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Air Filters, Dehumidifiers, and Humidifiers 

Here are some tips for understanding the air in your house and the air-quality appliances that can alter it.

Air filters

Purpose: Air-filtration systems pull most mold, pollen, dust mites, and other particulates out of household air.

The best systems use HEPA (high-efficiency particulate absorption) filters that have multiple pleats that trap tiny particles. Such a filter can be installed in the duct adjacent to the furnace fan. They also are available as portable room units.

Another kind of unit, the electronic cleaner, is especially good at sweeping mold and pollen particles from the air by catching them on an electrically charged plate.

The type of air cleaner known as an ionizer produces electrically charged ions that bond to particles in the air and causes them to cling to walls, ceilings, and drapes.

Filter systems improve air quality by removing pollutants. HEPA systems can be up to 99.9 percent efficient in removing floating particles from the air. Compare this with regular furnace filters, which are only 10 percent efficient in removing lint from the air. For a filtration system to be fully effective, it needs to run 24 hours a day.

Health benefits: A good air-filtration system can make a difference for people with severe allergies or asthma, but it's not a necessity for the average person. Keep in mind that studies have not proved that any filters dramatically reduce allergy or asthma symptoms. The best possible benefit may come from HEPA filters. So before you invest a lot of money, make sure you take other steps first. In general, families with allergy problems should first look for and eliminate or contain the source of the problem: pets, rugs, dust mites, and moldy areas in the home are the first suspects.

Disadvantages: Air-filtration systems need to be meticulously maintained. Changing the filters according to the manufacturers' instructions is critical to the success of the system. Also, these systems are no substitute for good indoor hygiene. If you have a cat, dog, or old rugs, your house is dusty, or you leave your windows open, the filtration system cannot do its work well.

Some systems with smaller motors are noisy, especially for the bedroom. They don't turn the air over as quickly and as efficiently as larger units.

In the case of ionizers, the particulates that were sent clinging to your walls, ceilings, and drapes fall off in a few days, back into your breathing environment.


Purpose: Dehumidifiers remove moisture from the air. This curbs the growth of mold and dust mites. They are particularly useful in parts of the house where humidity collects, such as damp basements.

Dehumidifiers draw air over cold coils, condensing out its moisture, before passing the air over warm coils and back into the room. (Air conditioners also take a certain amount of moisture out of the air, but dehumidifiers do so much more efficiently.) The condensed water drips into a container in the unit that has to be emptied. The water can be routed directly to a drain by means of a hose.

Home dehumidifiers remove between 10 and 50 pints of water from the air each day, depending on the relative humidity. The capacity of a unit is measured by the number of pints it can remove in a 24-hour period at 60 percent relative humidity and at 80 degrees.

Harmful dust mites, those microscopic organisms that particularly aggravate allergy and asthma sufferers, thrive in high humidity. They live in your bedclothes, your drapes, your rugs, and the air in your home. Removing excessive moisture from indoor air helps control these pests. Dehumidifiers also can help limit mold and bacterial growth.

Health benefits: Dehumidifiers are critical for households in humid climates with very old people or very young children, or for families with a history of allergies or asthma. In their first two years of life, children spend a lot of time on the floor or rug. If you have a 10-year-old rug, it likely has a host of dust mites that thrive in the high humidity. And the more you are exposed to something to which you have a genetic tendency to be allergic, the more likely you are to become allergic to it, whether it's mold, bacteria, or dust mites.

Disadvantages: Mold can grow in the drainage areas of a dehumidifier, so regularly clean the water basin with bleach. Also, smaller units may not dry out the air satisfactorily all the time. You should consider choosing a larger capacity unit, one rated at 50 pints a day or more. It can always be turned down.


Purpose: Humidifiers are of great use during winter in cold climates where home heating systems are in constant use. This dry air can dry out and irritate your eyes, throat, and lungs. Dry skin is another hazard. Besides that, you may find the wood in your house drying out, creating gaps between floorboards, loose furniture joints, and windows that rattle in their frames.

Humidifiers come in two types:

  • The evaporator type forces air over water inside the unit and blows the evaporated water into the house.

  • The atomizer type of humidifier breaks up water droplets and produce a mist that then evaporates as it is distributed throughout the house. To break up water into a mist, some of these humidifiers use a rotating device, like a blade or brush. In the case of an ultrasonic humidifier, a disc that oscillates about 1.6 million times per second does the job. The water is mechanically agitated into fine droplets.

Humidifiers can be built in to your central heating system and use the furnace ducts to distribute moist air throughout your home. If you have a closed heating system, such as electric baseboards, a central humidifier can be installed with its own fan and duct for distribution of humidified air.

Portable humidifiers vary in size and efficiency. Tabletop units can usually handle only single rooms. Larger console models can be set up in central locations to distribute moisture to a large area of the house.

Health benefits: Humidifiers are recommended for people who live in areas where houses must be heated for a good portion of the year, or for those who live in very dry climates. Humidifiers will help those suffering from sore throats, headaches, nose bleeds, and coughs related to dry air. In general, a moist house is more comfortable than a dry one.

Disadvantages: Humidifiers are not easy to use. All units, portable as well as those installed in central heating systems, must be cleaned rigorously. Otherwise, they tend to become contaminated with mold and bacterial growth that may be blown through the house.

You should clean portable units every two weeks and central units once a month. Remove any residue, then let a solution of chlorine bleach sit in the basin for 30 minutes to kill any fungus or bacterial growth.

Ultrasound humidifiers tend to leave a fine white dust about your rooms, as the tiny droplets they dispense evaporate and leave behind calcium carbonate and other minerals present in the water. This may be prevented by using distilled (not de-ionized) water in the humidifier. Distilled water, however, is fairly expensive.

Ideal humidity

  • The air in your home should range from 30 to 55 percent humidity.

  • If the humidity is too high, mold and dust mites may thrive. Both are common causes of allergies. Mold also causes an unpleasant smell and can discolor surfaces.

  • If the humidity falls too low, on the other hand, the people in your home may suffer dry eyes and may develop throat and sinus irritations. The simplest way to measure humidity is with a gauge called a humidistat. These gauges are often packaged with thermometers. You can usually buy one in a hardware store for less than $10.