Human respiratory tract aerodynamic diameter

The probability of deposition of particles in the various parts of the human respiratory tract depends on their aerodynamic diameter, Dae- This parameter in turn is a function of the physical dimensions, shape, and density of the particles. For spherical particles, the aerodynamic diameter is simply... 

Much of our early understanding of deposition of inhaled particles in the human respiratory tract as a function of aerodynamic diameters and breathing parameters... 

Since both gravitational and inertial transport are dependent on (pd ), the aerodynamic diameter concept also applies in principle to inertial transport of particles larger than 1 pm in aerodynamic size. Inertial transport contributes to particle deposition in the human respiratory tract for particles larger than 2 pm in aerodynamic diameter. Therefore, particle deposition for spheres of different density is an unique function of the aerodynamic diameter. 

Very often inertial deposition in impactors is used to characterize the aerodynamic behavior of aerosol particles. However, much larger inertial forces are applied for particle deposition in impactors than are available for particle deposition in the human respiratory tract. The particle size obtained by this technique is the inertial diameter. This diameter is defined in the same way as the aerodynamic diameter but based on inertial rather than gravitational particle transport. When a particle is not only inertially but also gravitationally transported its inertial diameter is identical with its aerodynamic diameter. 

It must also be recognized that when only the aerodynamic diameter of a particle is known, its deposition efficiency in the respiratory tract can be estimated but it remains unknown what mass this particle delivers to the surfaces of the respiratory tract. This mass can be estimated only when either the density or the geomeui-cal diameter of this particle is known. For instance, the mass of spheres that are 2 pm in aerodynamic size decreases by more than a factor of 5 when their density increases from 0.1 to 3 g cm (Table 1). Although the particles are deposited with equal probability in the human respiratory tract, the mass they deliver to airway and airspace surfaces is far from being equal. For aU spheres of equal aerodynamic size, the sphere with the lowest density carries the greatest mass into the lungs. 

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