Bioaerosols in the lungs of subjects with different ages-Part 2: clearance modeling.

BACKGROUND: The present contribution deals with theoretical aspects regarding biogenic particle clearance from various lung structures of probands with different ages (1, 5, 15, 20 y). With reference to part 1 of the study, particles varying in size and shape are subject to a detailed analysis. The main goal of the investigation consists in an increase of our knowledge concerning the clearance behaviour of bioparticles and its dependence upon various physiological and anatomical factors.

METHODS: Theoretical clearance of biogenic particles was subdivided into four main phases, namely fast bronchial clearance, slow bronchial clearance, fast alveolar clearance, and slow alveolar clearance. All of these phases were simulated by using a well validated stochastic modeling approach, where the main focus is set on the randomly varied particle mass transfer between main compartments of the human respiratory tract. Whilst effects of particle geometry on clearance were approximated by application of the projective-diameter concept, age dependence of the particle removal process was expressed by the experimentally proven relationship between bronchial mucus velocities and morphometry of the airway tree.

RESULTS: According to the results of the theoretical simulations efficiency of fast bronchial clearance, expressed by the 24-h-retention value, exhibits a negative correlation with proband's age, whereas the other clearance phases are characterized by a rather conservative behaviour among the different age categories. Highest clearance rates may be observed for very fine (<0.01 µm) and very coarse particles (>5 µm) preferentially deposited in the upper bronchial airways, whilst large particles accumulated in the alveoli may be stored there for several months to years.

CONCLUSIONS: The study comes to the conclusion that infants and children dispose of an enhanced bronchial clearance efficiency with respect to adolescents and adults, which results in a faster removal of particulate substances accumulated in the upper bronchial regions. Particles escaping from the natural filtering process in the upper airways and undergoing alveolar deposition are subject to identical clearance scenarios among the age groups and may represent remarkable health hazards.