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Air purifiers and cleaners:
This article explains the types of air cleaners or air purifiers used to improve indoor air quality in homes. We include a table of types of air cleaners and the types of particles they handle, comparing their ability to remove materials from the air, their ease of maintenance, and their operating cost.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
Our page top photo shows that even the naked eye can see comparatively large airborne particles indoors. But many indoor contaminants are simply too small to see, or are not particles at all but rather gases or chemicals.
There are three main types of particulate air cleaners on
the market: mechanical filters, electrostatic filters, and ion
generators. In addition, there are filters with adsorbents,
such as activated carbon, that are capable of removing certain
gases (see Table 7-2above) Most filters are available as standalone
units or as add-ons to the home’s HVAC or ventilation
system. Some hybrid systems on the market combine two
or more strategies, such as a filter to remove particles plus
activated carbon to remove odors and organic gases.
Mechanical Filters. These use a matrix of fiberglass or
synthetic fibers and resin to filter particles out of air passing
through. Some are coated with an oil or adhesive to help
trap particles, and others have a static electrical charge.
Some types of mechanical filters can be cleaned, but most
need to be replaced when full.
As mechanical filters clog,
they become more efficient at trapping particles, but airflow
is reduced. They can either work in stand-alone units
or be incorporated into the HVAC or ventilation system.
Flat filters are the standard fiberglass elements found
in furnaces and air handlers. They are designed to
catch large dust particles and have little effect on the
smaller respirable particles that affect health.
Pleated filters, or “extended media” filters, have
smaller pores designed to capture small- and medium-
sized particles. They are pleated like automobile air
filters to provide greater surface area for improved
airflow. Some will fit into a standard furnace filter
slot, but the higher-efficiency types are generally too
thick and require ductwork modifications. For good
performance, they need to be replaced regularly.
Electrostatic filters use a plastic element that is permanently
charged with static electricity or captures an
electric charge from the passing air. They are a little
more effective than a standard furnace filter at capturing
larger particles such as pollen and mold spores,
and can be washed and reused when full.
HEPA stands for “high-efficiency particulate accumulator.”
These filters range from 95% to over 99% efficient
for particles over 0.3 microns, including mold
spores, mites, pet dander, and some viruses. Because
of their high resistance to airflow, HEPA filters typically
require a separate fan and housing.
Replacement HEPA Filter
elements last a year or longer but cost well over $100
versus $10 to $20 for a medium-efficiency pleated
filter. Most have a prefilter to catch large particles
that would prematurely clog the filter. Prefilters need
to be changed regularly.
Electronic (Electrostatic) Air Cleaners
Electronic air cleaners
(EACs) use a series of electrically charged metal plates or
media filters to pull particles out of the air stream. They
are either portable units designed to clean the air in one
or two rooms or central systems connected to the return
ductwork of the HVAC system. EAC systems create little
resistance to airflow but require a separate fan, which
along with the electrical charging device use about 30 to
50 watts of electricity.
Electronic air cleaners are highly efficient at removing
both small and large particles, but require more maintenance
than many homeowners would like. To remain
effective, the filters must be removed regularly and hosed
down in a large sink or tub. Some are designed to fit in a
In charging the particles with high voltage, EACs also
produce small amounts of ozone, which can be an eye or respiratory
irritant at high levels. Most people are not bothered
by the amount produced. If this a concern, however, look for
a unit with an activated carbon filter to remove the ozone.
Negative Ion Generators to Clean Indoor Air
These work by releasing
electrically charged ions, which attach to dust particles
in the air causing them to settle on walls, ceilings, furniture,
and draperies. Placed too near a wall, they might leave
a smudge of particles. Some units contain an optional collector
to trap the charged particles in the unit, functioning
similarly to an EAC.
Over time, however, the particles can
lose their charge and reenter the air. Like EACs, they produce
small amounts of ozone. There is little scientific
evidence supporting the effectiveness of these units.
Watch out: Do not rely on ozone generators to correct indoor air quality issues, and beware of ion generators that may be adjusted to produce high levels of indoor ozone.
The turbulent flow
precipitator (TFP) is a new proprietary technology from
Canadian-based Nutech Energy System. The device, which
attaches to the return ductwork of an HVAC or ventilation system,
contains a fan and a labyrinthine core made of aluminum
plates and synthetic fibers.
Turbulence in the air flings the suspended
particles out of the airstream where they are trapped
by a primary and secondary core, which need replacement
in one and three years, respectively. Nutech claims that TFPs
will capture 99% of particles larger than 5 microns, 97%
from 2 to 3 microns, and 90% from 0.5 to 0.9 microns. A
TFP unit with HEPA filtration is also available.
Gas Removal Filters to Clean Indoor Air
To remove gases, such as formaldehyde,
combustion fumes, or volatile organic compounds, from
the air requires the use of special adsorption media. These
media contain materials, such as activated carbon or aluminum
oxide, which trap the gases in tiny pores.
chemical adsorbents are effective with different gases,
and none is effective with every gas found in the typical
home. Relatively small quantities of activated charcoal
can be very effective at reducing odors, but how well they
filter out the low levels of multiple chemical compounds
typically found in household air is unclear.
In general the rate of adsorption of a gas is reduced as
more of the target gas is captured in the filter media. Researchers
have also found that, in many cases, some of the
gas is reemitted from the filter back into the air. Scientific
evidence about the real-life usefulness of these filters in
homes is very limited.
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