Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
experimental and statistical problems in the data as well as the logical basis for
extrapolation of those particular data to humans. Relatively high confidence might be
placed in a model value based on animal data if fairly extensive interspecies
comparisons have been made and include observations on the species expected to be
most human-like; these comparisons suggest a strong basis for interspecies
extrapolation, either because the data are species-invariant or because the
physiological processes governing the biokinetics of the element in different species
have been reasonably well established; the model structure allows meaningful
extrapolation to man, usually on the basis of physiological processes; and such
processes have been well quantified in humans (i.e., the central value for humans has
been reasonably well established). A fairly wide uncertainty interval is indicated if
data are available only for species that frequently exhibit qualitative differences from
man (e.g., if data were available only for rats) or if no meaningful basis for
extrapolation to man has been established with regard to the quantity of interest.
Whatever the quality of the animal data, the uncertainty interval should reflect the fact
that some confidence in the predictive strength of the data is lost when the data are
extrapolated across species.
Uncertainty in inter-element extrapolation of biokinetic data
(385) Biokinetic models for elements often are constructed partly or wholly from
data for chemically similar elements, on the basis of empirical evidence that chemical
analogues often exhibit close physiological similarities. For example, the alkaline
earth elements, calcium, strontium, barium, and radium, exhibit many physiological
as well as chemical similarities (ICRP, 1993, 1995a), and the alkali metals rubidium
and caesium closely follow the movement of their chemical analogue, potassium.
(386) There are, however, counterexamples to the premise that chemical analogues
are also physiological analogues. For example, the alkali metals potassium and
sodium share close physical and chemical similarities but exhibit diametrically
opposite behaviours in the body, with potassium being primarily an intracellular
element and sodium being primarily an extracellular element.
(387) Moreover, chemically similar elements that behave in a qualitatively similar
fashion in the body may exhibit quite different kinetics. For example, caesium
appears to follow the behaviour of potassium in the body in a qualitative sense but is
distributed somewhat differently from potassium at early times after intake and
exhibits a substantially longer whole-body retention time.
(388) The level of confidence that can be placed in a model value based on human
data for a chemically similar element depends on the quality and completeness of the
data for the analogue, as well as the expected strength of the analogy for the given
situation. Whatever the quality of the data for the chemical analogue, the confidence
interval should reflect the fact that some confidence in the predictive strength of the
data is lost when the data are extrapolated across elements.
(389) The strength of the chemical analogy for a given element depends largely on
the extent to which the chemically similar elements have also been found to be
physiologically similar. That is, the analogy would be considered strong for a pair of
elements if a relatively large set of experimental data indicate that these elements
have essentially the same qualitative behaviour in the body and their quantitative
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