Whether air filters — used with any type of household heating, ventilating, and air conditioning system or as part of a portable room unit — are necessary tools in the arsenal against allergens in the home has been part of an extended debate among asthma and allergy experts. While allergens are found mostly in surface dust, questions have arisen as to whether removing those materials from the air in a home make any appreciable difference. But others have argued that any reduction in allergens in the home environment is a positive step forward for the asthmatic child — especially those homes with pets.

Using filters in the home may also have its benefits when opening windows at any time during the year is not an option. When the “fresh air” outside contains, for instance, pollens and molds, filtered indoor air could provide your child with relief from his asthma and allergy symptoms.

But getting the full benefit of filtered air requires some diligence on your part — including cleaning and maintenance of filters and air cleaning units — to make sure your child gets the full benefit from the products you do use. And, as the asthma and allergy experts advise, air filtering should not replace a thorough cleaning to remove allergens found in dust in your home.

Household Heating and Cooling System Filters

Your child's respiratory defense system, which includes her nose, sinuses, and windpipes, are designed to filter out particles she breathes that are about three to five microns in size (or about 5 to 7 percent the size of a human hair). However, surrounding her are even tinier particles — less than 1 micron in size — that make up to 99 percent of the particles flowing in the air through your home.

Breathing in these particles could pose problems for your asthmatic child — such as allergic reactions or asthma attacks. As part of a strategy to lower your home's particle count, air filters that work within your home's heating and cooling system may be considered.

These filters generally have a rating from one to twelve: The higher the MERV rating, the more efficient the filter is and the more particles (such as various allergens and mold spores) it can filter. High-efficiency furnace filters with at least a MERV rating of ten or higher will be more efficient in capturing indoor pollutants.

Filter performance is influenced by the size and density of the material used, along with the size and volume of the particles being filtered. Generally, the more efficient the filter is at capturing small particles and allergens, the more expensive it is.

The types of filters available for forced air heating and cooling systems include:

• Less expensive panel filters that have a layer of fibers, which are often fiberglass, that are covered by a honeycomb-shaped grille. The primary function of these filters is to capture larger particles from the air and to protect the forced air system's fan, while decreasing the amount of dust on heating and cooling coils.

• Reusable filters, which are designed to be washed and reused, are generally less ineffective at capturing small particles.

• Pleated filters that are not electrostatically charged are made of a variety of materials and are somewhat more expensive — but more efficient — than fiberglass filters because the pleats provide a greater surface area to trap air contaminants.

• Electrostatically charged pleated filters are designed to assist in attracting and holding microscopic particles such as dust, pollen, mold, pet dander, and smoke.

• High Efficiency Particulate Air (HEPA) filters are designed to remove submicron particles (99.97 percent of all particles 0.3 microns in diameter or larger) with high efficiency. The filter, made of fine submicron glass fibers within a matrix of larger fibers, generally will not fit within forced air heating and air conditioning systems found in most homes. Instead, it will need a separate setup with a fan and filter.

• A step beyond using the furnace filters is a whole-house electronic air filtration system that attaches to — and is distributed through — your home's heating and cooling system. It requires professional installation. While it generally has a high MERV rating, it is far more expensive than purchasing individual filters and could require frequent (for example, weekly to monthly) cleaning to maintain its effectiveness.

Whatever air filters you use to help protect against allergens, be extra diligent about keeping those filters clean. They need to be changed at regular intervals — from about once a month for the inexpensive models to about every two or three months for pleated varieties. Labels on the products should indicate time frames.

To remind yourself to change the filter, schedule those changes to coincide with other household activities, such as paying a monthly bill, or post a sticky note near the thermostat or on the outside of the heating and/or cooling unit — so you'll be less likely to forget.

Also, filters are available that can be cut to size and placed over room vents — such as those in your child's bedroom — to provide another layer of filtration. However, carefully follow manufacturer's directions on installation on selected vents so you don't accidentally strain your home's heating and/or cooling system.

Portable Air Filtration Devices

For filtering air just within a room or small section of a home, portable devices are available on the market — either as tabletop models or floor units. Since your child is likely to spend at least a third of his day in the bedroom, that would be a good location to keep it if you purchase the air filter.

The filters fall into several categories:

• HEPA filters, which are designed to remove 99.97 percent of airborne pollutants (including tobacco smoke, household dust, and pollen) 0.3 microns or larger from air that go through the filter.

• ULPA filters, which are ultra-HEPA filters that are designed to capture 99.999 percent of all airborne particles 0.3 microns or smaller that pass through the filter — including tobacco smoke, household dust, and pollen.

