Ozone produced by ionizing air purifiers may increase respiratory problems.Researchers at the University of California, Irvine, have determined that in poorly-ventilated rooms, air purifiers that produce even a few milligrams of ozone per hour may exacerbate health problems. The Mayo Clinic has also found that ozone can make asthma and other respiratory problems worse and recommends against ozone generators and ionizing air purifiers that produce the molecules. Instead, people should use HEPA air purifiers, which do not emit ozone.

What is Ozone?

The Environmental Protection Agency differentiates between good ozone and bad ozone. Good ozone is found in the upper atmosphere, the ozone layer, which protects the Earth from the sun’s harmful radiation. Bad ozone is found close to the Earth’s surface. This low level ozone is regarded by the EPA and the World Health Organization as an environmental pollutant.

The ozone molecule contains three oxygen atoms. Two of them form the familiar oxygen molecule. The third atom may detach and react with other molecules in the environment. As noted by the Mayo Clinic, inhaling even small amounts of ozone can be irritating to one’s lungs. Ozone can cause coughing or shortness of breath, can irritate one’s throat, and can increase the risk of respiratory infection.

What Produces Ozone?

Ozone may be emitted as a byproduct of an air purifier’s operation. Ionic air purifiers are the most common culprit. Ionic air purifiers charge airborne particles, attracting them to metal electrodes. In the process, they emit ozone as a byproduct. Though it varies by purifier, some ionizers may produce a few milligrams every hour. The study found that this ozone accumulates indoors, leading to higher concentrations than is considered safe. HEPA air purifiers, however, do not emit ozone.

Study Findings

The UCI study, published in the Journal of the Air & Waste Management Association, found that in rooms with poor ventilation, an air purifier that emits even a tiny amount of ozone per hour can lead to levels that exceed public safety guidelines. Significantly, it also demonstrated that ozone emitted from air purifiers adds to the ozone already present, resulting in a cumulatively higher ozone level. This means that people using some ionic air purifiers to reduce allergens and other pollutants may actually be causing themselves more harm.

To come to this conclusion, the researchers tested several kinds of indoor air purifiers at 40% or 50% relative humidity, cataloguing their levels of ozone production. They tested the purifiers in various indoor environments, including bedrooms, bathrooms, offices, and cars. They placed the air purifier in a room, turned it on, and recorded the increasing ozone concentration until it leveled off. In many cases, the indoor level significantly exceeded outdoor safety guidelines. Some even reached a level that would have triggered a Stage 2 smog alert if detected outside. The largest increases in ozone concentration occurred in small, poorly-ventilated rooms, especially those with glossy ceramic tiles and other materials that react slowly with ozone.

Other Research

In a 2009 study published in the New England Journal of Medicine, “Long-Term Ozone Exposure and Mortality,” researchers found that people who live in cities with high concentrations of ozone have a 30% greater annual risk of dying from respiratory illness. The study was especially notable because it looked at the long-term health impact of ozone concentrations, cataloguing deaths from 1977 through 2000. It’s the first study to quantify the deleterious health effects of ozone exposure. It’s clear that people should try to reduce their exposure to ozone, so they need to choose an air purifier accordingly.

HEPA Air Purifiers are a Better Alternative

Many people use air purifiers to improve indoor air quality, to potentially provide allergy relief, or to help alleviate symptoms of asthma. While the UCI study determined that ionic air purifiers may cause unsafe levels of ozone, there are alternatives. As the Mayo Clinic notes, “High-efficiency particulate air (HEPA) filters are more effective in removing allergens from the air, without posing any ozone concerns.” The American Lung Association also declares that HEPA filters are the most efficient at removing airborne irritants, getting rid of 99.97% of particles larger than .3 microns. For those wanting an air purifier that provides better indoor air quality without causing unsafe levels of ozone, HEPA air purifiers are a superior alternative.