Mother Earth Living

A Guide to Air Filters

Armed with a little knowledge, you can find a filter that helps to create a healthier, cleaner indoor environment for you and your loved ones.
By Dan Chiras
September/October 2001
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Particulate filters are the best way to trap pollen and other allergens.
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According to John and Lynn Bower, founders of the Healthy House Institute, virtually every home in the country is plagued by indoor air pollution ranging from mild to severe. Filtration is an effective solution, but only after preventive measures are taken to eliminate as much indoor air pollution as possible. Although the American Lung Association and the Environmental Protection Agency recommend air filtration, they say that controlling allergy-causing pollution sources and ventilation are more important. Once these options are exhausted, it is time to think about air filtration.

Portable and Whole-House Filters

While there are many models of air filters, only two basic types of filtration systems exist: portable and whole-house. Portable filters cost between $80 and $500. Whole-house filters cost as little as $1 and as much as $1,000 (including installation).

Portable filters purify the air in a ­single room, and work best when run continuously in a room that is closed off from the rest of the house. Here’s the rub: If you have to seal off a room, portable filters won’t be of much value during the heating and cooling season.

Whole-house filters are designed to clean air in an entire house. One of the easiest and cheapest ways to install a whole-house filter is to insert a filter or several filters into the duct system or onto the heat registers of a forced-air heating system or a central air-conditioning system. Air circulating through the system passes through the filters, which remove impurities. For the system to work optimally, the fan must run at least several hours each day. In addition, you will need a special air filter designed to trap gases and/or particles. Ordinary furnace and air conditioner filters are insufficient, as they’re typically designed to remove only very large particles.

Even better is an air-filtration unit installed in a whole-house mechanical ventilation system. Ventilation systems are rare in older homes but can be installed, albeit for a hefty sum. In houses equipped with a ventilation system that works in conjunction with a forced-air heating/ central air-conditioning system, a single air filter can do double duty. If properly placed, this filter can cleanse fresh air entering the house through the ventilation system and help remove pollutants from air circulating through the heating and/or central air-conditioning system.

Particulate Filters

Whole-house air filtration systems are the most effective, but what kind of filter is best? Particulate filters physically trap suspended particles such as mold, pollen, dust, dander, bacteria, and soot from tobacco smoke or combustion sources such as fireplaces and furnaces. Particulate filters come in three basic types, ranging from $1 to $15. The most commonly used filters contain fiberglass or other synthetic fibrous materials. Some models contain a permanently charged plastic film or fiber material that removes particulates from air passing through the filter. Finally, there’s the panel filter, containing a pleated fabric material. All of these remove about 10 to 40 percent of the large particulates from room air and must be replaced every month or so.

Electronic particulate filters can scrub up to 95 percent of large particulates—including dust, dirt, and smoke—from a room. In most electronic units, particulates passing through the filter are given an electrical charge. These charged particles, in turn, stick to oppositely charged surfaces in the room itself. Particulates removed from room air are deposited on walls, ceilings, floors, drapes, tabletops, pets, and people, which may be a nuisance, and the particles could become airborne again.

These electronic models, also known as ion generators, may be equipped with a fan and a mechanical filter. Ion generators cost between $50 and $150. Because they’re portable, you will need one in nearly every room of the house. Filters in most of these units require periodic replacement, and the units themselves may require occasional cleaning.

Yet another electronic filter is the electrostatic precipitator. Particulates passing through this unit are given a charge and then captured by the air cleaner on a series of flat plates or cells inside the unit, or on charged-media surfaces. Electrostatic precipators are available in portable and whole-house models and are the most expensive particulate filters on the market, running from $150 to more than $1,000. They remove up to 95 percent of dirt, dust, and smoke, but only about 10 percent of smaller particles. The collector plates, which trap the charged particles, can be cleaned in a dishwasher or a bathtub, and can be reused indefinitely.

Gas Filters

Gaseous pollutants from newly installed carpeting, paint, furniture, and wood sheathing must be removed by a gas filter, which typically contains an adsorbent material, most commonly activated carbon or activated alumina. These porous substances contain a huge surface area to which gas molecules adhere as they pass through.

Activated carbon filters, the most common, work best on larger volatile organic molecules such as benzene and acetone, which are not as prevalent as formaldehyde in indoor air. For formaldehyde problems, filters that contain a “treated” activated carbon substrate are best. Gas filters featuring activated alumina impregnated with potassium permanganate also destroy formaldehyde molecules through oxidation. For best results, some manufacturers sell gas filters containing activated carbon and activated alumina. These also filter out particulates, but dust can clog them, so a mechanical filter is usually placed upstream.

Many manufacturers sell hybrid air cleaners equipped with both gas and mechanical filters.

Judging Effectiveness

Unfortunately, there are no government standards for reporting efficiency. As a result, consumers must rely on manufacturers’ claims or assessments made by the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) or the Association of Home Appliance Manufacturers (AHAM).

In February 2000, ASHRAE announced a new standard for rating particulate filters. The test now measures efficiency for a variety of particle sizes, so there can be no misleading claims. ASHRAE has also developed a minimum efficiency reporting value (MERV). According to Charles Rose, a member of ASHRAE’s technical committee, “The MERV is a number from one to sixteen to compare air filters on the basis of the percent of dust they remove from the air. The higher the number, the more efficient the filter.”

Portable air filters are often rated by AHAM. Its label on a product lists the clean air delivery rate (CADR), a measure of the effectiveness of particulate ­filters in removing pollutants such as tobacco smoke, dust, and pollen. Ac­cord­ing to AHAM, CADR is a measure of the amount of air filtered by a unit, measured in cubic feet per minute, for a specific material. For example, if an air cleaner has a CADR of 380 for tobacco smoke, it reduces levels to the same concentration as would be achieved by adding 380 cubic feet of smoke-free air every minute. The higher the CADR, the better the filter.

The Asthma and Allergy Foundation of America recommends buying a model that recirculates eight or ten room volumes per hour. “This doesn’t guarantee completely clean air,” it states, “but it will be much cleaner than with systems that recirculate less.”

If you suffer from asthma, you should also purchase a system that removes more than 90 percent of all particles larger than 0.3 microns in diameter. Look for a unit with a HEPA filter, but be cautious: some filters claim to be HEPAs, but are only half as efficient.


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