For human food chains, the transfer of radionuclides to milk and meat has previously been commonly quantified using the transfer coefficient defined as the equilibrium ratio between the radionuclide activity concentration in milk (Fm; dl-1 or dkg-1) or meat (Ff dkg-1) and the daily dietary radionuclide intake. Transfer coefficients for smaller animals are higher than those for larger animals, and those for adults are lower than those for (smaller) young livestock. Beresford et al.32 suggested that much of this difference observed in transfer coefficients arises because they incorporate dry matter intake which increases with animal size, and suggested that the concentration ratio between the activity concentration in an animal product and diet may be a less variable and more generic parameter (later substantiated in the IAEA handbook28). Mean concentration ratios reported in IAEA28 for milk are highest for Cs (0.15) and for essential elements including I (0.46).68,69 The transfer of caesium and/or iodine isotopes to milk has required mitigation after the Windscale, Chernobyl and Fukushima accidents.

For wildlife assessment, most approaches use concentration ratios for at least some organisms. The CR is based on the whole organism activity concentration for terrestrial animals most usually compared to the soil activity concentration, see Equation (2).

The ‘‘whole organism’’ generally excludes the outer parts such as the skin and feathers, and the gut which can contain ingested material which is more highly contaminated than other body tissues.

Ratios approaches assume equilibrium, but there can be considerable tem­poral variation in an animal’s intake of radionuclides and hence tissue con­centrations may be constantly changing. Equilibrium will often not have been reached within an animal’s lifetime, especially for radionuclides with long physical and biological half-lives in tissues (e. g. plutonium). Dynamic models describing the behaviour of radionuclides within animal tissues have been developed for human food chains which can be used to predict radionuclide activity concentrations in different tissues following continuous, single or varying intakes.33 36

Differences in the quantification of transfer in the currently available assess­ment tools have resulted in large variation in predicted whole organism activity concentrations and resultant internal doses.37 40 In response, CRwo values have been collated for terrestrial, freshwater, marine and estuarine ecosystems in an online database and the data reported in broad wildlife groups by the IAEA in a Technical Report Series Handbook currently in preparation.41 Since much of the reported data for activity concentration in organisms are for edible fractions used in the human food chain, the handbook also provides tables to enable the conversion of data for edible fractions to whole organism values.42

In the IAEA handbook, all the CR values are based on reported data. As an example of the CRwo data available, the values for the selected radionuclides for some terrestrial wildlife groups are shown in Figure 1.

Given the large number of potential radionuclide-organism combinations which may require consideration within an environmental assessment many CRwo values cannot be derived from the literature. Various methods have been proposed to extrapolate from the available data to provide values for missing combinations, such as described by Beresford et al.43 and Higley.44