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Air cleaners are usually classified by the method employed to remove particles of various sizes from the air. There are three general types of air cleaners on the market: mechanical filters, electronic air cleaners, and ion generators. (Note: Because they may reduce some pollutants present in indoor air through condensation, absorption, and other mechanisms, devices such as air conditioners, humidifiers, and dehumidifiers may technically be considered air cleaners. However, this discussion includes only those devices specifically designed and marketed as air cleaners.)
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Mechanical filters may be installed in ducts in homes with central heating and/or air-conditioning or may be used in portable devices which contain a fan to force air through the filter. Mechanical filters used for air cleaning are of two major types.
Flat or panel filters generally consist either of a low packing density of coarse glass fibers, animal hair, vegetable fibers, or synthetic fibers ... (a flat filter in use in many homes is the typical furnace filter installed in central heating and/or air-conditioning systems.) Flat filters may efficiently collect large particles, but remove only a small percentage of respirable size particles.
Flat filters may also be made of "electret" media, consisting of a permanently-charged plastic film or fiber. Particles in the air are attracted to the charged material.
Pleated or extended surface filters generally attain greater efficiency for capture of respirable size particles than flat filters. Their greater surface area allows the use of smaller fibers and an increase in packing density of the filter without a large drop in air flow rate.
Electronic air cleaners use an electrical field to trap charged particles. Like mechanical filters, they may be installed in central heating and/or air-conditioning system ducts or may be portable units with fans. Electronic air cleaners are usually electrostatic precipitators or charged-media filters. In electrostatic precipitators, particles are collected on a series of flat plates. In charged-media filter devices, which are less common, the particles are collected on the fibers in a filter. In most electrostatic precipitators and some charged-media filters, the particles are deliberately ionized (charged) before the collection process, resulting in a higher collection efficiency.
Ion generators also use static charges to remove particles from indoor air. These devices come in portable units only. They act by charging the particles in a room, so they are attracted to walls, floors, table tops, draperies, occupants, etc. In some cases, these devices contain a collector to attract the charged particles back to the unit.
(Note: The latter two types of devices may produce ozone, either as a byproduct of use or intentionally.)
Some newer systems on the market are referred to as "hybrid" devices. They contain two or more of the particle removal devices discussed above. For example, one or more types of mechanical filters may be combined with an electrostatic precipitator or an ion generator.
In addition to particle removal devices, air cleaners may also contain adsorbents and/or reactive materials to facilitate removal of gaseous materials from indoor air. Air cleaners which do not contain these types of materials will not remove gaseous pollutants.
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