Background

Air purifiers evolved in response to people’s reactions to
allergens like pollen, animal dander, dust, and mold spores. Reactions
(sneezing, runny nose, scratchy eyes, and even more severe consequences
such as asthma attacks) are the result of antigens found in the home.
These antigens are major triggers of asthma, and there are more than 17
million asthmatics in the United States alone. Air purifiers remove a
portion of these particles, thus reducing allergic-type responses.

Due to their extremely small size, allergens are able to pass through a
standard vacuum cleaner bag and redistribute into the air where they stay
for days. Even a single microgram of cat allergens is enough to invoke an
allergic response in most of the six to 10 million Americans who are
allergic to cats. Other airborne particles—such as bacteria and
viruses—can cause illnesses, some of which are fatal. There are
many reasons—allergies, asthma, fatal illnesses—that
millions of air purifiers are sold in the United States every year.

There are two common types of air purifiers that can remove some or all of
the disease and allergy-causing particles in the air: mechanical
filters—the most effective are classified as High Efficiency
Particulate Air filters (HEPA filters)—and electrostatic
precipitators.

HEPA filters are made out of very fine glass threads with a diameter of
less than 1 micron (a micron is 0.00004 in, 0.001 mm). By comparison, a
human hair has a diameter of about 75 microns (0.003 in, 0.07 mm). The
fine glass threads are tangled together and compressed to form a filter
mat. Because the individual threads are so microscopic, most of the mat
consists of air. The openings in the mat are very small, generally less
than 0.5 micron (0.00002 in, 0.0005 mm). HEPA filters will collect
particles down to 0.3 microns (0.00001 in, 0.0003 mm) in diameter. Even
though the filter may only be 0.10 in (2.5 mm) wide, it would consist of
2,500 layers of glass threads.

Electrostatic precipitators rely on electrostatic forces to remove
particles from the air. They work by creating a cloud of free electrons
through which dust particles are forced to pass. As the dust particles
pass through the plasma, they become charged, making them easy to collect.
Electrostatic precipitators can collect particles down to a diameter of
0.01 microns (0.00001 mm).

Neither HEPA filters nor electrostatic precipitators can remove volatile
organic compounds from the air, therefore do nothing to reduce odors. For
this reason, most air purifiers are equipped with a pre- or post-filter
composed of activated carbon. Activated carbon is produced by heating a
carbon source (coconut shells, old tires, bones, etc.) at very high
temperatures in the absence of oxygen, a process also known as pyrolysis
or destructive distillation. Pyrolysis separates the pure carbon from the
other materials contained in the raw material. The pure carbon is then
exposed to steam at 1,500°F(800°C). The high temperature steam
activates the carbon. The activation process forms millions of cracks in
the carbon grains. These cracks have diameters of about 0.002 microns
(0.000002 mm). Because there are so many cracks, the activation process
provides the carbon with an enormous surface area per weight—about
6.5 acres/oz (1,000 m

²

/g). The millions of cracks provide locations where organic compounds can
be adsorbed. In addition, the surface of the carbon carries a residual
electrical charge that attracts non-polar chemicals (chemicals that do not
have separated positive and negative charges) to it. Activated carbon is
very effective at adsorbing odor producing compounds.

History

Air purity has been a concern as long as human beings have lived in
groups. One of the reasons that hunter-gatherers are nomadic is that they
periodically need to move away from their garbage dumps and latrines. In

A.D.

61, the Roman philosopher Seneca complained about the miasma of chimney
smoke that constantly hung over Rome. In 1306, King Edward I of England
banned the burning of coal in London due to the heavy pollutants left in
the air.

The Industrial Revolution of the eighteenth and nineteenth century only
worsened the problem. Burning coal to produce electricity and fuel trains
produced a dark cloud of smoke over every major center of industry in the
world and covered entire cities with soot. To deal with this problem,
engineers built higher smoke stacks to move airborne waste further away
from the source. Regardless of how high the stacks got, the people down
wind complained about the ashes and the acid gases from coal combustion
(the source of acid rain) destroying their crops. Air pollution took
another turn for the worse after World War II when automobiles became the
primary means of transportation in the industrialized world. Automobile
smog has provided Los Angeles with the worst air quality in the world.

