Hazardous Ozone-Generating Air Purifiers

Some devices that are advertised as air purifiers purposely emit large amounts of ozone, the main component of smog. CARB recommends that ozone generators not be used,  except for approved industrial purposes where harmful exposure to ozone is prevented. Not only are ozone generators ineffective at cleaning indoor air, but inhaling ozone poses serious health risks for humans and animals. This fact sheet discusses these health risks and provides effective, alternative solutions to address indoor air quality problems.

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What are ozone-generating air cleaners?

Indoor "air purifiers" or air cleaners that intentionally emit ozone are often called 'ozone generators." Manufacturers sometimes inappropriately refer to ozone as 'activated oxygen,' 'super oxygenated' or 'energized oxygen,' which implies that ozone is a healthy kind of oxygen. Because ozone reacts with some other molecules, manufacturers claim that the ozone produced by these devices can purify the air and remove airborne particles, chemicals, mold, viruses, bacteria, and odors. However, ozone is only partially effective at cleaning the air when it is used at extremely high, unsafe levels that pose a serious health risk.

Air cleaners that utilize ionizers and electrostatic precipitators are other types of devices that emit ozone, but do so as a by-product of their design and function. These devices are designed to electrically charge particles in the air and cause them to attach to surfaces in the room, such as walls or floors. Ozone is released through the charging process, although these devices typically emit much less ozone than ozone generators.

Are ozone generators effective at cleaning air?

Some devices are marketed with advertising claims that they will kill viruses, bacteria, mold and other biological contaminants, and remove chemical contaminants and odors. However, when ozone concentrations are below the health standards, it does not effectively remove biological contaminants. Ozone also does not remove particles (e.g. dust and pollen) from the air, including the particles responsible for most allergies.  Ozone generated by air purifiers does little to remove chemical pollutants. In fact, ozone has been found to react with existing chemicals in the air to create additional toxic pollutants, most notably formaldehyde and ultrafine particles. Some consumers purchase air purifiers to eradicate odors. There is scientific evidence that ozone concentrations below the health standards are not effective in removing many odor-causing chemicals. Ozone is also known to deaden one's sense of smell. Not only does this disguise rather than eliminate odors, it can also have the dangerous effect of decreasing a person's ability to detect high ozone levels. Unlike the situation in air, ozone can be used successfully to purify water in some applications. This is because high levels of ozone can be used in the water, most of the ozone reacts in the water, and people typically are not present when the ozone is used.

How much ozone do ozone generators produce?

Ozone generators can produce indoor ozone levels several times higher than the State's outdoor 1-hr and 8-hr health standards of 90 parts per billion (ppb) and 70 ppb respectively. Many commercial ozone generators emit more than 5,000 mg of ozone per hour of operation, which could result in unhealthy levels of ozone in indoor air.

What are the adverse health effects from exposure to ozone?

People who buy ozone generators may not be aware that ozone can harm the cells in the lungs and respiratory airways. Exposure to ozone irritates and inflames the lining of the respiratory system. This causes symptoms including coughing, chest tightness, shortness of breath, and impaired breathing. Ozone can worsen asthma symptoms, and may contribute to the development of asthma. Elevated exposures to ozone can cause permanent lung damage, and repeated exposure can even increase the risk of dying among persons already in poor health. Persons especially vulnerable to health problems from breathing ozone include children and those who already suffer from asthma or other respiratory diseases, including the elderly. There are many experimental studies on animals, including dogs, cats, hamsters and guinea pigs, that show respiratory effects from exposure to ozone.  Birds are especially sensitive to the effects of air pollutants, including ozone.

Why are ozone generators still on the market?

The unfortunate answer is that misleading advertising by manufacturers is very effective, and no government agency has the authority to fully regulate these devices. Thus, CARB is actively working to educate professionals and the public about the dangers of using ozone generators.

What does CARB recommend?

CARB strongly advises against the use of ozone generators in spaces occupied by people or animals. Other governmental agencies agree with this advice. CARB provides a list of potentially hazardous ozone generators sold as "air purifiers", which is periodically updated. If an ozone generator is not on this list, it does not mean that it is safe for use. 

