Theoretical simulation of diesel exhaust particle clearance from the human respiratory tract

Robert Sturm


Background: Diesel exhaust particles (DEP) deposited in the human respiratory tract are commonly seized by an innate defense system including a multitude of clearance mechanisms. Whilst in the larger tracheobronchial airways fast mucociliary particle transport predominates the clearance process, in the small non-ciliated airway tubes and alveoli much slower mechanisms of particle removal occur. The contribution includes a detailed theoretical description of the clearance routes being most significant in association with DEP.
Methods: DEP clearance was simulated by using (I) a probabilistic model of all known particle transport paths in the bronchial and alveolar structures and (II) a stochastic architecture of the human lung. Preceding deposition scenarios were computed after assumption of sitting breathing conditions (tidal volume: 750 cm³; breath-cycle time: 5.0 s; breath-hold: 1 s) with inhalation through the nose. Clearance predictions were conducted for irregularly shaped DEP aggregates ranging in size from 50 to 250 nm.
Results: According to the results obtained from the clearance simulations retention of DEP in different lung regions to a high extent depends on particle size. Therefore, 24-h retention, 10-d retention and 100-d retention negatively correlate with the aerodynamic diameter of the inhaled DEP and amount to 79.8–84.5%, 33.9–36.8%, and 7.2–7.9%, respectively. DEP with a size of 50 nm are completely cleared from the lung structures after 365 d, whereas clearance of 150- and 250-nm particles theoretically requires 342 and 324 d.
Conclusions: Based upon the results presented in this study it can be concluded that small DEP differ from larger ones by their slower clearance from the airways and alveoli. According to this essential circumstance, which is closely related to the deposition patterns generated by the DEP, individual risk assessments have to be carried out for single DEP size categories.