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E.P.A. on whole house air filtration


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 ventilate a home with clean outdoor air. But opportunities for ventilation may be limited by weather conditions or by contaminants in the outdoor air.

If the usual methods of addressing indoor air pollution are insufficient, air-cleaning devices may be useful. Air filters and other air-cleaning devices are designed to remove pollutants from indoor air. Some are installed in the ductwork of a home’s central heating, ventilating, and air-conditioning (HVAC) system to clean the air in the entire house. Portable room air cleaners can be used to clean the air in a single room or in specific areas, but they are not intended to filter the air in the whole house. Air-cleaning devices are categorized by the type of pollutants — particulate and gaseous — that the device is designed to remove or destroy.

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

Mechanical air filters, such as high efficiency particulate air (HEPA) filters, remove particles by capturing them on filter materials. Most mechanical air filters are good at capturing larger airborne particles — such as dust, pollen, some mold spores, and animal dander — and particles that contain dust mite and cockroach allergens. But because these particles settle rather quickly, mechanical air filters are not very good at completely removing them from indoor areas.

Electronic air cleaners, such as electrostatic precipitators, use a process called electrostatic attraction to trap particles. Ion generators, or ionizers, disperse charged ions into the air. These ions attach to airborne particles, giving them a charge so they can attach to nearby surfaces such as walls or furniture, or to one another, and settle faster. However, some electronic air cleaners can produce ozone, a lung irritant.

Another type of air-cleaning device is a gas-phase filter designed to remove gases and odors by either physical or chemical processes.

Gas-phase air filters remove gaseous pollutants by using a material called a sorbent, such as activated carbon, to adsorb pollutants. Because these filters are targeted at one or a limited number of gaseous pollutants, they will not reduce concentrations of pollutants for which they were not designed. None are expected to remove all of the gaseous pollutants in the air of a typical home. Gas-phase filters are much less common in homes than are particle air filters. One reason may be the filter can become overloaded quickly and may need to be replaced often.

Three types of air cleaners on the market are designed to deactivate or destroy indoor air pollutants: ultraviolet germicidal irradiation (UVGI) cleaners, photocatalytic oxidation (PCO) cleaners, and ozone generators sold as air cleaners.

UVGI cleaners use ultraviolet radiation from UV lamps that may destroy biological pollutants such as viruses, bacteria, and molds that are airborne or growing on HVAC surfaces (e.g., cooling coils, drain pans, or ductwork). UVGI cleaners should be used with, but not as a replacement for, filtration systems. 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 exposures than a typical home unit provides.

PCO cleaners use UV lamps along with a substance, called a catalyst, that reacts with the light. These cleaners are designed to destroy gaseous pollutants by changing them into harmless products, but they are not designed to remove particulates. The usefulness of PCO cleaners in homes is limited because currently available catalysts are ineffective in destroying gaseous pollutants in indoor air.

Ozone generators use UV lamps or electrical discharges to produce ozone that reacts with chemical and biological pollutants and transforms them into harmless substances. Ozone is a potent lung irritant, which in concentrations that do not exceed public health standards, has little potential to remove indoor air contaminants. Thus ozone generators are not always safe and effective in controlling indoor air pollutants.

Portable air cleaners generally contain a fan to circulate the air and use one or more of the air-cleaning technologies discussed above. They may be an option if a home is not equipped with a furnace or a central air-conditioning system. Many portable air cleaners have moderate to large air delivery rates for small particles. However, most of the portable air cleaners on the market do not have high enough air delivery rates to remove large particles such as pollen and particles that contain dust mite and cockroach allergens from typical-size rooms.

Several other factors should be considered when making decisions about using air-cleaning devices.

* Installation: In-duct air-cleaning devices have certain installation requirements that must be met, including sufficient access for inspection during use, repairs, and maintenance.

* Major costs: These costs include the initial purchase price and the cost of maintenance (such as cleaning or replacing filters and parts) and operation (electricity).

* Odors: Air-cleaning devices designed to remove particles cannot control gases and some odors. The odor and many of the carcinogenic gas-phase pollutants from tobacco smoke, for example, will remain.

* Soiling of walls and other surfaces: Typical ion generators are not designed to remove from the air the charged particles that they generate. These charged particles may settle on, and soil, walls and other room surfaces.

* Noise: Noise may be a problem with portable air cleaners that contain fans. Portable air cleaners that do not have fans tend to be much less effective than units that have them.

The ability to remove some airborne pollutants, including microorganisms, is not, in itself, an indication of an air-cleaning device’s ability to reduce adverse health effects from indoor pollutants. Although air-cleaning devices may help reduce levels of smaller airborne particles including those associated with allergens, they may not reduce adverse health effects, especially in sensitive populations such as children, people who have asthma and allergies, and the elderly. For example, the evidence is weak that air-cleaning devices are effective in reducing asthma symptoms associated with small airborne particles such as those that contain cat and dust mite allergens. There are no studies linking the use of gas-phase filtration, UVGI systems, or PCO systems in homes to reduced health symptoms in sensitive populations.

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