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Excerpted from: "Residential Air Cleaning Devices: A Summary of Available Information", Office of Air and Radiation, U.S. Environmental Protection Agency 400/1-90-002, February 1990
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. 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 filters may efficiently collect large particles, but remove only a small percentage of respirable size particles.
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. 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. 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. Concerns about ozone production are discussed in more depth later.) Some newer systems on the market are referred to as "hybrid" devices. They contain two or more of the particle removal devices discussed above.
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.