Assessment of long-term bronchiolar clearance of particles from measurements of lung retention and theoretical estimates of regional deposition

Twelve healthy nonsmokers inhaled monodisperse Teflon particles labelled wi th Cr-51 (half-life 27.8 days) with an aerodynamic diameter (d(ae)) of 6.1 mu m, 5 at a normal flow, 0.5 L/s, and 7 at an extremely slow flow, 0.05 L/ s. Lung retention after 24 hours was measured for about 6 months and could be well described by a 2-component exponential function. After the normal i nhalation, 14% of the particles retained after 24 hours cleared with a half -time of 3.7 days and 86% with a half-time of 217 days. After the slow inha lation, 35% of the particles retained after 24 hours cleared with a half-ti me of 3.6 days and 65% with a half-time of 170 days. Deposition was calcula ted using 3 different models including the recent Human Respiratory Tract M odel (HRTM), adopted by the International Commission on Radiological Protec tion (ICRP), and a model based on Monte Carlo particle transport, together with an asymmetric lung model. Generally, the 3 models agreed fairly well a nd predicted a considerably higher deposition in the bronchiolar region (ge nerations 9-15) at the slow flow than at the normal flow. Together, the exp erimental data and the predictions of the deposition models indicate that a bout 40% of the particles deposited in the conducting airways during the sl ow inhalation were retained after 24 hours. They also strongly indicate tha t the particles which cleared with a half-time of about 4 days mere mainly deposited in the bronchiolar region, and that about 25% of the particles de posited in the bronchiolar region cleared in this phase. The experimental d ata agreed guile mall with the HRTM Predictions made using its default para meter values for slow clearance in the bronchial tree.