
extent of sampling required for complete evaluation of the
atmosphere.
Biological Distribution
The chanical similarity of strontium and calcium makes it
desirable to use the Sr-90:Ca ratio in tracing Sr-90 from soils to
man.
A useful unit of Sr-90 contemination, first proposed by Libby,
is the .micromicrocurie . of Sr-90 per gram of calciim (ppe/gnCa).
For convenience this mit is frequently referred to as the
Sunshine Unit.
The biological distribution of Sr-90 is being studied in its
entirety and these investigations have produced a wide variety of
data.
For this discussion, which is concerned primarily with an
evaluation of the potential mmman hazard from Sr-90, the presentation will be simplified greatly by reviewing only the principal
sources of human calcium.
The presently observed and predicted
future levels of Sr-90 in these foods offer a convenient meens of
estimating the amomts of Sr-90 that the human skeletons of the
future are expected to contain.
In the United States, end mmy western coumtries, the mein
calcium source is milk, with leafy vegetables supplying the
majority of the balance.
In some esreas, such es southeastern
United States, this ratio may be reversed.
The sources of
calcium in countries of the world is under study by
the Food and