• Electronic air filters, such as electrostatic filters, which use a static charge on the filter that gets airborne particles to stick to it.

• Ion generators, which use electrical charges to take allergen particles out of the air. The ions stick to particles, causing them to attach to nearby surfaces, such as walls or floors, or to a filter with a magnetic-type attraction.

• Gas-phase filters, which cannot eliminate allergenic particles but remove odors and non-particulate pollution such as cooking gas, gases emitted by paint or building materials, or perfumes or fragrances.

• Hybrid filters, which contain two or more components of the particle-removing devices.

Another filtration device — ozone generators — have been marketed to clear the air of various particles. However, many consumer groups have opposed use of these types of devices because of the high amount of ozone they emit in the air — which could end up irritating an asthmatic child's lungs.

The other devices, such as electronic air filters and ion generators, for the most part do produce what is called an “ozone byproduct.” This byproduct should be similar to what other household devices in your home — such as ceiling fans and hair dryers — produce. The level of ozone byproduct produced by most of these air filtration devices should be well within the range of current federal standards — if the products are properly installed, and kept continually clean and well maintained.

The Association of Home Appliance Manufacturers has created one rating system for air cleaners called the Clean Air Delivery Rates (CADR) (www.cadr.org). Three CADR numbers are usually found on the back of an air cleaner's box: one for tobacco smoke, one for pollen, and one for dust. The CADR indicates volume of filtered air delivered by an air cleaner. The higher the tobacco smoke, pollen, and dust numbers, the faster the unit filters the air.

To make sense of the CADR numbers, follow the two-thirds rule — using your room's area size. For instance, if you had a room that was 10 feet by 12 feet (or 120 square feet), you would want one with at least a dust CADR of 80. If your room size is smaller, the unit will clean the air more often or faster; if the room is larger, it will be less effective. If you have ceilings higher than 8 feet, you'd need an air cleaner rated for a larger room.

Another measurement, called the Air Changes Per Hour (ACH) rating, can let you know how frequently the air purifier exchanges all the air in a given room.

Also the Environmental Protection Agency (EPA) recommends that you consider other factors as well when considering the use of air cleaners in your home:

• Gases and odors from particles collected by the devices could be redispersed into the air.

• Some devices scent the air to mask odors, which may lead you to believe that the odor-causing pollutants have been removed.

• Ion generators, especially those that do not contain a collector, may cause soiling of walls and other surfaces.

• Noises from some portable air cleaners may be bothersome, even at low speeds.

• Maintenance costs, such as costs for the replacement of filters, could be significant, and should be considered with the initial cost of purchase.

Also like the filters used with a forced-air heating, cooling, and ventilation system, proper installation, use, care, and maintenance is necessary to ensure that you get the most benefit from the air cleaners. They should be placed away from a corner of a room, and at least a few feet away from the walls to support maximum air flow. This means frequent cleaning and filter replacement to ensure proper functioning.

Tobacco Smoke and Filters

If you smoke, will using filters cut down on the amount of secondhand smoke generated by smoking? According to various researchers, the most effective way to eradicate environmental tobacco smoke — or “secondhand” smoke — is to eliminate the main source of smoke: Persuade any smokers in your home to quit smoking.

Some air cleaners can help to reduce secondhand smoke to a limited degree. However, no household or portable air filtration systems can completely absorb secondhand smoke particulates. These invisible particles are of concern since they can be inhaled deeply into the lungs and could trigger asthmatic symptoms.

Air Duct Cleaning

The cleaning of air ducts in a heating, cooling, and ventilation system has been heavily marketed as a way for families like yours to help control a child's asthma. While research in this area is limited, asthma and allergy experts have pointed to other targets that should be considered first in lowering allergens in the home — particularly cleaning surfaces such as floors and furniture, while keeping humidity low to discourage mold and dust mites growth.

Duct cleaning generally refers to the cleaning of various heating and cooling system components of forced air systems, including the supply and return air ducts and registers, grilles and diffusers, heat exchangers, heating and cooling coils, fan motor, and the air handling unit housing.

Duct cleaning has not been shown to actually prevent health problems, nor have studies conclusively demonstrated that particle (e.g., dust) levels in homes rise because of dirty air ducts or go down after cleaning, the EPA said. This is because most of the dirt that accumulates inside air ducts adheres to duct surfaces and usually does not enter your home's living space.

Therefore, unless the ducts are infested with vermin, (e.g., rodents or insects) or clogged with excessive amounts of dust and debris (such as from pets or from a remodeling project) that are being released into your home from the registers, the EPA does not recommend that air ducts be cleaned, except on an as-needed basis, because of the continuing uncertainty about the benefits of duct cleaning.