Biomedical Engineering Reference
In-Depth Information
size range of OIP-produced aerosols (0.5- to 10-μm aerodynamic diameter) are as
follows (in no particular order of priority) [
2
]:
1. Gravitational sedimentation
2. Turbulent deposition to adjacent surfaces
3. Particle-particle agglomeration (if the particle concentration is sufficiently high)
4. Flash evaporation of highly volatile propellants associated with the aerosol for-
mation process with MDIs
5. Evaporation/condensation of associated low-volatility substances (e.g., ethanol
cosolvent incorporated with some MDI formulations as well as ambient mois-
ture, if present)
6. Molecular (Brownian) diffusion
The latter process is only significant with the finest particles < ca. 0.5-μm physical
(geometric) diameter. The influence of these processes on CI-measured APSDs is
in APSD may or may not be detected by the efficient data analysis (EDA) metrics.
This topic is concerned with the CI method that determines the aerodynamic
rather than physical (e.g., geometric) size of such particles, as would be measured
by means of a microscopy-based technique [
1
]. As a general rule, particles in the
size range from about 0.5 to 10 μm in aerodynamic diameter will deposit some-
where in the human respiratory tract (HRT) and larger particles in the upper airways
(oropharyngeal region), with finer particles penetrating progressively further into
the 23 generations of the airways of the lungs before the finest ultimately reach the
alveolar sacs in which gas exchange takes place.
Detailed descriptions of aerosol mechanics associated with transport through the
HRT can be found in the topics by Hinds [
1
] and Finlay [
2
]. The explanation given
here is intended to provide the basics in order to understand the capability and limi-
tations of the inertial impaction method for size-characterizing aerosols emitted
from OIPs.
Particle motion throughout the airways of the respiratory tract is assumed to take
place following Stokes's law, in that the relative velocity of the gas at the surface of
the particle is zero. Under Stokesian motion in a stagnant support gas, the force (
F
d
)
acting on a particle in a fluid (air) comprises both form and frictional components,
such that
F
=
3
ph
v d
(2.1)
d
a
tp
where
h
a
is the air viscosity which is a function of the temperature of the air,
v
t
is
particle terminal velocity, and
d
p
is the particle volume-equivalent diameter [i.e., the
diameter of a spherical particle of the same volume and the same as the physical
(geometric) diameter for particles possessing spherical geometry]. Particles rapidly
reach their setting velocity (
v
t
) when released from rest, and the drag force (
F
d
) is
balanced by the gravitational force
F
g
, where
F
g
= mg
, so that
2
r
dg
pp
v
=
(2.2)
t
18
h
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