Which air purifiers emit ozone: ionizers, plasma, ozone generators, and UV-C

What is ozone and why it harms lungs

Ozone (O3) is a molecule made of three oxygen atoms. It is created naturally in the upper atmosphere by ultraviolet light. At ground level, ozone is a pollutant that irritates the respiratory system.

Ozone reacts with lung tissue, causing inflammation, reduced lung function, and increased susceptibility to respiratory infections. People with asthma, allergies, or heart disease are at highest risk. Even in healthy people, ozone exposure increases cough and shortness of breath.

The EPA has set safe limits: 70 parts per billion (ppb) averaged over 8 hours is the standard for outdoor air. Indoors, where ozone is not diluted by outdoor air, the concentration can climb to 100 ppb or higher in minutes if a device is running. This is a known health risk that air purifier manufacturers avoid mentioning.

Ozone generators: intentional production and harm

Ozone generators deliberately produce ozone and are marketed with pseudoscientific claims: "kills bacteria", "neutralizes odors", "purifies air". The ozone does neutralize some molecules and kill some microbes, but the cost is respiratory damage to anyone in the room.

California, the most populous US state, classifies ozone generators as pollutant sources and banned them for residential use under CARB regulations. Some states followed suit. However, they are still sold online and in big-box stores, often with misleading marketing.

Never buy an ozone generator for use while you are home. If you must use one, evacuate the building during operation and ventilate well before returning.

Ionizers: intended benefit, uncertain ozone risk

An ionizer applies an electrical charge to particles, making them stick to nearby surfaces or each other. The idea is that charged particles are easier to remove.

Some ionizers emit ozone as a byproduct of the ionization process, while others do not. The risk depends on design, voltage, and airflow. Many manufacturers do not disclose ozone levels.

If you own a purifier with an ionizer feature, check the specifications or ask the manufacturer: does this model produce measurable ozone? If the answer is unclear or if the manufacturer lists ozone output, consider turning off the ionizer and running the HEPA filter alone.

Plasma and corona discharge technologies

Plasma purifiers use electrical discharge to ionize air and create reactive species that damage microbes and break down pollutants.

Some plasma designs (dielectric barrier discharge, or DBD) emit ozone as a byproduct. Others do not. Like ionizers, the risk varies by manufacturer and design.

If you have a plasma purifier, check for ozone output in the specifications. Some CARB-certified models keep ozone emissions below 10 ppb. Uncertified models may not have been tested.

UV-C light and ozone emission

UV-C light kills bacteria and viruses by damaging their DNA. There are two main types: mercury lamps (254 nm wavelength) and excimer lamps (222 nm wavelength).

Mercury lamps at 254 nm can produce ozone as a byproduct when they run. The amount depends on the lamp design and surrounding materials. Some purifiers with UV-C include an ozone filter to absorb any ozone produced.

Excimer lamps at 222 nm do not produce ozone, making them a safer choice if your concern is ozone. However, excimer lamps are newer and more expensive than mercury lamps.

If your purifier has UV-C, check the specifications: does it mention ozone output or an ozone-absorbing filter? If not, it may be producing ozone that you cannot see or smell.

CARB certification and ozone limits in California

California CARB (Air Resources Board) regulates indoor air cleaning devices and maintains a list of certified models that meet strict ozone limits: 10 ppb or lower in California, with some models at even lower levels.

If you live in California, look for a CARB-certified model: it has been tested and verified to emit less ozone. If you live elsewhere, CARB certification is still a sign of environmental responsibility, but it is not a legal requirement.

The CARB certified list is publicly available at arb.ca.gov. You can search by product name or manufacturer to see if a model is certified.

Identifying ozone-safe purifiers

The simplest way to avoid ozone is to choose a purifier with only mechanical HEPA and activated carbon filters. These technologies produce no ozone.

Look at the specifications: if the purifier mentions "ionizer", "plasma", "active oxygen", "ozone", or "UV-C", it may emit ozone. Ask the manufacturer for the ozone output in ppb or look for CARB certification.

Do not trust marketing language like "safe", "eco-friendly", or "natural" on a device with ionizer or plasma technology. The term "active oxygen" is a marketing name for ozone.

Worth knowing before you buy