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HEPA filters have
been recognized in the CDC Guidelines for the Prevention
and Transmission of TB in Health Care Facilities to
play an important role in the containment of airborne
infectious pathogens. However, what principles or collection
mechanisms are utilized and applied that can allow
a tiny particle, such as M. tuberculosis, an
airborne pathogen that is rod shaped and 0.4 - 1.4
microns in size, to be effectively captured by a HEPA
filter or, for that matter, any bacteria, mold spore,
fungi, or microbe?
HEPA Media
The heart of any HEPA filter lies in the media
and the fiber formulation. The media or matrix is comprised
of a blend of 100% micro-glass (vitreous) fibers of various
diameters that are randomly dispersed in a filter mat
and bonded together with acrylic resin binders, water
and chemicals in a wet-laid process. The binder provides
the strength for the fibers to allow for filter fabrication,
such as pleating. These man made micro-glass fibers are
unique in their cylindrical shape; they are straight
and uniform in diameter. Higher percentages of fine diameter
glass fibers in the media yield higher media filter efficiencies.
These precise formulas of ingredients and the exact process
control of the formulations yield duplicable physical
characteristics of product. This is one reason HEPA filters
are so consistently uniform in their performance.
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Figure
1:
HEPA filter magnified
500 times. |
Figure 1 is a photograph
of what the HEPA filter media looks
like when magnified 500 times. The fibers being randomly
dispersed create a very tortuous path for the air to
follow. You can observe there is no controlled pore
size and various fiber diameters are attached to each
other by the binder. This irregular pathway through
this maze of vitreous fibers allows for depth loading
and capture of the smallest of particulates. The reason
that HEPA filters cannot be cleaned out and reused
is because the captured particles are bound to the
fibers.
Filters are measured
by efficiency levels and efficiencies are only relevant
when compared to a particular particle size. For example,
a filter can claim to be 100% efficient, but to what? Chicken
wire is 100% efficient on chickens, but 0% on flies.
The point being that you always need to relate efficiency
to particle size.
HEPA filters have a minimum efficiency of 99.97% on particles 0.3 microns
in size or larger. The most widely recognized standard test method for
HEPA filters uses 0.3 thermally generated DOP particles as a challenge
agent and is based upon this size being viewed as the most penetrating
particle size and being near the minimum efficiency level and therefore
efficiency will be greater for all other sizes. As a comparison, the
diameter of a human hair measures 75 microns across, we're talking about
pretty small stuff, particle sizes that are invisible to the naked eye.
To understand the
efficiency level of a HEPA that is 99.97%, means that
for every 10,000 particles .3 microns in size or larger
that are challenged by the filter, only 3 particles
will not be captured. HEPA filters are also available
in 99.99% and 99.999% efficiency levels, but 99.97%
on 0.3 microns defines a HEPA. The testing and certification
of HEPA filters is another topic all together, but
HEPA filters with penetration levels above .030% do
not qualify, and all HEPA filters must be individually
tested and certified by prescribed industry and federal
standards.
HEPA Efficiency
HEPA filters never become less efficient than
their initial efficiency rating (unless damaged). It
is sometimes a misunderstanding that these filters need
to be replaced, because they have lost their efficiency.
HEPA filters actually increase in efficiency as they
become loaded because the tiny particles continually
build up and the entrapped particles act as tiny filters.
Operating HEPA filters at lower air flow velocities will
also improve efficiency levels. HEPA's need replacement
because they load up with contaminates which gradually
decrease airflow and increase resistance (delta P) to
a point that maintaining a prescribed cfm becomes diminished.
That is why pressure sensing devices are utilized, which
are a measure to determine filter life.
Now that we have
outlined what makes a HEPA filter and what determines
a HEPA filter, we will look at why HEPA filters are
so efficient.
Technology
of Capture
HEPA filters use four different capture mechanisms.
Straining or sieving is the easiest to understand because
it provides a simple principal (Figure 2-A). The object
is larger than the opening through which it can pass and,
therefore, it comes to a barrier and is stopped. Due
to the physical fiber matrix structure of the filter
media, this principal of filtration decreases the life
expectancy of a HEPA as these larger particles tend to
clog up the pathways and cause surface loading. Prefilters
are usually used to filter out the large particulate,
allowing the HEPA to contain the smaller particles, the
purpose for which it was designed
Inertia Impaction
Inertia impactment or impingement is very effective
for particles usually larger than one micron in size
(Figure 2-B). These larger size particles are a mass
in an air stream and collide with the fibers in the fiber
media head-on. They are large enough not to be able to
maneuver or travel around the fiber bed, so they become
impaled and entrapped on the fiber through a collision.
Figure 2: HEPA
filters use four different capture mechanisms: A-straining,
B-inertia impaction,
C-interception, and D-diffusion.
Interception
Interception is very effective on particles larger
than 0.1 micron in size (Figure 2-C). As these lightweight
particles travel in the air stream, they will flow around
a fiber or obstruction. When they make contact with a
fiber, the particle is captured. The bonding or attraction
to the particles to the fiber is due to an intermolecular
surface adhesion known as van der Wael's forces.
Diffusion
The final capture mechanism is known as diffusion
or Brownian Motion (Figure 2-D). Particles that are smaller
than 0.1 micron in size are bombarded by air molecules.
In 1827, Robert Brown, a Scottish botanist, reported
on small particles called molecules. These molecules
were in a continually random motion. As a result of this
motion, particles migrated from areas of high concentration
to areas of low concentration, a process called diffusion.
As the molecules collide with airborne submicron size
particles, they create a spontaneous intermingling. This
causes them to travel in an erratic path within the air
stream (Brownian Movement), thereby improving their chances
of colliding with the filter fibers, at which point they
are retained by the intermolecular van der forces. Diffusion
is a result of velocity. The lower the air flow
velocity, the greater the possibility of a particle colliding
with the fibers. This approach, therefore, works even
when the space between the fibers may be larger than
the captured particle. Particles more than one micron
in size have virtually no effect on this capture mechanism.
The relation of inertia to interception/diffusion is
based not only on particle size retention, but airflow
velocity. Inertia is based on high-velocity impaction
of the particle, while the others depend on lower velocities.
Filter efficiency increases with decreasing particle
size and decreases with the increase of air velocity.
Conclusion
When it comes to airborne infection control,
the HEPA filter is a highly effective, reliable, refined,
and dependable airpurification device. It has proven
its worth for protection of individuals, property, and
process in countless applications throughout the world
for more than 40 years. There are virtually no restrictions
or cautions as to its use. Humidity, heat, cold, component
exposure, or creating ideal operational conditions for
its optimum use play no part in its effectiveness. HEPA
filters improve with use, most other systems deteriorate
with continued use. Improvements to the filter components,
materials, and testing procedures continue unabated in
a variety of industries. Efficiency levels for HEPA filters
reach 99.97% on 0.3 micron, and for ultra-low penetration
air (ULPA) filters as high as 99.9999999% on 0.12 micron.
Like any device, it needs to be properly installed, maintained,
and correctly applied. Beyond the healthcare field,
the electronics industry would cease to exist without
the ability to create particulate-free, controlled environments
with the HEPA filter as its heart.
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