Implications of airway retention for radiation doses from inhaled radionuclides

A slow phase of bronchial and bronchiolar clearance ('airway retention') has been included in the revised dosimetric model for the human respiratory tract recently adopted by the International Commission on Radiological Protection (ICRP), based primarily on the results of the bolus clearance experiments conducted by Stahlhofen et al. The treatment of airway retention in the new ICRP model is described. An analysis was carried out of the sensitivity of the equivalent lung dose, and the effective dose, to assumptions made in the new model about the extent of airway retention. It was shown that inclusion of airway retention of material deposited in the bronchial tree does have a significant effect on both end-points (of the order of 10-50%) for a wide range of radionuclides. In some cases, notably moderately soluble forms of long-lived alpha-emitters, its inclusion increases these doses by up to a factor of three, so that it makes the dominant contribution to the calculated dose. The model assumes that material in transit through the bronchial tree, having initially deposited in the alveolar region, is not subject to airway retention. Including retention of this material would typically increase doses by a further factor similar to that resulting from the airway retention of the bronchial deposit.