Mechanisms of Particle Deposition - Aerosols: Measurement, Dosimetry, and Health Effects - page 54

3.3.2 c ontinuuM r egiMe
In the continuum regime, the gas in which the particles are suspended can be assumed to act as
a continuous, viscous luid. The continuum regime may also be called the Stokes regime. 38 This
regime is alternately deined as Kn < 0.1, 38 Kn < 0.4, 7 and d p < λ ( Kn < 2). 4 A value of Kn < 0.l cor-
responds to particle diameters of greater than 1.3 μm in air at STP.
In the continuum regime, particle motion within the gas is governed by the momentum (Navier-
Stokes) equations for a viscous luid, and the particle drag force and terminal settling velocity
can be calculated from Stokes's Law. In this region, no-slip conditions are assumed to exist at the
surface of the particle.

3.3.3 s liP -F low r egiMe
The slip-low regime falls between the free-molecule and continuum regimes and has been vari-
ously deined as including particles having a value of Kn of 0.1-0.3 (with a transitional regime
deined having Kn of 0.3-10), 38 or 0.4-20. 4
In the slip-low regime, the assumption of no-slip conditions at the surface of the particle is not
applicable. In this case, there exists a quantiiable velocity of the gas relative to the particle at its
surface. In the slip-low regime, the drag force exerted on a particle is overestimated (and the termi-
nal settling velocity is underestimated) by Stokes's Law. Therefore, within this regime Stokes's Law
must be corrected by the Cunningham slip correction factor (Equation 3.5).

3.4 INHALABILITY
To understand the deposition of particles in the respiratory system, one must irst understand the
inhalability of a particle, or the ability of the particle to enter the nose or mouth during breathing.
Many experimental studies have used “breathing” mannequins to determine the inhalability of
various particle sizes and types. These mannequins act as “samplers” and the inhalability is actu-
ally the aspiration eficiency of the mannequin breathing system. 39,40 Mathematical models such as
the ICRP 66 41 and ACGIH 42 have been used to predict inhalability of particles through the nose. 43
Computational luid dynamics (CFD) has also been used to estimate the inhalability of particles. 44
Breysse and Swift studied nasal inhalability from still air by four human subjects; the only human
study of inhalability from still air in the literature at this time. 45 A comprehensive overview of inhal-
ability work is provided by Millage et al. 43
Inhalability ( I ), or the ratio of the concentration of particles of a particular aerodynamic diameter
that enter the nose or mouth to the ratio of the concentration of particles of the same aerodynamic
diameter in the inhaled ambient air, 41 may be basically deined by Equation 3.19 43 :

= ⎡

Cinhaled d
Cambient d

( ) ⎤

⎥

ae

I d ( ) (3.19)

⎢

ae

( )

ae

Inhalability is dependent on wind speed with different inhalability fractions resulting from still
air than from low or moderate wind speeds. 41,43,46 Inhalability is also affected by face orientation
to the wind and facial feature dimensions, 43,46-48 as well as breathing mode (nasal versus oral) and
breathing rate. 43 Because of the dependence of inhalability fraction on each of these factors, var-
ious inhalability eficiency equations have been developed primarily for the Industrial Hygiene
community. 41-43,46,49
The overarching message from all of this work is that there is no single equation that describes
all situations when it comes to inhalability; there are so many variables (e.g., wind speed, particle
size, and breathing rate) that different equations must be used for different situations. Finally, it is