Investigations of Retention of Inhaled Particles in the Human Bronchial Tree

Two distinct phases of particle clearance from the human lungs are usually observed. It was generally assumed that the rapid phase, complete within about a day, represents clearance of the material deposited in the tracheo-bronchiolar (TB) airways, and the much slower phase alveolar deposition. This assumption was used as the basis for measurements of regional lung deposition. In a series of experiments, radiolabelled particles were administered to volunteers as a small 'bolus' at a preselected point in the breathing cycle, to deposit them at a chosen lung depth. When the bolus was administered near the end of the breath ('shallow'), in order to deposit the particles in the bronchial region, only about 50% of the material cleared rapidly. This finding was reproducible in different subjects, and with particles of different materials. Experiments in which the particles were administered with the subject horizontal, or with the particles inhaled rapidly to increase deposition in the large airways, gave similar results. Complementary measurements of bolus deposition as a function of particle size and breath-holding time did not support the explanation that a large fraction of the bolus was reaching the alveolar region. The results, with those of further bolus clearance experiments, using particles of different sizes and densities, suggest that there is a slow phase of TB clearance, and that the slowly cleared fraction of the TB deposit decreases with increasing particle geometric diameter. On the basis of these observations a particle size dependent slow phase of bronchial and bronchiolar clearance was included in the new ICRP dosimetric model for the human respiratory tract.