Jeff Wald's patients are forever talking about room air cleaners.

They ask the allergist whether the units work, where to put them in their homes, and which one they should buy. Wald gives them an answer in line with what most doctors and researchers say.

"Use one with HEPA [high-efficiency particulate air] filters if you buy one," said Wald, who has offices in Olathe and Overland Park, Kan. "They've done the best in controlled studies, sometimes improving symptoms and decreasing medication use."

Consumers should be able to rely on air cleaners to help deal with pet dander, dust, pollen and smoke particles. But the units can't be expected to keep away viruses, odors or harmful gases such as carbon monoxide. Or dust mites, which are too heavy to float in the air.

But Wald cautions that even the best HEPA filter system won't be enough for allergy sufferers, who should keep pets out of bedrooms and shut windows at the very least.

Room air cleaners with HEPA filters usually work with a fan. But fanless units are available. You've seen the commercials: Electrostatic precipitators -- with models made by Honeywell, Hoover and Sharper Image -- use an electrical charge to stick particles to oppositely charged collector plates. Hybrid devices use a combination of mechanical and electric debris-removal devices.

Prices range from about $100 to $600 for room air cleaners. The portable cleaners typically weigh 25 to 35 pounds.

Air cleaners perform most effectively when they're on a high setting, and they should be on at all times. Some run quietly, some sound like the hum of a computer, and others have a wind-gushing noise. "You don't hear a motor or machine noises, but you hear air coming out," said Michael Doman, sales manager for Austin Air Systems, an air cleaner manufacturer in Buffalo. "People get used to the white noise."

Ariella Winograd, general manager of the U.S. office of Stockholm-based Blueair, said customers often tell the company about how comfortable a room air cleaner makes them feel. "They say, 'My kid can breathe and hasn't had an asthma attack in a long time,' " she said. "They've reported that it's limited their snoring and helped clear their allergies."

Tips

Here's what to consider if you want to buy an air cleaner:

* Room size. Air cleaners are priced according to their square-footage capacity. Whole-house systems are also available, but they are more expensive than room cleaners and can be used only in houses with forced-air heating and cooling.

* Location. Doctors say bedrooms are the most effective places to have them. But they also might need to be in the living room, home office or kitchen to have a role in alleviating allergies, said Harold S. Nelson, an allergist for the National Jewish Medical and Research Center in Denver.

* Performance. Go to the Association of Home Appliance Manufacturers' Web site, www.aham.org. It compares models using the clean air delivery rate -- the amount of contaminant-free air delivered by air cleaners.

* Energy efficiency. Later this summer, the Environmental Protection Agency's Energy Star program will begin certifying room air cleaners. The label will measure product quality and energy efficiency. A consumer with an Energy Star air cleaner could save up to $27 a year on utility bills. People with air cleaners older than five years might not have an energy-efficient system, says Kate Lewis, an Energy Star spokeswoman.

* Maintenance. Air cleaners should be wiped down with a wet cloth a few times a week. Filters should be changed according to the recommended schedules.

* Return policy and warranty. Doctors often suggest to patients who want a room air cleaner to keep it for three weeks. That's usually long enough for someone to decide whether it's placebo or a panacea.