Indoor air is more polluted than the air on the other side of the window, according to estimates by the U.S. Environmental Protection Agency. Further, the American Lung Association cites indoor pollution as a health hazard for millions of Americans with asthma or allergies. Indoor pollutants may include visible particles of dust, pollen, and smoke as well as invisible combustion by-products such as carbon monoxide and nitrous oxide, along with other gaseous invaders such as fumes from carpet adhesives and upholstery.

Two commonsense solutions are insuring proper ventilation and controlling the pollutant at the source. If dust is a problem, you might want to replace wall-to-wall carpeting with bare floors or area rugs. Frequent vacuuming may help, though some vacuum cleaners stir up dust. You can lessen the effects of pet dander by designating pet-free rooms, particularly bedrooms. A properly vented range hood can rid kitchen air of smoke and odor, while an exhaust fan in a bathroom can help squelch mold, mildew, and odor.

Air cleaners may be the next step when those measures aren't enough. But only people with respiratory problems are likely to benefit from using air cleaners. Even then, experts say, air cleaners may not be consistently effective.

If your house has forced-air heating and cooling, choose an appropriate whole-house furnace filter or professionally installed cleaner for your system. If your house doesn't forced-air heating and cooling, your only option is a room air cleaner.

WHAT'S AVAILABLE

Whole-house air cleaners. Major brands include Aprilaire, Honeywell, Lennox, Trane, and Trion. Whole-house cleaners range from inexpensive fiberglass furnace filters to electronic precipitators, which must be installed professionally in a home's duct system.

Furnace filters range from plain matted-fiberglass (about $1), meant to trap large particles of dust and lint up to electrostatically charged filters ($15 to $25) designed to attract pollen, lint, pet dander, and dust.

Electronic-precipitator air cleaners impart an electrical charge to particles flowing through them, then collect the particles on oppositely charged metal plates or filters. These more elaborate systems must be fitted into ductwork and wired into the house's current. Most have a collector-plate assembly that must be removed and washed every one to two months.

Price: about $400, plus $200 or more for installation.

Room air cleaners. Sharper Image has surpassed other brands as the market leader. Other notable brands include Bionaire, Friedrich, Holmes, Honeywell, Hunter, and Whirlpool. Most room air cleaners weigh between 10 and 20 pounds. They can be round or boxy, and can stand on the floor or on a table.

Room air cleaners can work quite well, even on dust and cigarette-smoke particles, which are much smaller and harder to trap than pollen and mold spores. They aren't good at trapping gases, however. Two technologies predominate. The most common is a filter system in which a high-efficiency particulate air (HEPA) filter mechanically strains the air of fine particles. The other dominant technology uses an electronic precipitator that works like those in some whole-house systems, with a fan to move air through them. Honeywell, Hoover, and Sharper Image sell a type of electrostatic precipitator. It typically has no fan or has an ineffective one.

The Association of Home Appliance Manufacturers (AHAM), a trade group, tests and rates room air cleaners using a measurement known as clean air delivery rate (CADR), which is determined by how well a filter traps particles and how much air the unit moves. Separate CADRs are listed for dust, tobacco smoke, and pollen. (While most manufacturers participate in this voluntary program, some models do not have an AHAM-certified performance rating.) We've typically found the AHAM-certified CADRs to be accurate. If whole-house air cleaners and filters are labeled, they carry a minimum efficiency reporting value (MERV). The higher the MERV, the better for trapping small particles.

Price range: $110 to $600. Annual filter cost: $30 to $220.

IMPORTANT FEATURES

Whole-house air cleaners are generally available in a range of standard sizes or can be adapted to fit. Some manufacturers say their filters are treated with a special antimicrobial agent, presumably to prevent bacterial growth on the filter. We've not evaluated those claims.

Room air cleaners typically use a fan to pull air into the unit for filtration. Some models with an electronic precipitator or a HEPA filter incorporate ionizing circuitry that uses powered needles or wires to charge particles, which are then more easily trapped by the filter. But this ionization may also make the particles stick to walls or furnishings, possibly soiling them. An indicator in most models let you know when to change the filter.

HEPA filters are supposed to be replaced annually and can cost more than $100--sometimes as much as the room air cleaner itself. Prefilters, which are designed to remove odors and/or larger particles, are generally changed quarterly, while washable prefilters should be cleaned monthly. An electronic precipitator's collector-plate assembly must be removed and washed every month or so; it slides out like a drawer, and you can put it in a dishwasher or rinse it in a sink.

Most room air cleaners have a handle, while some heavier models have wheels. Fan speeds usually include low, medium, and high. A few cleaners use a dust sensor and an air-quality monitor designed to raise or lower the fan speed automatically, depending on conditions. Our tests of one model with this feature found that it didn't respond well to very small particles in the air.

HOW TO CHOOSE

Performance differences. Consumer Reports tests of furnace filters and whole-house air cleaners found that the better ones were effective with dust but not smoke. The filters may also restrict air flow through the system, adversely affecting the performance of your furnace or air conditioner. Electronic precipitator filters were most effective against dust and smoke, and they restrict airflow much less.

Room air cleaners provided varying levels of performance in our tests, with no one type--HEPA filter or electronic precipitator--clearly outdoing the others. When set at high, the best did a very good job of clearing a room of dust and smoke; other models were only good or fair. Most of the room air cleaners were easy to use, we found. Electronic-precipitator models cost less to run than HEPA units because they don't require you to replace an expensive filter. The Sharper Image Ionic Breeze, Honeywell Environizer, and Hoover Silent Air were far less effective than any other air cleaner tested.

Recommendations. Choose an air cleaner based on the size of your air-quality problem. Among whole-house models, one of the better pleated electrostatic filters may be all you need; consider an electronic precipitator if someone in your home smokes or has a chronic respiratory problem. Also be sure a whole-house filter fits snugly in its mount, since leaks can make it less effective. (You can seal gaps with weather stripping.)

If you decide to get a room air cleaner, choose one that's appropriately sized for the room. We suggest looking for a model with a clean air delivery rate of at least two-thirds of the room's area, assuming an 8-foot ceiling. For example, a 12x15-foot room--180 square feet--needs a model with a CADR of at least 120 for the contaminant you want to remove (dust, smoke, or pollen, for example). The CADR printed on the packaging assumes you'll run the air cleaner at high speed. If you think you'll use medium or low speed to cut noise, compensate by getting a model with a CADR that is a bit higher than suggested for the room size. Also note that a room with a high ceiling requires a model with a correspondingly higher CADR. Follow instructions when placing a room air cleaner to ensure it will work effectively. Some models can sit against a wall; others need to go in the middle of the room.