The air pollution outside our homes may be a cause for worry, but the air inside may not always he quite as pure as we'd like to believe it is. Enter air purifiers. These units are designed to remove a variety of pollutants - including smoke, dust, pollen, mold, animal dander, and some chemical odors and gases from the air. Do they work? Yes and no. Studies show that some types of air purifiers are effective against some types of pollutants. The key to buying one is knowing which pollutants you want to target and how much money you want to spend; prices and maintenance costs vary greatly.

Choosing the right unit

To rid your home of noxious odors particularly those that emanate from formaldehyde, pesticides, perfume, and tobacco smoke the best choice is a unit that uses an activated carbon filter. (Note: Air cleaners that do not contain special media, such as activated carbon, will not remove gaseous pollutants, including radon, or reduce their associated health effects.)

Electronic air purifiers may also reduce cigarette smoke, but they may not eliminate the odor. Different varieties, called electrostatic cleaners, electric cleaners, and negative ionizing cleaners, charge particles in the air so that they can be collected, either by a special filter or by your walls, floors, and furniture. These units neutralize some offensive odors but won't remove odorless gases like carbon monoxide. Neither will they alleviate your allergy to cats, pollen, or dust.

If allergies to dust, pollen, and mold are your biggest complaint, look for a mechanical filtration system that traps large particles. A so called HEPA filter (high efficiency particulate air filter) is a good bet. But there's a catch: these units filter only particles that are airborne; most allergens quickly settle on surfaces, where air purifiers can't touch them. (In fact, you may not even need an air purifier to filter allergens from the air; at least one study showed that air conditioners do the job almost as well.)

To remove gases and odors in addition to pollen and dust, you'll need a hybrid unit that uses more than one type of filtration system. If you have central heating or air conditioning, you may not need to buy an air purifier at all: you can have an electronic or mechanical filter installed in the ducts.

Recognize the drawbacks

Electronic air purifiers and negative ionizing cleaners may produce ozone, a lung irritant, especially if they are not properly installed and maintained.

And negative ionizing cleaners, especially those that lack a collection system, may eventually create an ugly black film on your walls and furniture. Also, many even most units designed to remove gases may actually send the gases right back into the air over time. And some devices fool you into thinking that they work better than they do by releasing scents in order to mask odors. Know which pollutants you want to remove.

Match the purifier to the room size. Look for the CADR (clean air delivery rate) to learn how many cubic feet oft air per minute the unit can clean.

Tabletop units are relatively ineffective, unless used in small areas. No air purifier is 100 percent effective on its own. You must also control the source of pollution. For people with allergies, an air purifier with a HFPA system is a fair bet but not a surefire solution.