Purpose. Air-filtration systems pull 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. This filter can be installed professionally in the duct next 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. They catch them on an electrically charged plate.

The type of air cleaner known as an ionizer makes 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% efficient in removing floating particles from the air. Compare this with regular furnace filters. These are only 10% 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. It’s not as necessary 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 control the source of the problem like pets, rugs, dust mites, and moldy areas in the home.

Disadvantages. Air-filtration systems need to be strictly maintained. Changing the filters according to the manufacturers' instructions is key 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, if your house is dusty, or if you leave your windows open, the filtration system can't 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 well 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. Then they are back into your breathing environment.

It is important to note that some electronic filters like an ionizer can produce ozone. Ozone is a lung irritant. 

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 like 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 this much more efficiently.) The condensed water drips into a container in the unit that has to be emptied. The water can be directed to a drain by means of a hose.

Home dehumidifiers remove between 10 pints and 50 pints of water from the air each day. The amount depends 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% relative humidity and at 80°F (27°C).

Harmful dust mites, those microscopic organisms that particularly make allergy and asthma sufferers feel worse, 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 2 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 an inherited leaning to be allergic, the more likely you are to become allergic to it. It could be mold, bacteria, or dust mites.

Disadvantages. Mold can grow in the drainage areas of a dehumidifier. 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 like 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 problem. Besides that, you may find the wood in your house drying out. This can create gaps between floorboards, loose furniture joints, and windows that rattle in their frames.

Humidifiers come in 2 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 makes a mist that then dries up as it is spread 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 moves back and forth at about 1.6 million times per second does the job. The water is changed into fine droplets.

Humidifiers can be built in to your central heating system and use the furnace ducts to spread moist air throughout your home. If you have a closed heating system like electric baseboards, a central humidifier can be installed with its own fan and duct for spreading the 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 spread 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. They are also recommended for those who live in very dry climates. Humidifiers may help those suffering from sore throats, headaches, nosebleeds, 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 thoroughly. Otherwise, they tend to become contaminated with mold and bacterial growth that may be blown through the house. Stop the humidifier and call your health care provider if you develop any respiratory symptoms that you feel are related to the use of a humidifier. 

Clean portable and central units according to manufacturers' instructions.

Ultrasound humidifiers tend to leave a fine white dust about your rooms. The tiny droplets they give out evaporate and leave behind calcium carbonate and other minerals present in the water. Use distilled (not de-ionized) water in the humidifier to prevent this. Distilled water, however, is fairly expensive.

Aim for the right amounts of humidity:

• The air in your home should range from 30% to 55% 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, 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.