Air purifiers like the Oreck XL Tower (*Est. $600) are electrostatic air cleaners. They reverse the electric charge in airborne particles, so allergens either fall to the floor or stick to the walls -- removing them from the air but not necessarily capturing them on the air purifier's collection plates or filters (though they may also include other air-filtering technologies like filters and a fan). A byproduct of this technology is ozone gas. In sufficient quantities, ozone can be toxic; in smaller amounts, it can cause lung damage and other problems.
"Relatively low amounts of ozone can cause chest pain, coughing, shortness of breath and throat irritation," according to the U.S. Environmental Protection Agency (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 high-efficiency particulate arresting (HEPA) models on the market. Although there are no federal guidelines for ozone levels in the home, the Food and Drug Administration (FDA) does regulate ozone for medical devices (the FDA does not recognize home air cleaners as medical devices), and air-purifier manufacturers may voluntarily submit their air cleaners for ozone-emission testing. The FDA mandates a limit of 50 parts per billion (ppb) when measured at 2 inches from the unit.
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 parts per billion) are not your best choice."
As of October 2010, California banned all air purifiers that produce ozone levels greater than 50 ppb. For now, this regulation will mostly affect air purifiers that are strictly ozone generators rather than electrostatic precipitators such as the Oreck XL Tower. The California Air Resources Board has decided that further testing is needed to determine whether the ozone levels now considered safe should be lowered.