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Ozone is a chemically reactive natural gas. It is usually created by sunlight high in the stratosphere where it takes part in the normal cycle of atmospheric chemistry. Unfortunately, ozone is harmful to living things at ground level, especially over long-term, repeated exposure. Sometimes air purification methods produce ozone as a byproduct or even use ozone as a primary method of cleaning the air.

Ozone is classified as a pollutant by the EPA, the WHO, and many other health organizations, so improving the indoor air quality in your home should not include adding ozone. What are the different types of air purifiers that produce ozone? Before getting to that, here is a look at why some air purifiers create ozone.

Why do air purifiers create ozone?

Studies have found some acute negative effects of short-term ozone exposure and increased mortality rates in cities during periods of increased ambient ozone levels (Gryparis et al., 2004). A review of ozone health effects reports that acute exposure may can cause “changes in lung capacity, flow resistance, epithelial permeability, and reactivity to bronchoactive challenges…Repetitive daily exposures over several days or weeks can exacerbate and prolong these transient effects.” (Lippmann, 2012).

While certain air purifiers release ozone as a by-product, other release it intentionally, claiming that ozone can be a method of purifying the air. Ozone is simply three oxygen atoms bound together to form an O3 molecule. It is a highly reactive molecule, which means that it binds with other compounds in the air or in your lungs, forming new types of molecules. Proponents of ozone air purification say this means the ozone breaks down harmful toxins in the air, converting them to other, less harmful types of molecules.

The EPA has done an extensive breakdown of how ineffective ozone actually is at air purification. It can be effective at breaking down certain chemical compounds in the air, such as some of the toxins found in tobacco smoke. However, ozone released from an ozone generator cannot remove other compounds in the air such as dust, pet dander or other allergens. It may take months for ozone to have any effect.

Ozone may even break compounds down into more harmful chemicals. Some professional cleaners use ozone to purify rooms, removing smells and killing bacteria, but they use extremely high concentrations of ozone and they have to air the room out, removing the ozone, before anyone can occupy it again. It is also well-known for its tendency to damage anything in the home with a rubber component, cracking and breaking the rubber into dust. The amount of ozone a home air purifier produces may not do enough good to outweigh the harmful effect of ozone exposure.

Ozone air purifiers to watch out for

Here are some air purifier technologies that can produce ozone, either as a byproduct or as the primary method of cleaning the air.


Ozonators, or ozone generators, are often marketed as a “natural” way to remove odors or purify air. They generate ozone and directly release it into the air, creating high concentrations. Molekule agrees with the EPA that these should not be used in occupied spaces.

Electrostatic Precipitation

Electrostatic precipitation air purifiers remove dust and other particles from the air by imparting an electrical charge to these contaminants. Within the air purifiers are plates (or sometimes fibers) that are given an opposite charge. Opposites attract, so the charged particles are attracted to the oppositely charged plate, removing the particles from the air. While these air purifiers are somewhat effective at this, the process of adding an electrical charge to the air causes some free oxygen molecules to become ozone. Studies have found that electrostatic precipitation air purifiers can increase indoor ozone concentrations up to six times the outdoor level (Rim et al., 2014).


Ionizers are very similar to electrostatic precipitation air purifiers: they use a coronal discharge to put electrically charged ions into the air, which attach to particles, giving the particles a charge. The particles are then attracted to each other (because some of them will have positive and some negative charges, causing electric attraction). As the particles clump together they become too heavy to remain suspended in the air, and they precipitate out of the air to land on the floor, sofa, curtains or whatever other surface attracts them. In addition to this somewhat messy solution to air purification, ionizers have the same ozone generation problems as electrostatic precipitation air purifiers.

UV-C Light

Air purifiers that use ultraviolet light to kill microorganisms (also known as UVGI, ultraviolet germicidal irradiation) only work against certain species. More importantly, they may also have a hidden danger. Small home UV-C air purifiers use a fan to blow air through the purifiers, where pollutants are briefly exposed to UV light while passing through. The hidden danger is not from the radiation (although you do need to be careful when using UV-C light). The danger comes from oxygen particles that are broken apart by the high-energy UV-C light. These atoms then combine again with other oxygen atoms to form ozone. Therefore, UV-C air purifiers are not ozone-free.

Not only do they produce ozone, they are not very effective in a residential setting. While studies have shown that high-energy ultraviolet light kills bacteria, viruses and some mold spores, to do so effectively requires very high-intensity UV light and a long duration of exposure. So these units offer neither the intensity nor duration necessary to be really effective. Plus, they do nothing to remove particles like dust from the air, so they have to be combined into hybrid systems with HEPA filters.


Photocatalytic oxidation (PCO) air purifiers are designed to remove gaseous pollutants from the air, though the effectiveness of this method has not been demonstrated. The filter surface is coated with a chemical catalyst which is activated by UV-C light. The resulting photochemical reaction creates hydroxyl radicals, and these molecules oxidize the pollutants that come in contact with the filter surface. Ideally, the chemicals break down into harmless CO2 and water. However, unless the PCO device is known to very efficiently produce the radicals in large amounts, it is impossible to control or predict exactly how completely chemicals will break down, so PCO air purifiers can form incomplete byproducts like formaldehyde, nitrogen dioxide and other harmful compounds (Hodgson et al., 2007). In addition, without a proper coating, the UV lamps used in PCO air purifiers can create ozone, just like UV-C air purifiers. Without such a coating, PCO air purifiers are not ozone-free. And even with them, they may be ineffective and produce toxic byproducts.

Which air purifiers are ozone-free?

Here are the types of air purifier technology that do not produce any ozone.

  • HEPA filters – These filters are designed to remove 99.97 percent of all particles that are 0.3 microns in diameter from the air. While they are not effective against odors, VOCs or other gaseous pollutants, they do not produce any ozone.
  • Carbon filters – A combination of high surface area and activated carbon allows these filters to remove VOCs and other gaseous pollutants from the air. They are not effective at removing dust particles, but may help with gases. Carbon filters are completely ozone-free.
  • PECO – This technology, Photo Electrochemical Oxidation, may appear similar to PCO in concept, but is distinct from the outdated PCO process. PECO was developed to be orders of magnitude more efficient than PCO. The Photo Electrochemical Oxidation process creates the right concentrations of hydroxyl radicals that completely destroy pollutants, instead of generating new pollutants in the form of incomplete byproducts like the PCO process. PECO uses safe UV-A light instead of harmful UV-C light. This prevents any ozone from being produced. PECO has even been found to reduce levels of ozone in the air because ozone molecules are converted into breathable oxygen. To learn more about the PECO process, which is the technology contained in the Molekule air purifier, see here.

What to do if your air purifier produces ozone

If you have one of the air purifiers that produces ozone, there are a few steps you can take to limit the harm that it may do.

  • Switch to an ozone-free solution. Yes, this is an obvious answer and might be tough to swallow if you spent a lot on an air purifier that creates ozone. But it is a clear way to eliminate the risk of ozone health problems from your home.
  • Check the manufacturer information on the amount of ozone generated. This might be of limited use, since you do not know how the manufacturer tested or what standards they tested against, but it will give you some information about the potential risk.
  • Do not use the air purifier in a very small enclosed space. Keep a window open to disperse any ozone that may be forming in the room.
  • Only operate it when no one is present. If you want to clean the air in a room, turn on the air purifier, then stay out of that room while it is running. Later, turn off the air purifier and give the ozone time to disperse before using that room again. Of course, this will not protect any rubber components that are exposed.

Given their limited effectiveness and potential hazards, searching for an ozone-free air purifier whenever possible is the best option for you and your family.

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