Raw Materials

The materials that go into both HEPA filters and electrostatic
precipitators are: a case made out of plastic, an electric fan to induce
air flow through the filter, the filter itself, and control switches to
control the speed of the fan and turn the air purifier on and off. The
HEPA filters are made of borosilicate glass fibers or plastic fibers
(e.g., polypropylene) bound together with up to 5% acrylic binder (the
same compound that binds latex paint to a house). Electrostatic
precipitators generate ions by running extremely high positive direct
current voltages through steel wires set between grounded steel charging
plates. Cases are almost universally made from plastic, usually
high-impact polystyrene, polyvinyl chloride, high-density polyethylene, or
polypropylene. Most air purifiers are also usually equipped with a
post-filter composed of activated carbon.

Design

HEPA filters are designed based on the size of particles to be removed and
the required air flow rate. The finer the pores in the HEPA material, the
finer the particles removed from the air. However, collecting finer
particles means the filter material will clog sooner and need replacing on
a more frequent basis. The designer will specify the diameter of the glass
fibers and the mat density of the filter fabric that fixes the filter pore
size. HEPA filters can contain binders that provide additional strength,
but this also produces a filter that clogs sooner.

Design of an electrostatic precipitator is considerably more complex. Home
electrostatic precipitators usually are designed to have two components,
an ionizing component (where the electron cloud is created) and a
collecting component (where the charged dust particles are pulled out of
the air). The collecting component consists of a series of parallel steel
plates—half are grounded and half carry a positive direct current
voltage—thus alternate plates are either positively or negatively
charged. The ionizing unit consists of thin wires strung between a
separate set of grounded steel ionizing plates parallel to, but set in
front of, the collector plates. The thin wires carry a very high positive
voltage direct current (up to 25,000 volts in a home air purifier). The
positive charges in the wires induce a flow of electrons between the wires
and the adjacent ionizing plates. Because there is a very high voltage on
the wire, electrons are pushed toward it by an acceleration of around
1,000 times the acceleration of gravity, which accelerates the electrons
to very high velocities. For example, as a particle of dust mite excrement
floats past the wire, the high-speed electrons collide with the electrons
in the molecules of the particle, knocking

An example of an electrostatic precipitator and its components.

some of them free. As these molecules lose electrons, they take on a
positive charge and are thus attracted to the negatively-charged collector
plate. The designer must select a voltage high enough to produce
sufficient numbers of electrons to ionize the particles passing through
the precipitator, and space the collector plates close enough together so
that the ionized dust particles will be captured on the plates before the
precipitator fan can pull them completely through the air purifier.

The Manufacturing

Process

The case

• 1 Pellets of the raw material (high-impact polystyrene, polyvinyl
  chloride, high-density polyethylene, or polypropylene) are fed into a
  hopper and heated to the melting point, 300-590°F (150-310°C).
• 2 The molten plastic is injected under high pressure into a mold of the
  case. The mold is usually made from tool steel by a highly skilled mold
  maker. Vents in the mold allow the entrained air to escape as the
  plastic enters. The mold designer must assure that the mold fills evenly
  with plastic and that all of the entrained air is allowed to escape,
  otherwise the final part might contain small air bubbles or even voids.
• 3 Water is forced through channels built into the mold to transfer heat
  from the molten plastic into the environment. Once the part is
  sufficiently cool, which can take up to two minutes, the mold opens.
  Hydraulically-operated pins push the part out of the open mold into a
  receiving bin.

The fan

• 4 An electric fan is used to pull air through the air purifier. The fan
  is usually purchased from a small-parts supplier. The fan consists of a
  small electric motor with metal fan blades attached to the
  motor’s power take-off. The fan blades are usually spot welded to
  a collar, which is slipped onto the power take-off and bolted in place.
• 5 The fan is usually attached to the case with steel screws.

HEPA filters

1. The glass fibers that make up a HEPA filter are created by passing
   molten glass or plastic through very fine pores in a spinning nozzle.
   The resulting glass fibers cool and harden almost instantly because of
   their tiny diameters.
2. The spinning nozzle moves back and forth (causing the glass fibers to
   form a web) above a moving conveyor belt onto which the fibers are
   collected. The speed of
   the conveyor belt determines the thickness of the filter
   material—a slow conveyor belt allows more glass fibers to build
   up on the belt.
3. The melting and cooling of the fiber produces some bonding of the
   fibers. As the conveyor progresses, a latex binder is sprayed onto the
   fabric to provide additional strength. The fabric can be any width up to
   the practical size of the machinery and can be cut down to the size
   specified by the customer before the fabric is taken up on rollers.
4. Once the HEPA mats are formed, they are folded into an accordion pattern
   in an automatic folder. The accordion pattern allows up to 50 ft
   
   ²
   
   (5 m
   
   ²
   
   ) of filter material to be enclosed in a small space.
5. The accordion-shaped filter is then enclosed in a filter case, usually
   consisting of an open wire grid. The purpose of the filter case is to
   support the filter.

