Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
behaviour either is similar or differs in a predictable fashion. In view of
counterexamples to the premise that chemically similar elements are necessarily
physiologically similar, the chemical analogy does not provide high confidence if the
elements in question have not been compared in animals or man.
(390) If a chemical analogue has been shown to be a good physiological analogue,
then application of human data on the chemical analogue (H2 data) may be preferable
to application of animal data on the element of interest (A1 data). For example, for
purposes of constructing or evaluating a biokinetic model for americium in humans,
use of quantitative human data on the physiological analogue curium seems preferable
to use of the best quantitative animal data on americium. Similar statements can be
made for radium and barium, rubidium and potassium, or other pairs of close
physiological analogues. On the other hand, if two chemically similar elements show
only broad physiological similarities, the animal analogy may be preferred to the
chemical analogy, particularly if element-specific data are available for a variety of
animal species (as is the case, for example, for uranium and calcium). In general,
lower confidence would be placed in animal data for a chemical analogue than in
animal data for the element of interest.
Uncertainty in central estimates stemming from variability in the population
(391) ‘Uncertainty’ refers here to lack of knowledge of a central value for a
population, and ‘variability’ refers to quantitative differences between different
members of a population. Although uncertainty and variability are distinct concepts,
the variability in biokinetic characteristics within a population is often an important
factor contributing to the uncertainty in a central estimate of a biokinetic quantity.
This is because such variability complicates the problem of identifying the central
tendency of these characteristics in the population due to the small number of
observations generally available and the fact that subjects usually are not randomly
selected.
(392) Variability in the biokinetics of radionuclides, pharmaceuticals, or chemicals
in human populations appears to result from many different physiological factors or
modulating host factors of an environmental nature, including age, sex, pregnancy,
lactation, exercise, disease, stress, smoking, and diet. Large inter-individual biokinetic
variations sometimes persist in the absence of appreciable environmental differences
and suggest that these variations may be genetically controlled. In real-world
situations, genetic and environmental factors may interact dynamically, producing
sizable variations in the behaviour of substances taken into the human body.
6.5.4 Uncertainties in Dosimetric Models
(393) Dosimetric models are used to estimate the mean absorbed dose resulting
from radiations emitted by nuclear transformations of radionuclides present in the
body. The absorbed dose is computed for target regions (organs, tissues, or regions of
tissues) considered to be radiosensitive. Radiation and tissue weighting factors are
applied to the mean absorbed dose to determine the equivalent and effective dose. The
weighting factors are assigned reference values and as such are not regarded as
uncertain quantities. Thus, the uncertainties associated with an estimated equivalent
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