Annex E

(E1) In the HRTM for radiological protection (ICRP, 1994a,b), the oral cavity was included with the other airways of the head and neck in a composite region, the ‘extrathoracic airways’ (ET airways). The oral cavity was not considered in the Publication 30 model of the gastrointestinal tract (ICRP, 1979), with which the HRTM has been used to date. However, since the oral cavity is identified in the HATM, consideration is given here to consistency between the two models. As described below, separating the oral cavity from the ET airways has no effect on the behaviour of inhaled materials predicted by the HRTM, nor on the resulting calculated doses. It is therefore proposed that when used with the HATM, the oral cavity should not be included in the ET airways of the HRTM. (E2) In the HRTM, the respiratory tract is divided into two main regions: the ET airways and the lungs. Each of these regions is further subdivided on the basis of considerations of anatomy, physiology, radiobiology, deposition, and clearance. The ET airways are subdivided into: ET1, the anterior nasal passage; ET2, which consists of the posterior nasal passage, oral cavity, nasopharynx, oropharynx, and larynx; and LNET, the associated lymphatic tissue (Fig. E.1). (E3) The HRTM assumes that deposition in the ET airways of inhaled particles that enter the respiratory tract through the mouth only takes place in the larynx (ICRP, 1994a,b, Annex D, Paragraphs D63, D69, and D83). Similarly, deposition in the ET airways of inhaled particles that enter through the nose is assumed to take place in the nasal passages (ET1 and the posterior nasal passage) and in the larynx. (E4) In performing calculations with the HRTM of the deposition of inhaled activity in the thoracic regions, account has been taken of the volume of the oral cavity because of its effect on the fraction of inhaled air that reaches the lungs, and hence the fraction of inhaled activity available for deposition (ICRP, 1994a,b, Paragraphs 163–168). The volume of the oral cavity is included in that of the ET airways [termed the ‘extrathoracic dead space’, VD(ET), in the HRTM]. As VD(ET) has only a second-order effect on regional deposition, the same value is already assumed for mouth breathing as for nose breathing. Values of VD(ET) for each reference age group are given in Table 15 of Publication 66 (ICRP, 1994a,b). The approach used to scale for body mass and hence age is described in Publication 66 (ICRP, 1994a,b, Paragraph 162). (E5) Regardless of the initial site of deposition within it, activity deposited in ET2 is assigned to two compartments: 0.05% is retained in its wall (ETseq), from which it clears slowly to LNET; and the rest is retained in compartment ET 0 2 which clears rapidly to the alimentary tract. With the Publication 30 gastrointestinal tract model, material is transferred directly from ET2 to the stomach compartment, and with the HATM, material is transferred to the slow oesophagus compartment. There is simultaneous absorption into body fluids of material from all the compartments, except ET1, at a rate normally determined by the absorption type of the inhaled material.