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In an effort motivated largely by the Chernobyl nuclear accident, the International Commission on Radiological Protection (ICRP) is developing age-specific biokinetic models and dose coefficients for environmentally important radionuclides. This paper describes the ICRP's age-specific biokinetic model for uranium. The model is constructed within a physiologically based framework originally developed for application to the alkaline earth elements but sufficiently general to apply to the larger class of bone-volume-seeking elements. Transfer rates for a reference adult are based mainly on: 1. measurements of uranium in blood and excreta of several human subjects who were intravenously injected with uranium; 2. postmortem measurements of uranium in tissues of some of those subjects; 3. postmortem measurements of uranium in tissues of occupationally and non-occupationally exposed subjects; 4. data on baboons, dogs, and smaller laboratory animals exposed to uranium for experimental purposes; and 5. consideration of the physiological processes thought to control retention and translocation of uranium in the body. Transfer rates for the adult are extended to children by application of a set of generic assumptions applied by the ICRP to calcium-like elements. These assumptions were derived mainly from observations of the age-specific biokinetics of the alkaline earth elements and lead in humans and laboratory animals but are consistent with available age-specific biokinetic data on uranium.

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Information on the biokinetics of the 15 elements of the lanthanide series, 57La to 71Lu, is too sparse to permit individual development of meaningful biokinetic models to describe the behaviour of each of the elements in humans. The lanthanides show a regular gradation in chemical properties across the series, and animal studies indicate that this is reflected in regular differences in their deposition in tissues such as the liver and skeleton. These regular differences in chemical and biological behaviour have been utilised to construct a generic lanthanide biokinetic model and to define element-specific parameters for each element in the series. This report describes the use of the available biokinetic data for humans and animals to derive the parameters for each of the elements.

Various factors have been offered to explain and/or predict the wide variation in the retention of Cs among humans. For example, the biological half-time of Cs has been expressed as an increasing function of body mass and also as an increasing function of age throughout life. Some early investigators attempted to describe accumulation of Cs in the body in terms of discrimination factors between Cs and the chemically similar element K, but such factors were found to oversimplify the relation between Cs and K and they were soon abandoned. In this paper it is shown that K may be a useful index, after all, for predicting the retention patterns of Cs in individuals. A predictive model for individuals is constructed, using observed relationships between the mass of total-body K (Kt) and the parameters in the standard two-exponential retention model for Cs.