Indoor air cleaners claim relief for people wishing to rid their homes of smoke, dust and pollen. About 4 million sell in this country annually, says the Association of Home Appliance Manufacturers. But many are ineffective, if not dangerous, says the magazine Consumer Reports. Here's a quick look at each type:

1. High Efficiency Particulate Air (HEPA) filters: Consumer Reports says air cleaners with these filters are among the best performers. But homeowners can more easily improve their indoor air with other methods, such as banning indoor smoking, keeping pets out of bedrooms, removing carpeting and other dust-mite havens and opening windows, the magazine says.

It says manufacturers' claims of germ removal by cleaners are oversold, quoting a Houston physician as saying, "Stick with the basics instead. Wash your hands with soap and water for at least 15 seconds and cover your mouth and nose when you sneeze and cough."

In response, Blueair Inc., a manufacturer of HEPA cleaners, says that taking these steps don't make air cleaners unnecessary and that a good cleaner will always make the room better. A cleaner "can definitely help" with airborne germs and allergens, even if soap and water is a better way to wash germs off hands, Blueair said.

2. Electrostatic precipitators: They apply an electrical charge to particles and deposit them onto filters. They commonly emit small amounts of ozone as a byproduct. This month, Consumer Reports withdrew its endorsement of them after 15 years of recommending some models, saying, "We now believe that air purifiers that emit even small amounts of ozone are not your best choice."

Ozone is a common air pollutant, an indicator of smog's presence. The Environmental Protection Agency says that relatively low ozone levels can cause respiratory problems.

In a statement, Sharper Image Inc., a big seller of electrostatic air cleaners, says its models don't emit ozone.

"We have been the leader in energy efficient air purifiers since we introduced our product over seven years ago. … We believe that consumers appreciate our air cleaners for the following characteristics: effective air cleaning. Continuous 24/7 cleaning. No replacement filters. Low energy costs. Low maintenance. Attractive design. Silent air circulation."

Consumer Reports ranked Sharper Image fourth out of nine electrostatic precipitator models, saying its cleaner had "lackluster air cleaning performance in our tests."

3. Ozone generators: Manufacturers say that ozone will purify the air, but Consumer Reports recommends against such machines, saying its tests have found that their ozone production generally exceeded federal Food and Drug Administration limits of 50 parts per billion set for medical devices. The California Air Resources Board just approved new rules, requiring such generators to limit ozone to 50 ppb by Dec. 31, 2008.

The Consumer Product Safety Commission is weighing a similar limit.

The manufacturer of models that California said exceed the 50 ppb limit, Prozone Inc., said California's testing was not legitimate, adding, "It is really hard to measure ozone at those levels." When Prozone had its machines tested at a private lab 2 1/2 years ago, "we passed," said Ronald Barnes, founder of the Huntsville, Ala.-based company.

An executive of a manufacturer whose models are rated potentially hazardous, Air-Zone Inc. of Suffolk, Va., said the California tests aren't valid because they don't account for the fact that emission levels depend on the room size.