Guide to Air Filters and Air Cleaners in the Home - How to Select an Air Filtering Device for Your Home

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Guide to Air Filters and Air Cleaners in the Home

Guide to Air Cleaners in the Home

EPA 402-F-08-004, May 2008

A PDF Version of this document is available (PDF, 12 pp., 238 K, about PDF)


See also: Residential Air Cleaners (Second Edition) A Summary of Available Information
HTML Version | PDF Version (PDF, 36 pp., 606 K)

Introduction

Our houses are filled with items that release chemicals, such as formaldehyde from carpeting and insulation, dust and dander from pets, and organic compounds from cleaners and fresh paints. Dust and pollen from outdoors also enters the home.  These all aggravate allergies, respiratory and other health problems. These all contribute to "Sick Building Syndrome".  Indoor air pollution is among the top five environmental health risks.

Usually the best way to address this risk is to control or eliminate the sources of pollutants, and to ventilate a home with cleaner outdoor air. The ventilation method may, however, is only as good as the outdoor air. In many cases, an air cleaning device, commonly called an air flter, may be useful. Air cleaning devices are intended to remove pollutants from indoor air. Some air cleaning devices are designed to be installed in the ductwork of a home’s central heating, ventilating, and air-conditioning (HVAC) system to clean the air in the whole house. Portable room air cleaners can be used to clean the air in a single room or specific areas, but they are not intended for whole-house filtration. This page will provide information on different types of air cleaning devices and how they work.

Indoor Air Pollutants

Pollutants that can affect air quality in a home fall into the following categories:

Understanding the Types of Air Cleaning Devices

Before deciding whether to use an air cleaning device, several questions should be considered:

What Types of Pollutants Can an Air Cleaner Remove?

There are several types of air cleaning devices available, each designed to remove certain types of pollutants.

Particle Removal

Two types of air cleaning devices can remove particles from the air — mechanical air filters and electronic air cleaners.

Mechanical air filters remove particles by capturing them on filter materials.

High efficiency particulate air (HEPA) filters are in this category.

Electronic air cleaners such as electrostatic precipitators use a process called electrostatic attraction to trap charged particles. They draw air through an ionization section where particles obtain an electrical charge. The charged particles then accumulate on a series of flat plates called a collector that is oppositely charged. Ion generators, or ionizers, disperse charged ions into the air, similar to the electronic air cleaners but without a collector. These ions attach to airborne particles, giving them a charge so that they attach to nearby surfaces such as walls or furniture, or attach to one another and settle faster.

Gaseous Pollutant Removal

Gas-phase air filters remove gases and odors by using a material called a sorbent, such as activated carbon, which adsorbs the pollutants. These filters are typically intended to remove one or more gaseous pollutants from the airstream that passes through them. Because gas-phase filters are specific to one or a limited number of gaseous pollutants, they will not reduce concentrations of pollutants for which they were not designed. Some air cleaning devices with gas-phase filters may remove a portion of the gaseous pollutants and some of the related hazards, at least on a temporary basis. However, none are expected to remove all of the gaseous pollutants present in the air of a typical home. For example, carbon monoxide is a dangerous gaseous pollutant that is produced whenever any fuel such as gas, oil, kerosene, wood, or charcoal is burned, and it is not readily captured using currently available residential gas-phase filtration products.

Pollutant Destruction

Some air cleaners use ultraviolet (UV) light technology intended to destroy pollutants in indoor air. These air cleaners are called ultraviolet germicidal irradiation (UVGI) cleaners and photocatalytic oxidation (PCO) cleaners. Ozone generators that are sold as air cleaners intentionally produce ozone gas, a lung irritant, to destroy pollutants.

Ozone is a lung irritant that can cause adverse health effects.

The chart below provides a brief summary of air cleaning technologies and the pollutants they are designed to control.

Air Cleaning Technologies Pollutants Addressed Limitations
Filtration Air filters Particles Ineffective in removing larger particles because most settle from the air quickly and never reach filters.
Gas-phase filters Gases Used much less frequently in homes than particle air filters. The lifetime for removing pollutants may be short.
Other Air Cleaners UVGI Biologicals Bacterial and mold spores tend to be resistant to UV radiation and require more light or longer time of exposure, or both, to be killed.
PCO Gases Application for homes is limited because currently available catalysts are ineffective in destroying gaseous pollutants from indoor air.
Ozone generators Particles, gases, biologicals Sold as air cleaners, they are not always safe and effective in removing pollutants. By design, they produce ozone, a lung irritant.

In addition to understanding the different types of air cleaning devices, consumers should consider their performance, as explained in the next section.

How is the Performance of an Air Cleaner Measured?

There are different ways to measure how well air cleaning devices work, which depend on the type of device and the basic configuration. Air cleaning devices are configured either in the ductwork of HVAC systems (i.e., in-duct) or as portable air cleaners.

In-duct Particle Removal

Most mechanical air filters are good at capturing larger airborne particles, such as dust, pollen, dust mite and cockroach allergens, some molds, and animal dander. However, because these particles settle rather quickly, air filters are not very good at removing them completely from indoor areas. Although human activities such as walking and vacuuming can stir up particles, most of the larger particles will resettle before an air filter can remove them.

Consumers can select a particle removal air filter by looking at its efficiency in removing airborne particles from the air stream that passes through it. This efficiency is measured by the minimum efficiency reporting value (MERV) for air filters installed in the ductwork of HVAC systems. The American Society of Heating, Refrigerating and Air-Conditioning Engineers, or ASHRAE developed this measurement method. MERV ratings (ranging from a low of 1 to a high of 20) also allow comparison of air filters made by different companies.

