Some air purifiers produce large amounts of ozone, although the Association of Home Appliance Manufacturers conducts independent testing to verify whether air purifiers meet the recommended ozone emission limits of the U.S. Food and Drug Administration (FDA) of 50 parts per billion (ppb) or less.
However, manufacturers must pay a fee to be tested by this organization, and some of the top-rated air purifier manufacturers have not opted to do so. Ozone in large amounts can neutralize strong odors (such as the smoke odor from fire damage), but according to the EPA this is dangerous for human health, because ozone can cause lung damage, worsen asthma and cause other lung problems.
Air purifiers that use ionization tend to produce ozone gas as a byproduct. In sufficient quantities, ozone can be toxic; in smaller amounts, it can cause lung damage and other problems. The Blueair 650E (Est. $900) , for example, uses an ionizer (as do most Blueair models) and earns some criticism from owners. However, this particular model -- as well as two other air purifiers by Blueair -- meets FDA ozone emission standards.
"Relatively low amounts of ozone can cause chest pain, coughing, shortness of breath and throat irritation," according to the EPA. "It may also worsen chronic respiratory diseases such as asthma as well as compromise the ability of the body to fight respiratory infections."
Although some reviews say that the ozone produced by electrostatic air cleaners is safe for those in normal good health, both the EPA and American Lung Association advise against using these devices because there are many ozone-free HEPA models on the market.
But even smaller amounts of ozone can be harmful over the long term, studies say. A study published in The New England Journal of Medicine in March 2009 followed nearly 450,000 people in 96 U.S. cities from 1977 through 2000. It found that long-term exposure to low levels of outdoor ozone -- under the 50 ppb generally considered safe -- can be fatal.
For every 10 ppb increase in ozone concentration, the risk of death from respiratory causes (primarily pneumonia and chronic obstructive pulmonary disease) went up 4 percent. That "can translate into thousands of excess deaths each year," says study lead author Michael Jerrett, a University of California-Berkeley associate professor of environmental health sciences.
There have been a lot of studies on the health effects of outdoor ozone levels, but few long-term tests have measured the effects of ozone produced by indoor devices like air cleaners. One 2006 study published in the Journal of the Air & Waste Management Association tested 13 air purifiers, including several personal air cleaners designed to be in close proximity to the user. Research analysts concluded that even these are capable of producing steady levels of indoor ozone that exceed health standards. Indeed, some people say they can smell ozone, and some report headaches.
In the past, ConsumerReports.org recommended some air purifiers that emit small amounts of ozone as a byproduct, but now editors say, "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 ppb) are not your best choice."