Some electronic air purifiers emit ozone, either intentionally or as a byproduct. Underwriters Laboratory Safety Standard 867 establishes 50 parts per billion as the maximum safe limit -- the same level that's been established by the U.S. Food and Drug Administration (FDA) as the maximum allowable ozone emissions for medical devices. The Association of Home Appliance Manufacturers (AHAM) does not independently test for ozone levels, but does post which of the air purifiers it lists are certified to meet UL standards.
Ozone in large amounts can neutralize strong odors (such as the smoke odor from fire damage), but, according to the EPA, using an ozone air purifier can be dangerous for human health because ozone can cause lung damage, worsen asthma and cause other lung problems. "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."
Further, the EPA states that ozone levels that fall within safety standards are not effective in removing "indoor air contaminants" based on currently available scientific evidence. In addition, safe levels of ozone are not effective in "removing many odor-causing chemicals," nor can it "effectively remove viruses, bacteria, mold, or other biological pollutants."
There are also concerns that even smaller amounts of ozone could be harmful over the long term. 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 ozone air purifiers or ionic air purifiers. 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."
Although some reviews say that the ozone produced by electrostatic air cleaners is safe for those in normal good health, the EPA, American Lung Association and most other expert sources recommend against using these devices. California has banned the sale of air purifiers that exceed the UL standard of 50 parts per billion, though they can still be bought elsewhere. Good Housekeeping offers this blunt advice: "If you already own an air purifier, make sure it doesn't produce ozone and if it does, stop using it and cut the cord before you discard it so no one else can use it either."