Particle Transport onto Airway Surfaces

Marcel Dekker, Inc. 270 Madison Avenue. New York, New York 10016 

Aerosol particles of dimensions comparable with the mean free path of gas molecules (about 0.06 pm) recognize their gaseous surroundings as composed of individual molecules, and every collision of a particle with a gas molecule changes its kinetic energy and direction of motion as a result, the particle moves at random through the gas (Brownian motion or diffusion). The random displacement a particle covers by this transport increases with time and with decreasing particle diameter. It is independent of the particle density. 

In the respiratory tract, only ultrafine particles (particles smaller than 0.1 pm in diameter) are deposited solely due to diffusion, since those particles cover more than 30 pm s by diffusional transport (Fig. 2). For all ultrafine particles of the same size, deposition is the same regardless of their density. Because of the time-dependence of diffusional particle transport, it is anticipated that diffusional deposition of ultrafine particles occurs mainly in lung regions of maximum residence time of the tidal air—i.e., in small airways and in the lung periphery. 

In the branching network of airways, the inspired air is changing its velocity and direction of motion all the time while it is penetrating into the lungs. Particles carried with the air are therefore exposed to inertial forces aU the time. For particles of sufficient mass, these forces result in an inertial displacement and thus in a particle transport toward airway surfaces. This displacement increases with particle 

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Figure 1 Illustration of particle transport onto airway surfaces.

In summary, particle transport onto airway surfaces depends on four parameters particle diameter, particle density, particle velocity, and the time available for... 

The models introduced by Findeisen (117), Altshuler (118), and Taulbee and Yu (119) are considered primary deposition models, in that each proposed original and independent mathematical formalisms to describe particle transport onto airway surfaces. Incidenfly, they are also based on different models of the geometric structure of the human lung. Later, the formalisms of the primary models were adopted and modifled by many authors, resulting in so-called secondary deposition models (120-134). Recent models have been proposed by the U.S. National Council on Radiation Protection and Measurements (NCRP 136-137) and by the Task Group on Human Respiratory Tract Models for Radiological Protection (ICRP 137,138). 

When an aerosol enters the respiratory tract, its particles first experience inertial transport onto airway surfaces in the extrathoracic and upper bronchial region. 

Pharmaceutical particles are gravitationally and inertially transported onto airway and airspace surfaces. Extrathoracic and upper bronchial surfaces collect them by impaction and lower bronchial and peripheral airspace surfaces by sedi-... 

Heyder J. Particle transport onto human airway surfaces. Eur J Respir Dis 1982 63(suppl 119) 29-50. 

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