Prior to purchasing an air cleaner, consumers are encouraged to eliminate or reduce indoor pollution sources and to ventilate areas with outdoor air. The most effective method of controlling indoor air pollution is through prevention: eliminating pollution at its source. To minimize the release of pollutants indoors:

• carefully follow directions on consumer products, such as cleaning agents, paints, and glues
• properly maintain and operate gas- and wood-burning appliances
• restrict smoking to outdoor areas
• purchase building materials and wood furniture that do not emit formaldehyde
• use candles and incense sparingly, if at all
• clean frequently and thoroughly to prevent dust and mold build-up.

Use plenty of ventilation: be sure there is adequate airflow to/from the outdoors. This can be achieved by opening windows, using exhaust fans near pollutant sources (e.g. above stoves while cooking), and increasing airflow through the use of mechanical ventilation systems. If your home is equipped with a central forced air system, you should also consider upgrading the filter to at least MERV 13.

California Certified Air Cleaners

In , CARB adopted a regulation to limit ozone emissions from indoor air cleaning devices. Over 300 manufacturers have submitted test results and obtained CARB certification of their air cleaning devices as required under our regulation. Certification is based on a device's low (usually near-zero) ozone emissions and electrical safety. The following information provides the list of CARB certified air cleaning devices and additional information on how to choose a safe and effective air cleaner. 

Additional information on ozone generators

Ozone generator machines are popular tools to remove odors, pollutants, and viruses. But what does the science say about the effectiveness of ozone machines?

The biggest downside of using an ozone generator is that ozone is harmful to humans! But hey, maybe ozone could be useful in an empty room? Scientists have tested this. Here's what they found.

1. Do Ozone Generators Remove Particles?

Researchers found no effect of an ozone machine on particulates.

What's worse, ozone generators can actually create particles in the air. This can happen if there are scents in the air, like from cleaning products or air fresheners.

Here's what happened when researchers at the University of Texas put an ozone generator in a room. They ran it for a while, and then they opened up some Pine Sol. The ozone reacted with the Pine Sol to create air pollution particles.

2. Can Ozone Remove Harmful Chemical Gases?

OK, ozone is a harmful gas, but maybe it can remove other chemical gases? Researchers tested this by putting new carpet in a stainless steel test chamber. The carpet put out chemicals into the air, like formaldehyde and styrene.

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Then they turned on an ozone generator. Some chemicals went down (like styrene), and some chemicals went up (formaldehyde). The net effect was that these sorts of volatile organic compounds (VOCs) went up.

So ozone actually made VOCs worse (for the curious, ozone itself is not a VOC).

3. Can Ozone Generators Remove Odors?

The hardest question to test is whether ozone generators remove smells. This is harder to test because smells are harder to measure. Most researchers just ask human volunteers to use their noses and rate the smells. I feel bad for the people who had to test the effectiveness of carbon filters by sniffing farts.

But farts are just a side story here. Does ozone reduce smells? Back in the s, researchers made a room really smelly with body odor, filled it with ozone, and then sent people in to rate how smelly it was (described here).

The ozone didn't help. The only way the ozone worked was when the smell of the ozone itself was so strong that people couldn't smell the body odor anymore. Those results suggest ozone is not effective at removing odors.

Are Ozone Generator Machines Effective?

Besides being directly harmful to humans, these peer-reviewed studies have found no benefit of using ozone to reduce particles, gases, or odors.

Smart Air

P.S. If ozone doesn't work, what does?

HEPA filters are great at capturing particles, even tiny nanoparticles. For example, that earlier study of ozone generators also tested HEPA filters, and they did reduce particles:

HEPA filters don't remove gases, but activated carbon filters can remove chemical gases like formaldehyde.

And remember that study of farts? Activated carbon helped reduce the fart smell! So carbon works for farts too.

Recommendation: If you are looking to remove fart smells and dangerous particles, I recommend using a HEPA air purifier with an additional activated carbon filter such as the Sqair Chemical Warfare.

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