Air Purifiers and Ozone: Which Ones Emit It and How to Check
Which air purifiers can emit ozone, what the EPA says about ozone generators, California's 0.050 ppm limit, UL 2998 zero-ozone validation, and how to check a model.
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Air purifiers that only use a fan and filters, the HEPA and carbon kind, do not produce ozone. Ozone generators sold as air cleaners are designed to produce it, and the EPA warns against them. In between are electronic purifiers, ionizers, electrostatic precipitators, photocatalytic and bipolar ionization devices, some of which make ozone as a byproduct. To check a model, look it up on the California Air Resources Board's certified list, which classes every certified air cleaner as mechanical or electronic and limits electronic ones to 0.050 parts per million of ozone.
Ozone matters because it irritates the lungs. The EPA notes that relatively low amounts can cause chest pain, coughing, shortness of breath and throat irritation, may worsen asthma, and can weaken the body's ability to fight respiratory infections.
Three kinds of air cleaner, three answers on ozone
| Type | How it cleans | Ozone | What to look for |
|---|---|---|---|
| Mechanical (fan and filter) | HEPA or other filters, carbon | None | CARB "Mechanical" listing; AHAM CADR |
| Electronic (ionizer, ESP, PCO, bipolar ionization, UV) | Charges, oxidizes or irradiates particles and gases | Possible as a byproduct | CARB "Electronic" listing at or below 0.050 ppm; UL 2998 zero-ozone validation |
| Ozone generator | Produces ozone on purpose | By design | Avoid in occupied spaces |
What the EPA says about ozone generators
The EPA's page on ozone generators sold as air cleaners makes several points worth knowing:
- At concentrations that do not exceed public health standards, ozone has little potential to remove indoor air contaminants.
- Ozone does not remove particles such as dust and pollen.
- Ozone reacts slowly or not at all with many indoor chemicals, including carbon monoxide and formaldehyde, and in one study of carpet it produced aldehydes and increased total airborne organic chemicals.
- To stop mold and bacteria regrowing, ozone would need to be 5 to 10 times higher than public health standards allow.
- ASHRAE does not consider ozone useful for odor removal; it can mask odors rather than remove them.
The EPA's guidance on respiratory viruses is shorter: do not use ozone generators in occupied spaces.
California's limit and how to use its list
California requires indoor air cleaning devices sold in the state to be certified, and electronic air cleaners must meet an ozone emission concentration limit of 0.050 ppm (50 parts per billion), according to the CARB certified list. The list names each model with its type: "Mechanical" or "Electronic", where electronic covers ionizers, electrostatic precipitators, photocatalytic oxidation, hydroxyl generators and UV devices. Because California is a large market, most purifiers sold nationally are on it.
To check a model, search the list for the exact model number on the purifier's label. Examples from our reviews:
| Model | CARB type |
|---|---|
| Honeywell HPA300, HPA304 | Mechanical |
| Coway Airmega 400, 400S | Mechanical |
| Levoit Vital 200S (LAP-V201S) | Mechanical |
| IQAir HealthPro Plus XE | Mechanical |
| Winix 5510, 5520 | Electronic (PlasmaWave) |
| Blueair Blue Pure 211i Max, 411i Max | Electronic (HEPASilent charging) |
| Alen BreatheSmart 45i V2, 75i V2 | Electronic (ionizer) |
| Molekule Air Pro | Electronic (PECO) |
Not finding a model on the list does not prove it is uncertified, because names and model numbers vary. When in doubt, check the label.
UL 2998: zero-ozone validation
Some makers go further and validate their products under UL 2998, which UL describes as confirming ozone emissions below the test's quantifiable limit of 0.005 ppm, 5 parts per billion, a tenth of California's limit. The EPA's guidance on bipolar ionization recommends choosing a device that meets UL 2998 if you use one. Blueair, for example, cites zero-ozone verification by Intertek for its Blue Pure 411i Max. Check the specific certificate, not just the marketing.
Should you avoid electronic purifiers?
A certified electronic purifier stays within California's limit, and many let you switch the electronic stage off. But the filter does the work AHAM's CADR test measures, and an electronic stage adds a variable without adding certified cleaning. For homes with asthma, babies or anyone sensitive, a mechanical purifier is the simplest choice. Our ionizer guide covers the evidence on byproducts, and our asthma picks are all filter-only.
How to tell if a purifier is making ozone
- Smell. Ozone has a sharp, clean, chlorine-like smell. If a purifier smells like that, switch off its ionizer or stop using it.
- Marketing words. "Activated oxygen", "O3", "energized oxygen" and "trioxygen" all mean ozone.
- The CARB list. An ozone generator cannot be certified; a mechanical listing means no electronic stage.
Frequently asked questions
Do air purifiers produce ozone?
Fan-and-filter purifiers do not. Ozone generators are designed to, and some ionizers and other electronic purifiers can produce it as a byproduct. California certifies electronic air cleaners at or below 0.050 ppm.
Is ozone from air purifiers harmful?
The EPA says ozone can damage the lungs; relatively low amounts can cause chest pain, coughing, shortness of breath and throat irritation, and it may worsen asthma.
How do I know if my air purifier makes ozone?
Look up its exact model on CARB's certified list: "Mechanical" means no electronic stage. A sharp, chlorine-like smell or marketing terms like "activated oxygen" are warning signs.
What is UL 2998?
A UL validation that a product's ozone emissions are below 0.005 ppm (5 ppb), the test's limit of detection, a tenth of California's 0.050 ppm limit.
Do ozone generators remove odors or mold?
The EPA says ozone at public health limits has little ability to remove indoor contaminants, ASHRAE does not consider it useful for odor removal, and stopping mold regrowth would take levels 5 to 10 times above health standards.
Are HEPA air purifiers ozone free?
A purifier that uses only HEPA and carbon filters with a fan does not produce ozone. Some HEPA purifiers add an ionizer; check for an off switch or a mechanical CARB listing.
Sources
- Ozone Generators that are Sold as Air Cleaners, US Environmental Protection Agency
- Air Cleaners, HVAC Filters, and Respiratory Viruses, US Environmental Protection Agency
- List of CARB-Certified Air Cleaning Devices, California Air Resources Board
- Zero Ozone Emissions Validation, UL Solutions
- Guide to Air Cleaners in the Home, US Environmental Protection Agency
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