Electrostatic precipitators

1. The electrostatic precipitator collection system is manufactured by
   enclosing steel plates into a plastic casing, often by hand assembly.
   The plates are arranged parallel to each other in the case.
2. Wires are then connected to alternate plates through which the high
   voltage positive direct current will be applied to the plates. The other
   plates are grounded.
3. The ionizing unit is constructed by running small diameter wires in
   front of the collector plates.
4. A voltage transformer, which is used to convert 115 volt household
   alternating current into high voltage direct current, is fixed to the
   precipitator case. This voltage is run to both the positively charged
   collector plates and the ionizing wires.

The activated carbon filter

• 1 The activated carbon filter (for odor reduction) usually consists of
  carbon-impregnated cloth or foam. This is manufactured by infusing the
  raw material with powdered activated carbon.
• 2 The carbon filter is then wrapped around the inside or outside of the
  HEPA filter, or stretched in a frame at either the inlet or outlet of
  the electrostatic precipitator.

Assembly

• 3 There are very few components in an air purifier. For this reason,
  they are usually bench assembled. In bench assembly, moving conveyors
  bring the individual components or sub-assemblies (e.g., the fan already
  attached to the case) to a bench where a person then hand assembles
  them. In a typical HEPA air purifier, there may only be five components
  that require assembly: casing, fan, particulate filter, carbon filter,
  and the on/off switch.

Quality Control

Filter efficiency is the most important quality control test for air
purifiers. The American Society for Testing and Materials (ASTM) publishes
quality control tests that filters must meet before they can be used in
certain applications or be marketed as HEPA filters (e.g., ASTM-F50:
Standard Practices for Continuous Sizing and Counting of Airborne
Particles in Dust Controlled Areas and Cleanrooms Using Instruments
Capable of Detecting Single Submicrometer and Larger Particles). The
United States Department of Defense has promulgated a standard in which
dioctylphthalate (DOP) particles are blown through a filter. To pass, the
filter must remove 99.97% of the influent DOP.

Byproducts/Waste

The byproducts of manufacturing include the non-carbon materials that are
distilled from the manufacture of activated carbon, specification filter
material, and excess material that must be discarded in the production of
HEPA filters. Most of the other manufacturing wastes, plastic runners from
the injection machines and excess sheet metal, can be recycled.

Additional wastes are produced during the operation of air filters. The
ions produced by electrostatic precipitators interact with oxygen in the
air to produce ozone. At high concentrations, ozone is poisonous. The
ozone levels produced in a home electrostatic precipitator are unlikely to
reach dangerous levels, but some people are sensitive to even low levels
of ozone. The collector
plates in an electrostatic precipitator need to be cleaned periodically.

HEPA filters have limited lifetimes, depending on the amount of air that
is filtered through them and the amount of particulates in the air. Most
manufacturers recommend that they be replaced every few years. The used
filters cannot be recycled and thus end up in landfills.

Activated carbon can be recycled, but the cost of handling the small
amount of carbon contained in a home air purifier would be prohibitive.
Generally, it also ends up in landfills after it is used completely.

The Future

As scientists learn more about environmental pollutants and their impact
on human health, the need to provide cleaner air in homes and offices will
only grow. The current generation of HEPA filters can only remove
particles down to 0.3 microns (0.00001 in, 0.0003 mm) in diameter while it
is believed that particles down to 0.1 microns (0.0001 mm) in diameter can
cause mechanical damage to lung tissue. Viruses can be as small as 0.02
microns (0.00002 mm) in diameter. Clearly, there is still progress that
can be made in controlling indoor air pollution. The current direction of
technology is toward ever finer filter materials. The new standard in
filtration is the ULPA filter, which stands for Ultra Low Penetrating Air.
An ULPA filter is required to be able to remove particles down to 0.12
microns (0.00012 mm) in diameter, about one third of the diameter of the
smallest particle a HEPA filter can remove.