Removal of Particles

Air filters are designed to remove particulate pollutants from indoor air. Their performance depends not only on the airflow rate through the filter media and the filter efficiency, but also on factors such as the:

·    Particle size and mass.

·    Amount of dust on the air filter.

·    Airflow rate, velocity, path, and resistance through the filter media.

·    Mixing of air leaving the filter with the air in the room.

·    Leakage rate of air that bypasses the air filter.

Types of Particle-Removal Air Filters

Two general types of particle removal air-cleaning devices are available: mechanical air filters and electronic air cleaners. They are classified by the method employed to remove particles of various sizes from the air.

Mechanical air filters installed in a central HVAC system or in a portable air cleaner capture particles on filter media. Particles either become trapped in the fibers of the filter or stick to the filter because of an electrostatic charge. Mechanical air filters come in two major types: flat and pleated.

Flat or panel filters generally consist of coarse glass fibers, coated animal hair, vegetable fibers, synthetic fibers (such as polyester or nylon), synthetic foams, metallic wools, or expanded metals and foils. The filter media may be treated with a viscous substance, such as oil, that causes particles to stick to the fibers. Flat filters also may be made of three types of permanently electrostatically charged material: resin wool, a plastic film or fiber called “electret,” or an electrostatically sprayed polymer. Their static charge attracts and captures particles. The fibers of electret filters are somewhat larger than the fibers of other flat filters, resulting in relatively low pressure drop and greater efficiency in filtering smaller particles. The efficiency of electret filters decreases as the media become loaded with particles.

Pleated or extended surface filters are generally more efficient than flat filters in capturing respirable particles. Pleating the filter medium increases surface area, reduces air velocity, and allows the use of smaller fibers and increased packing density of the filter without a large drop in airflow rate. A wire frame in the form of a pocket or V-shaped cardboard separators may be used to maintain the pleat spacing. The media used in pleated filters are fiber mats, bonded glass fibers, synthetic fibers, cellulose fibers, wool felt, and other cotton-polyester material blends.

High efficiency particulate air (HEPA) filters are a type of extended surface filter. HEPA filters usually are made of submicron glass fibers and have a texture similar to blotter paper. They also have a larger surface area and remove respirable particles more efficiently than pleated filters.

Electronic air cleaners use a process called electrostatic attraction to trap charged particles. There are two types of electronic air cleaners: electrostatic precipitators and ion generators.

Electrostatic precipitators have an ionization section and a collecting plate section, both of which use an external power source. The air cleaner draws air through the ionization section, where particles obtain an electrical charge. The charged particles accumulate on a series of flat plates called a collector that is oppositely charged. Cleaning the collector plates is essential to maintaining adequate performance.

Ion generators, or ionizers, disperse charged ions into the air, similar to an electrostatic precipitator, but ionizers do not have collecting plates. They produce ions by means of corona discharge or UV light. The ions attach to particles and give them a charge so they adhere to nearby surfaces such as walls, furniture, and draperies, or combine with other particles and settle on room surfaces. Ion generators are the simplest form of electronic air cleaner and come in tabletop, portable, and ceiling mounted units.

Like mechanical filters, electronic air cleaners can be installed in HVAC systems or used in portable units. Although electronic air cleaners remove small particles, they do not remove gases or odors. And because electronic air cleaners use high voltage to generate ionized fields, they can produce ozone, either as a by-product or by design.⁸ Residential indoor ozone concentrations may be affected by the amount of ozone emitted by electronic air cleaners, which varies among models. Even at concentrations below public health standards, ozone reacts with chemicals emitted by such common indoor sources as household cleaning products, air fresheners, deodorizers, certain paints, polishes, wood flooring, carpets, and linoleum. The chemical reactions produce harmful by-products that may be associated with adverse health effects in some sensitive populations. The ozone reaction by-products that may result include ultrafine particles (smaller than 0.1 µm in diameter), formaldehyde, ketones, and organic acids.^{8, 9, 10} Concerns about ozone and ozone-generating devices are discussed in the EPA document "Ozone Generators that are Sold as Air Cleaners," posted on the EPA Website at www.epa.gov/iaq/pubs/ozonegen.html.

Defining Efficiency and Effectiveness

To choose air-cleaning devices and use them properly, it is important to understand the difference between efficiency and effectiveness. The efficiency of an air-cleaning device, usually expressed as a percentage, is a measure of its ability to remove airborne particles or gaseous pollutants from the air that passes through it. The effectiveness of an air-cleaning device is a measure of its ability to reduce airborne particle or gaseous pollutant concentrations in an occupied space.

