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BULLETIN OF THE TORREY BOTANICAL CLUB
{Vor. 91
soils (= DF,) may varyfrom 0.8 to 1.0. In the food-chain transfer from plant
to cow to milk, the discrimination factor (DF.) has been estimated as 0.18.
The discrimination factor for the transfer from plants to bone (DF;) and
that for transfer from milk to bone (DF’,) have both been estimated as 0.25.
If 80% of the calcium in the diet of a human population (e.g., the population of the United States) is derived from milk and 20% is derived from
other sources, the overall discrimination factor in going from soil to bone
(ORgone/soi) = (0.8x DF, x DF,x DF, + (0.2xDF,xDF,) = (0.81.0
x 0.18 x 0.25) = (0.2 x 1.0 x 0.25) = 0.058 and indicates an equilibrium Sr-90/
Ca ratio equal to 5.8% of the Sr-90/Ca ratio in soil or plants.
If the average concentrations of available Sr-90 and Cain soils are known,
this model can be used to predict the specific Sr-90 activity of Ca deposited
in the skeleton during a given period of environmental contamination. Allowances for dietary differences can be made by using a general expression,
ORbonessour = (My x 0.25 + (Rex 0.25), in which M; and R; are the fractions
of dietary Ca derived from dairy products and from other sources, respectively.
The admitted defects of this model are that it makes no allowance for
direct foliar contamination and assumes that both Sr-90 and Ca are uniformly mixed in the plow zone of cultivated soils. Models of this sort are
useful for predicting equilibrium or steady-state concentrations of radioisotopes in ecosystems which have been contaminated by fallout but are
receiving no new additions from sources outside the system.
2. Proportionality Constants for Prediction of Sr-90 in Human Diet. The
Health and Safety Laboratory (HASL) of the U. S. Atomie Energy Commission, the U. S. Public Health Service and manyother agencies in the United
States and throughout the world have established monitoring programs to
measure the levels of Sr-90 in soils, in milk and other humanfoods, and in
fallout—especially the fallout deposited in rain and snow. It is generally
agreed that the amount of Sr-90 in or on plants, in the diet and bones of
domestic animals and in the diet and bones of human populations may vary
from place to place in relation to: (a) the total (or available) concentration of Sr-90 in soil and (b) the rate of Sr-90 deposition in world-wide fallout.
Knapp (1961) in the United States and Kulp et al. (1960) have studied
the data derived from monitoring programs, and they have proposed empirical formulas to account for the observed levels of Sr-90 in human diets
and human bones and to predict the future levels of dietary and assimilated
Sr-90. The formula proposed by Knappto account for the Sr-90 in U. S. milk
supplies is based on empirically derived proportionality constants and seems
to fit the available data.