Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
kinetics'). Similarly, element-specific bound state parameter values would be expected
to apply to decay products formed in the respiratory tract. However, analysis carried
out by the Task Group showed that application of independent kinetics rather than
shared kinetics within the respiratory tract, to decay products of Type F radionuclides,
would make little difference to respiratory tract tissue dose coefficients (up to a factor
of two, but in most cases much less), and less difference to effective dose coefficients.
For Type F materials absorption to blood is rapid and doses from deposition in
systemic tissues will often make greater contributions to effective dose than doses to
respiratory tract tissues. The additional complexity involved in application of
independent kinetics was therefore considered to be unjustified. Furthermore, in many
practical exposure situations, an intake of a parent nuclide will often be accompanied
by simultaneous intakes of its decay products. Their activities (which, being treated as
separate intakes will be given absorption kinetics appropriate to the element) will
often be considerably greater than the activities of decay products formed within the
respiratory tract, because very little decay of the parent takes place before a Type F
material is absorbed into blood.
(167) Thus, in this series of documents, radioactive decay products formed within
the respiratory tract (with the exception of noble gases) are assumed by default to
follow the absorption behaviour of the parent nuclide, and are given the same
dissolution and uptake parameter values as the parent (shared kinetics). Following
absorption to blood, they are assumed to behave according to the systemic model
applied to the element as a daughter of the parent radionuclide.
(168) Nevertheless, where experimental results are available which allow direct
comparison between the absorption behaviour of a parent radionuclide, and that of its
radioactive decay products, they are summarised in the inhalation section of the
parent element (e.g. uranium, thorium). Such information may be of use to those
carrying out individual monitoring, especially if intakes of a parent are being assessed
by means of measurements on one or more of its decay products. The behaviour of
thorium and its decay products can be of particular importance in this context,
because there is generally significant long-term retention of thorium in the lungs
following its deposition in soluble form, whereas soluble forms of important decay
products, notably radium and lead, are absorbed relatively readily.
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