Even the best air cleaner can be a frivolous investment, according to medical experts. There's little evidence that they alone will reduce the effect of indoor pollutants for those with asthma or allergies, according to Consumer Reports magazine.

In its October report, "Air Cleaners: Some do little cleaning," the magazine presents the results of unbiased, independent tests for room and whole-house air cleaners. The report also advises consumers to try other simple indoor air-cleaning steps before considering an air cleaner.

Relatively few air cleaners excelled in the CR tests, especially among room models, which account for the most sales, as measured in dollars. Many room air cleaners scored fair or poor in at least one of CR's four cleaning tests for dust and smoke.

In May 2005, CR published "New concerns about ionizing air cleaners," which showed that some ionizing models did a poor job of cleaning the air and several can expose users to potentially harmful levels of ozone, an irritant that can worsen asthma and decrease lung function.

CR's October report features ratings of 30 room air cleaner models as well as 24 whole-house models, based on testing conducted for previous reports as well as for the current report.

"Our test findings should be a major concern for consumers who are bombarded with advertising for these air cleaners," said Dr. Jeff Asher, Vice President and Technical Director for Consumers Union, the publisher of Consumer Reports. "Consumers need to know that they may be spending money on models that don't clean the air and have no proven health benefits. Some may actually cause harm," he said.

Some room air cleaners that have electrostatic precipitators, which trap charged particles on oppositely charged plates or filters, are effective at cleaning the air. But other models that use this same basic technology are far less effective.

Consumer Reports rated four such models, including, for the first time, the Oreck XL Professional Signature Air8S, as Not Recommended. Ads claim the Oreck's XL Professional "cleans the air in your home of allergens, dust, and dirt."

However, in CR's tests, the Oreck performed poorly at such air-cleaning, as did other ionizing models that have a small fan or no fan. In separate tests for ozone, the Oreck passed the industry-standard (50-parts-per-billion) Underwriters Laboratories test.

There are three other models that are Not Recommended because they were poor at cleaning and also exceeded the Underwriters Laboratories ozone standard. These include the Ionic Pro CL-369, the Surround Air XJ-2000, and the Sharper Image Professional Series Ionic Breeze Quadra SI737 (and its similar S1637 model).

As CR's October issue went to press, Sharper Image began advertising a model (S1837) with an ozone-reducing catalyst. The Sharper Image model that CR tested did not have this catalyst; thousands of the S1637 and S1737 models remain in homes and on store shelves. CR is currently testing the new model.

CR suggests avoiding electrostatic room air cleaners with a small fan or no fan, which have cleaned poorly in our tests and can emit significant amounts of ozone. CR also advises consumers to avoid dedicated ozone generators, which were not tested for this report. Unlike electrostatic precipitators, which emit ozone as a by-product of their cleaning process, these niche products produce large amounts of ozone by design.

Professionally installed whole-house models performed best overall in CR's tests, but are pricey. Whole-house electrostatic precipitator models emitted little ozone and performed well. But the best cost $500 to $700 for the unit and another $200 or more to install and can only be used if your home has central hot-air heat or air-conditioning.

Among the top performing whole house models are the Aprilaire 5000 and Carrier AIRA. Some of the do- it-yourself whole-house filter models that CR recommends include American Air Filter Dirt Demon Ultra High Efficiency and 3M Filtrete Ultra Allergen Reduction 1250.

Don't assume that any air cleaner will improve your health. Medical societies say there's little evidence that air cleaners alone will reduce the effect of indoor pollutants for those with asthma or allergies.

While capable air cleaners can trap dust, smoke particles, pollen and pet dander, you can reduce all of those allergens without opening your wallet.