Knowledge of the behaviour of radioactive materials within the human body is essential for the assessment of intake or committed effective dose from measurements of activity in the body or in excreta. This Chapter gives a general description of the routes of intake of radionuclides into the body, and subsequent transfers within and out of the body.  It also gives an overview of the current ICRP biokinetic models used to calculate body or organ content and daily urinary or faecal excretion at specified times after intake. See the original reports of ICRP [28-33] for details.

Figure 8 summarises the routes of intake, internal transfers, and excretion. The respiratory tract, the gastrointestinal (GI) tract, the intact skin, and wounds are the princi­pal routes of entry to the body. A proportion of the activity is absorbed into blood and hence body fluids. Activity reaching body fluids (transfer compartment) in this way is known as systemic material. The activity then undergoes various transfers which determine its distri­bution within the body and its route and rate of elimination. The distribution of systemic activity in the body can be diffuse and relatively homogeneous, e.g. with tritiated water, or localised in certain organs or tissues, e.g. with iodine (thyroid), alkaline earth metals (bone), plutonium (bone and liver).

Removal of deposited material from the body occurs principally by urinary and fae­cal excretion. Urinary excretion is the removal in urine of material from the plasma and extracellular fluid. Faecal excretion has two components: systemic faecal excretion which represents removal of systemic material via the GI tract; and direct faecal excretion of the material passing unabsorbed through the GI tract.

The models for the major routes of intake (inhalation and ingestion) are described in the following Sections. For some radionuclides, it is also necessary to consider direct uptake from contamination on the skin. There is no general model of entry of radionuclides through the skin because of the large variability of situations which may occur. Many factors must be taken into account: the chemical form of the compound, the location and the surface of the contaminated area as well as the physiological state of the skin. Intact skin is a good barrier against entry of a substance into the body. Generally, radionuclides do not cross the intact skin to any sig­nificant extent. However, a few elements may be transferred rapidly. The most important is tritiated water and this is the only case considered specifically by ICRP [28, 33]. However, absorption through skin is not included in the derivation of the dose coefficient for tritiated water (ICRP, [33]).  Iodine may also be taken up through skin, but to a lesser extent.