The efficiency of air filters used in ducts of HVAC systems or in portable air cleaners varies based on the airflow rate and the particulate matter load. The effectiveness of an air-cleaning device in removing pollutants from an occupied space depends on three factors: its efficiency, the amount of air being filtered, and the path that the clean air follows after it leaves the filter. For example, a filter may remove 99 percent of the particles from the air that passes through it (i.e., have 99 percent efficiency). However, if the airflow rate through the filter is only 10 cubic feet per minute (cfm) in a typical room of approximately 1,000 cubic feet (e.g., 10’ x 12’ x 8’), the filter will be relatively ineffective at removing particles from the air (i.e., 10 times less effective than if the airflow rate were 100 cfm).

Higher efficiency filters remove larger and smaller airborne particles more efficiently. Homeowners should take care to properly install them in HVAC systems and make sure that leakage of air bypassing the filter is minimized. The higher a filter’s efficiency, the more attention must be paid to its sealed installation because increased airflow resistance is more likely to create leaks. Air filter effectiveness may be substantially reduced if air leaks through a poorly installed filter frame and its holding system.^{11, 12} Leakage of air bypassing a HEPA filter used in a portable, stand-alone unit may also reduce the filter’s expected efficiency. Effectiveness may be decreased if air exiting an exhaust grille of the HVAC system is not well mixed with room air before re-entering the system. This situation can occur if air return and intake vents are too close together.

Air Filters - Available Guidance for Their Comparison

Several standardized methods have been developed to measure the efficiency of different types of air filters installed in the ductwork of HVAC systems. They can be used to compare the performance of air filters made by different companies. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and the Institute of Environmental Sciences and Technology (IEST) have published voluntary standards for rating air filters. The IEST is now the recognized standard-setting organization for the former Military Standard 282 developed by the U.S Department of Defense for rating HEPA filters. The standards do not rate the air filters’ effectiveness; rather, they compare the performance of various filters.

Particle removal efficiency can be assessed by four standard methods: the weight arrestance test, atmospheric dust spot efficiency test, dioctyl phthalate (DOP) penetration test, and particle size removal efficiency (PSE) test.

* ASHRAE Standard 52.1.2992, Gravimetric and Dust-Spot Procedures for Method of Testing Air-Cleaning Devices Used in General Ventilation for Removing Particulate Matter was withdrawn in Spring 2009. Information previously found in this standard is now included via Addendum B to ANSI/ASHRAE Standard 52.2, Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size. The addendum mandates calculation of weight arrestance for filters with Minimum Efficiency Reporting Values (MERVs) of 1 to 4 and atmospheric dust spot efficiency for filters with MERVs of 5 to 6.

The weight arrestance test,¹³ defined in ASHRAE Standard 52.1-1992,* is generally used to evaluate low efficiency filters designed to remove the largest and heaviest particles. These filters are commonly used in residential furnaces and air-conditioning systems to protect system components, or as upstream filters to protect higher efficiency filters. In this test, a synthetic dust is fed into the air cleaner and the percentage by weight of the dust the filter traps, called “arrestance,” is determined. The weight arrestance test may be of limited value in assessing the removal of smaller, respirable particles because particles in the test dust are generally larger than those that can be inhaled deeply into the lungs.

The atmospheric dust spot efficiency test,¹³ also defined in ASHRAE Standard 52.1-1992,* is generally used to rate medium-efficiency filters in removing fine airborne dust particles that can soil walls and other interior surfaces. A naturally occurring atmospheric dust is fed into the air cleaner to test its ability to reduce soiling of a clean paper target as an indication of the cleaner’s capability to remove fine particles from the air.

The DOP penetration test,¹⁴ described in the IEST-RP CC001.4 test method, is used to rate true HEPA filters. A DOP cloud of uniform 0.3 µm particles is fed into the filter. The concentration of penetrating smoke measured upstream and downstream of the filter determines the filter efficiency, or the percentage of particles the filter removes.

The PSE test,¹⁵ described in ASHRAE Standard 52.2-2007, provides a composite minimum efficiency for removing particles of specific size by filters incrementally loaded with synthetic dust. The PSE test method does not eliminate the need for DOP penetration and arrestance testing. Very low-efficiency air filters, such as furnace filters, must also be tested in accordance with the weight arrestance method. The composite minimum efficiency values are averaged and used to determine the air cleaner’s minimum efficiency reporting value (MERV). The MERV ranges from a low of 1 to a high of 20. The PSE test may not be appropriate for evaluating electronic air cleaners because the dust used contains conductive carbon, which may cause electrical shorting and thus compromise the effectiveness of these devices and alter their MERV. The dust-loading procedure may also affect the efficiency of electrostatically charged filters.