Air Cleaning Technology

Technology does matter! You, the consumer, need to know about technology what air cleaning technology is used to make an informed choice. Study the pros and cons of each technology. First, choose the technology that fits your lifestyle preference.

Four Types of Air Cleaning Technologies

The following definitions are paraphrased from the American Lung Associations' "Residential Air Cleaning Devices: Types, Effectiveness and Health Impact" article. The complete article can be found at its web site at www.lungusa.org.

Electronic (Electrostatic Precipitators)

Electrostatic precipitators are the more common-type of electronic air cleaner. They employ a one-stage or a two-stage design for particle collection. In the less expensive but less effective single-stage design, a charged medium acts to both charge and collect airborne particles. A two-stage design employs a high-voltage electrode or wire, which places a charge on the incoming airborne particles. In the second stage, the charged airborne particles are drawn between a series of oppositely charged metal plates which attract the charged particles from the air causing them to precipitate onto the metal plates. Collection efficiency is a function of the area of the collecting plates, the flow rate, and the strength of the electrical field (Offerman et al., 1985). The airflow remains constant with use, but the particle capture efficiency declines rapidly as the charged collector plates become coated with particles.

Pros - The advantages of electronic filters are that (1) they generally have low energy costs because of low pressure drop, (2) low noise levels, (3) the airflow through the units remains constant with use, and (4) the precipitating cell is reusable, avoiding long-term filter replacement costs.

Cons - The major disadvantages are that (1) they become less efficient with use, cleaning must be done regularly (at least every few months) to maintain adequate performance (King, 1973). (2) precipitating cells require frequent cleaning, (3) they can produce ozone, either as a by-product of use or intentionally, and (4) Additionally, the charged particles produced by negative ion generators can sometimes soil room walls and furnishings. See cons of "Ionizers" below.

HEPA (Mechanical Filters)

High-efficiency particulate air (HEPA) filters, were originally developed during World War II to prevent discharge of radioactive particles from nuclear reactor facility exhausts. They have since become a vital technology in industrial, medical, and military clean rooms and have grown in popularity for use in portable residential air cleaners.

HEPA filters are mechanical filters, which utilize convention methods of impingement. HEPA media has been traditionally defined as an extended-surface dry-type filter having a minimum particle removal efficiency of 99.97% for all particles of 0.3 micron diameter and larger. To qualify as a "true" HEPA, the filter must allow no more than 3 particles out of 10,000 to penetrate the filtration media. More recently, filters made in the same physical style using less efficient filter paper are being referred to as HEPA filters or "HEPA-type" filters. Their actual efficiency may be 55% or less at 0.3 microns. These versions of the original HEPA filter have higher airflow, lower efficiency, and lower cost than their original version. The true HEPA has very high pressure drop performance and both versions require prefiltration for maximum life cycle.

Pros - The major advantages of the original HEPA filters include (1) extremely high efficiency, which actually increases with use, (2) a long maintenance-free life cycle of up to five years when used with proper pre-filtration, and (3) with other new and emerging technologies, HEPA is still the most relied upon technology used in critical environments today.

Cons - A disadvantage of HEPA filters is that the need for a powerful fan leads to increased noise and energy costs compared to less efficient filtration systems, and replacement filters are generally quite expensive (retail prices range from $50 to $100, depending on size).

Synthetic (Hybrid Filters)

A synthetic filters is a hybrid of a mechanical filter and electrostatic filter or an ion generator in an integrated single filter. An example of a hybrid filter is the "electret" media filter. The media filter, made from synthetic fibers, is inherently negatively charged in the manufacturing process and retains a charge which attracts airborne particles that are trapped and retained within the fibers in the conventional methods of impingement (used by HEPA filters).

In some cases, ionizers are used to increase the efficiency of the trapped media. Please see the "Ionizers" section below.

Pros - The advantages of a synthetic filter are (1) the filter's relatively low energy cost, and (2) they can also be relatively quiet.

Cons -The disadvantages are (1) high maintenance costs due to frequent need to replace filters, (2) if the synthetic filter uses an ionizer, see cons of "Ionizers" below, and (3) efficiency dramatically decreases with use. See chart 1 below.

Source: Hanley JT, Ensor DS, Sparks LE,. Aerosol filtration efficiency of in-duct air cleaners

Ionizers (Negative Ion Generator)

The simplest form of air cleaner is the negative ion generator. A variety of ionizers are available. The simplest types use static charges to remove particles from indoor air. They operate by charging the particles in a room, which become attracted to and deposit on walls, floors, table tops, curtains, occupants, etc., where they may cause soiling problems. With these simple types, 100% of the negative ions are released into the room.

More advanced units are theoretically designed to reduce soiling in a room. They generate negative ions within a space through, which air flows, causing particles entrained in the air to become charged. The charged particles are then drawn back into the cleaner by a fan, where they are collected on an electrostatically charged panel filter. Although this process involves a filter, 10-60% of the negative ions will still be released in the room. In other ionizers, a stream of negative ions is generated in pulses, and negatively charged particles are drawn passively back to the ionizer, which contains a positively charged sleeve or cover.

Pros - The advantages are that they are (1) relatively inexpensive, (2) no noise, (3) small - does not take up space, (4) and finally, no maintenance.

Cons - Unfortunately there are many disadvantages. (1) Ionizers do not trap particles - particles are charge and either stick to surface or large particle will drop to the ground. If these particles are not properly vacuumed, they will become airborne again through people casually in the room. (2) Room walls and furnishings can become soiled. (3) The EPA has expressed concern that these charged particles are more easily trapped in the human respiratory system. (3) Finally, they can produce ozone, either as a by-product of use or intentionally.