By Adem Lewis / in , , , , , , , , , , , /

The forces and phenomena that we have just
discussed are crucial in understanding aerosol
behavior in a variety of important settings, such as in the collection of particles by filters.
In filters, dirty air passes through a filter and
comes out clean. As you may have guessed, particle size plays
an important role in collection efficiency with some particles hitting the filter and being
collected, while other particles pass through. For a moment, I want us to consider a single
fiber in the filter. The airflow bends as it moves around the fibers,
much like the airflow moving up and over a car
on the highway. Large particles have a high probability of hitting
the fiber because inertia causes them to deviate
from the airflow streamlines. Small particles, in red, also have a high
probability of hitting the fiber because of
Brownian motion. However, medium-sized particles, in orange,
typically around 300 nanometers in diameter,
are affected minimally by diffusion or inertia and ‘go with the flow’ following the streamlines to the
other side of the filter. The combined effect of diffusion and impaction
results in a collection efficiency curve that is
typical of filters and other devices, as we’ll see. Here, I show particle collection efficiency by
size for two different kinds of filters. The purple curve is typical of a low-efficiency
filter, like a low-cost furnace filter. High collection efficiency is achieved for very
small particles (10 nanometers in size), due to diffusion and for large particles (larger
than 5 micrometers), due to impaction. However, the lowest collection efficiencies occur
for particles in the middle size range, (about 300 nanometers) because diffusion and
impaction have the least effect on these size
particles. In contrast, some filters, known as high
efficiency particulate air filters, or HEPA filters, shown in magenta, are designed
to have high collection efficiency. Even for these filters, however, the lowest
collection efficiency occurs for particles with a
diameter of 300 nanometers, a size associated with low diffusion and inertial
forces. With good design, collection efficiency of a
HEPA filter is typically 99% or greater, even for
particles of this size. Electret filters leverage electrical forces to
improve the collection efficiency of filters. In this collection efficiency curve, a filter with
uncharged fibers has a collection efficiency
curve resembling a poor home filter furnace, regardless of whether the particles are charged
or not. If instead the fibers of the filter are charged, then
performance is dramatically improved for both
charged and uncharged particles. These types of filters are sold commercially,
under the brand name of Filtrete, by 3M, and are
also used in respirators. The same forces and phenomena dictate if a
particle will transport through a tube. Big particles settle due to gravity; Small particles will diffuse due to Brownian
motion; and Medium-sized particles will tend to ‘go with the
flow’ and pass through the tube. Passing through a tube is critical in applications
like particle sampling or in local exhaust
ventilation ducts to avoid clogging. Transport through something is characterized by
penetration, which is 1 minus the collection
efficiency. So in this plot, a penetration of 100% means
that all of the particles pass through, or penetrate, the tube, and that of 0% means
that all of the particles hit the walls of the tube
and do not transport to the other side. Very small particles tend to diffuse to the walls,
resulting in low penetration. Medium-sized particles, in contrast, like the
orange particle, ‘go with the flow’ and have a high
penetration efficiency. Big particles settle due to gravity and wind up on
the bottom walls of the tube and have low
penetration again. We can now understand particle deposition in
the respiratory tract, which can be thought of as
a series of simple tubes. Airflow in the upper airways is fast moving with
few large tubes, where as it is very slow in the lower respiratory tract before terminating in the
alveolar region. Big particles tend to impact where velocities are
fast and the air curves, like in your nose, or
settle due to gravity where air slows down. Medium sized particles go with the flow, often
being breathed in and breathed out. Small particles tend to hit the walls where
dimensions are small and velocities are low,
because of Brownian motion. The net result is that the fraction of particles that
deposit in the human respiratory system looks
like a bad filter. So here, we show respiratory deposition
fraction, the fraction of particles that wind up
depositing in the respiratory tract. Here, a deposition fraction of one means that all
particles deposit, whereas a deposition fraction
of zero means that no particles deposit. Only about 15% of 300 nanometer sized
particles deposit (highlighted with the red arrow) because neither diffusion nor inertia do much to
move this particle size away from airflow
streamlines. Thus, a particle of this size, if inhaled, is
breathed back out 85% of the time. In contrast, nanoparticles deposit with higher
efficiency due to diffusion. For particles larger than 300 nanometers, inertia
causes deposition to increase until about 5
micrometers. Deposition then becomes progressively lower
because larger particles have sufficiently high gravity settling velocities to make them difficult
to aspirate into the respiratory system. So they aren’t even available to be deposited
inside the respiratory system.

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