Radionuclides present in the environment can enter the body through different routes (i.e. respiratory, digestive, and cutaneous). Depending on the nature of the element, its chemical characteristics (speciation), the contamination conditions (for instance single or repeated), and its path of entry, the element’s behavior in the human body and the biological consequences of internal contamination will be different. It is therefore useful to know its progress through the body, including: its transport from its entry to the sites of deposition (i.e. organs and tissues), its retention time, and finally its routes of excretion.
In the case of radionuclides, the International Commission on Radiological Protection (ICRP) has developed, in its recommendations, biokinetic models for the major elements and their isotopes.
After an internal contamination, the fate of these elements in the body can be evaluated using biokinetic models.
A radionuclide having entered the body can be:
- distributed homogeneously throughout the body;
- or it can be concentrated in one or more organs, which would then be referred to as retention or target organs.
Two mechanisms come into play to remove a radionuclide from the body: its radioactive decay, based on the half-life of each isotope; and its biological elimination, based on the toxicokinetics of the chemical compound.