Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
contents to right colon contents.
c
The degree of precision of the values given is for computational purposes and does
not reflect the certainty with which they are known.
Modifying factors
(179) The default regional transit times given in the HATM are central estimates
based on collected data for a given sex, age group, and type of material (e.g., solids,
liquids, caloric liquids, or non-caloric liquids). As extensively illustrated in
Publication 100 (ICRP, 2006), transit of material through each of the major segments
of the tract shows considerable inter- and intra-subject variability even under normal
conditions. Extremely large deviations from the norm may result from constipation,
diarrhoea, unusual diet, pharmaceuticals, and a variety of diseases that affect the
nervous system or increase energy requirements, for example.
Sex specific values
(180) The HATM provides sex-specific parameter values for adults for dimensions
and transit times of contents through the regions. Transit times and dimensions of the
stomach and intestines are generally greater and lower respectively in females
compared to males. In adults, mean transit times for the stomach and colon are about
one-third greater in females than males. However, for simplicity, parameter values for
the reference adult male are used in this report series.
Material entering from the respiratory tract
(181) Mucus and associated materials cleared from the respiratory tract enter the
oesophagus via the oropharynx. For ingested food and liquids, the HATM specifies
two components of oesophageal transit, representing relatively fast transfer of 90% of
the swallowed material (mean transit time of 7 seconds for total diet) and relatively
slow transit of the residual 10% (40 seconds for total diet). It is assumed that the
slower oesophageal transit times apply to all material cleared from the respiratory
tract.
3.3.3 Absorption from the alimentary tract
(182) Radionuclides may enter the alimentary tract directly as a result of ingestion,
or indirectly after inhalation and mucociliary escalation of particles from the
respiratory tract to the oropharynx and oesophagus. The absorption of radionuclides to
blood is specified in the HATM as a fraction of the amount entering the alimentary
tract, with total absorption denoted as fA (ICRP, 2006). The model structure allows for
the use of data on absorption in any region, where information is available. In most
cases, no information will be available on the regional absorption of radionuclides
and the default assumption is that all absorption takes place in the small intestine, i.e.
fSI = fA. As a default, it is also assumed there is no recycling from the wall to the
contents of the alimentary tract.
(183) Some fA values recommended in this report are the same as the f1 values given
previously for use with the Publication 30 model, since there is not sufficient new
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