For the past 15 years, Consumer Reports has recommended some electrostatic precipitators. The Friedrich C-90B and Aprilaire 5000 were effective air purifiers with low annual maintenance costs but produced small amounts of ozone. These portable and whole-house purifiers apply an electrical charge to the particles drawn into the device and produce some ozone as a byproduct.

These products' performance has not changed significantly, but our advice has. We now believe that air purifiers that emit even small amounts of ozone (less than 50 parts per billion) are not your best choice. Research is increasingly warning against adding ozone to indoor air. But more studies need to be conducted to determine the effects of low-level ozone exposure.

If you own one of these models, we suggest that you err on the side of caution. If a member of your household has asthma or allergy symptoms that aren't improving, even though you've removed allergy and asthma triggers and keep your home well ventilated, you might want to consider switching to one of our highly rated, filter-based models if you still want a purifier.