481Cs DEPOSITION AND DIETARY LEVELS
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AGE, YEARS
Fig. 6—Concentration of #°"Cs in human rib bone in Chicago, Ill.
gamma-ray spectrometry. The same general distribution pattern as a
function of age was shown for the two samplings, namely, higher concentrations in the 0- to 5-year age group followed by a decrease to minimum values in the 10- to 20-year age group, after which the concentration gradually increased with age. The high concentrations in the
very young are thought to be due to the highly cartilagenous quality of
the bone involved, and the decrease in concentration with age up to 20
years follows the pattern of ossification of cartilage. The increase with
age, also seen by Yamagata and Yamagata,’® has not as yet been explained. Yamagata and Yamagata found rib bone to be somewhat higher
in "Cs than other bone, and the recent findings of Rivera! bear this
out. The same general pattern is seen in the 1962-1963 sampling, with
some increase noted in all but the 0- to 5-year age group. This may
have been due to the larger proportion of bone from 0 to 2 year olds in
the 1961 sampling than were present in the later collection. The average increase in the 1962—1963 sampling was approximately 50% compared with an increase in whole-body burden during 1963 by a factor of
3 to 4 over that present in 1961. If the '*’Cs in rib is actually incorpor-
ated in bone, its biological half-time should be considerably longer than
that for soft tissue, being on the order of years. Hence increase in bone
content due to higher dietary levels will proceed much more slowly.
Rivera!’ has estimated that less than 5% of the whole-body ‘Cs was
present in bone during 1961. In light of the marked increase in whole-
body "Cs and the 50% increase in bone content, the skeleton may be
presumed to contain currently 1% or less of the total. Only after many