Effects on dose coefficients for workers of recent changes in internal dosimetry

The International Commission on Radiological Protection (ICRP) has recently reviewed and updated, in ICRP Publications 56, 67 and 69, some of the biokinetic models of ICRP Publication 30. As well as this, ICRP has adopted a new model for the human respiratory tract, the details of which were issued as ICRP Publication 66. A compendium of dose coefficients (dose per unit intake values) for workers based on the new lung model, and some new recommendations on systemic models, has been issued in ICRP Publication 68. Doses for inhalation of 1 and 5 μm Activity Median Aerodynamic Diameter (AMAD) aerosols and for ingestion are given here for a selection of important nuclides, using the latest lung and systemic models. The results are compared with those of previous models. The new Absorption Types F, M, S (Fast, Moderate and Slow) are compared with the old inhalation Classes D, W, Y (for retention periods Days, Weeks, Years). It is found that generally the dominating factor for Types F and S is the lower overall deposition in the new model, leading to lower doses. Type M material is retained in the lung for much longer than Class W material of the ICRP Publication 30 model. This overrides the lower deposition and lung doses are higher in most cases. For 5 μm aerosols (the new default for occupational exposure) the activity reaching blood for Types F and M is similar to that for 1 μm (the previous default) Class D and W aerosols. However, the amount for Type S is only around 10% of that for Class Y. Doses to systemic tissues are reduced accordingly. A detailed investigation of systemic uptake is given in the Appendix. Further changes in systemic tissue doses are caused by some substantial changes to biokinetic models. Committed effective doses are generally lower although there are some exceptions. Differences in doses by ingestion are more simply explained by changes to systemic models and some other relatively minor changes.