Mucociliary clearance of deposited particles in the conducting airways of the human lung was investigated using various symmetric and stochastically generated asymmetric models of the conducting tree. Mucous velocities in all airways of the conducting airways were calculated from the principle of mass balance for the mucus. These velocities were used to calculate particle residence time in all the airways of the conducting tree. Equations for the transport of particles by the mucous escalator were developed and solved numerically. The retained mass in the tracheobronchial region was calculated for a scenario of 1 h exposure followed by 2 days of post exposure. Initial deposition pattern of particles in the conducting airways was found to be crucial for the analysis of retention curves. Particles deposited in peripheral bronchiolar airways of asymmetric stochastic lungs cleared more slowly than those in more central airways. Consequently, the retention curves of the stochastic lungs with a greater number of bronchial generations exhibited longer tails than those of symmetric lungs. The results indicated that the asymmetric stochastic lung models may predict significant lung burdens even after 24 h. The extent of the difference in inter-subject variability in retained particle mass may partially explain the observation of investigators regarding greater than expected retained mass in the TB region after 24 h, without invoking any additional slow bronchial clearance mechanisms.