Some residential HVAC systems may not have enough fan or motor capacity to accommodate higher efficiency filters. Therefore, the HVAC manufacturer’s information should be checked prior to upgrading filters to determine whether it is feasible to use more efficient filters. Specially built high performance homes may occasionally be equipped with true HEPA filters installed in a properly designed HVAC system.

There is no standard measurement for the effectiveness of electronic air cleaners. While they may remove small particles, they may be ineffective in removing large particles. Electronic air cleaners can produce ozone — a lung irritant. The amount of ozone produced varies among models. Electronic air cleaners may also produce ultrafine particles resulting from reaction of ozone with indoor chemicals such as those coming from household cleaning products, air fresheners, certain paints, wood flooring, or carpets. Ultrafine particles may be linked with adverse health effects in some sensitive populations.

In-duct Gaseous Pollutant Removal

Although there is no standard measurement for the effectiveness of gas-phase air filters, ASHRAE is developing a standard method to be used in choosing gas-phase filters installed in home HVAC systems. Gas-phase filters are much less commonly used in homes than particle air filters. The useful lifetime of gas-phase filters can be short because the filter material can quickly become overloaded and may need to be replaced often. There is also concern that, when full, these filters may release trapped pollutants back into the air. Finally, a properly designed and built gas-phase filtration system would be unlikely to fit in a typical home HVAC system or portable air cleaner.

In-duct Pollutant Destruction

UVGI cleaners may not reduce allergy or asthma symptoms.

There is no standard measurement for the effectiveness of UVGI cleaners. Typical UVGI cleaners used in homes have limited effectiveness in killing bacteria and molds. Effective destruction of some viruses and most mold and bacterial spores usually requires much higher UV exposure than is provided in a typical home unit. Furthermore, dead mold spores can still produce allergic reactions, so UVGI cleaners may not be effective in reducing allergy and asthma symptoms.

There is no standard measurement for the effectiveness of PCO cleaners. The use of PCO cleaners in homes is limited because currently available catalysts are ineffective in destroying gaseous pollutants from indoor air. Some PCO cleaners fail to destroy pollutants completely and instead produce new indoor pollutants that may cause irritation of the eyes, throat, and nose.

Portable Air Cleaners

Portable air cleaners generally contain a fan to circulate the air and use one or more of the air cleaning devices discussed above. Portable air cleaners may be moved from room to room and used when continuous and localized air cleaning is needed. They may be an option if a home is not equipped with a central HVAC system or forced air heating system.

Portable air cleaners can be evaluated by their effectiveness in reducing airborne pollutants. This effectiveness is measured by the clean air delivery rate, or CADR, developed by the Association of Home Appliance Manufacturers, or AHAM. The CADR is a measure of a portable air cleaner’s delivery of contaminant-free air, expressed in cubic feet per minute. For example, if an air cleaner has a CADR of 250 for dust particles, it may reduce dust particle levels to the same concentration as would be achieved by adding 250 cubic feet of clean air each minute. While a portable air cleaner may not achieve its rated CADR under all circumstances, the CADR value does allow comparison across different portable air cleaners.

Many of the portable air cleaners tested by AHAM have moderate to large CADR ratings for small particles. However, for typical room sizes, most portable air cleaners currently on the market do not have high enough CADR values to effectively remove large particles such as pollen, dust mite, and cockroach allergens. Some portable air cleaners using electronic air cleaners might produce ozone, which is a lung irritant. AHAM has a portable air cleaner certification program, and provides a complete listing of all certified cleaners with their CADR values on its Website at www.cadr.orgexiting epa.

Will Air Cleaning Reduce Adverse Health Effects?

The ability to remove particles, including microorganisms, is not, in itself, an indication of the ability of an air cleaning device to reduce adverse health effects from indoor pollutants. The use of air cleaning devices may help to reduce levels of smaller airborne allergens or particles. However, air cleaners may not reduce adverse health effects completely in sensitive population such as children, the elderly, and people with asthma and allergies. For example, the evidence is weak that air cleaning devices are effective in reducing asthma symptoms associated with small particles that remain in the air, such as those from some airborne cat dander and dust mite allergens. Larger particles, which may contain allergens, settle rapidly before they can be removed by filtration, so effective allergen control measures require washing sheets weekly, frequent vacuuming of carpets and furniture, and dusting and cleaning of hard surfaces. (For more on allergen control, visit www.epa.gov/asthma). There are no studies to date linking gas-phase filtration, UVGI, and PCO systems in homes to reduced health symptoms in sensitive populations.

Additional Factors to Consider

When making decisions about using air cleaning devices, consumers should also consider:

Conclusion

Indoor air pollution is among the top five environmental health risks. The best way to address this risk is to control or eliminate the sources of pollutants, and to ventilate a home with clean outdoor air. The ventilation method may, however, be limited by weather conditions or undesirable levels of contaminants in outdoor air. If these measures are insufficient, an air cleaning device may be useful. While air cleaning devices may help to control the levels of airborne allergens, particles, or, in some cases, gaseous pollutants in a home, they may not decrease adverse health effects from indoor air pollutants.

Additional Information

More in-depth analysis of air cleaners is available in the EPA technical document "Residential Air Cleaners (Second Edition) A Summary of Available Information," EPA 402-F-09-002, Revised August 2009