Mechanical filters (HEPA, carbon) capture pollutants and add nothing to the air. "Active" technologies — ionizers, electrostatic precipitators, photocatalytic oxidation (PCO), plasma and some UV lamps — try to alter particles or molecules instead, and some can emit ozone or create chemical by-products. Ozone generators sold as air purifiers should be avoided in occupied rooms. If a device uses an active technology, check for certified low or zero ozone emissions and make sure the feature can be switched off.
Two families of air cleaning
| Passive / mechanical | Active / electronic | |
|---|---|---|
| Examples | HEPA and EPA filters, pre-filters, activated carbon | Ionizers, electrostatic precipitators, PCO, plasma, UV lamps, ozone generators |
| How it works | Air passes through media that captures or adsorbs pollutants | Energy (electric field, UV light, catalysts) charges particles or reacts with molecules |
| What happens to the pollutant | Held in the filter until it is replaced | Charged and deposited elsewhere, collected on plates, or chemically transformed |
| Can it add pollutants? | No (a neglected filter can smell, but it does not generate new chemicals) | Some designs can emit ozone or produce incomplete-oxidation by-products |
| How to verify | Filter class plus whole-device CADR | CADR plus ozone emission test, and by-product data where relevant |
Why ozone is the key concern
Ozone (O₃) is a reactive gas and a well-established respiratory irritant. Outdoors it is regulated as an air pollutant. Indoors, it also reacts with common compounds — for example terpenes from cleaning products and air fresheners — to form secondary pollutants, including fine particles and formaldehyde. Adding ozone to a room in the name of "cleaning" can therefore make the air chemically worse.
The California Air Resources Board (CARB) requires portable indoor air cleaners sold in California to be certified for electrical safety, and electronic air cleaners must meet an ozone emission concentration limit of 0.050 parts per million (50 ppb). CARB is explicit that its certification is about ozone and electrical safety, not about how well a device cleans the air. A stricter voluntary benchmark is UL 2998, which validates "zero ozone" claims at a threshold of 0.005 ppm (5 ppb).
Ionizers and "negative ion" generators
An ionizer emits charged molecules (ions) that attach to airborne particles. Charged particles are attracted to nearby surfaces — walls, floors, furniture, screens — or clump together and settle faster. Airborne concentration drops, but the particles are not captured; they are relocated, and some can be re-suspended when surfaces are disturbed. Visible grey deposits near an ionizer are a common side effect. Depending on design, ionizers can also emit ozone.
Some purifiers combine a mechanical filter with an ionizer that charges particles so the filter catches them more easily. That can improve single-pass capture, but it is worth confirming the ozone rating and whether the ion function can be disabled.
Electrostatic precipitators (ESP)
An ESP charges particles as they pass through the unit and collects them on oppositely charged plates inside the device. When clean and well maintained, ESPs can have high particle efficiency with low airflow resistance. Their performance falls as collector plates load, and the high-voltage charging section can generate ozone. The plates need regular washing — a maintenance task that is easy to neglect.
Photocatalytic oxidation (PCO)
PCO devices shine ultraviolet light onto a catalyst, usually titanium dioxide, to oxidise organic molecules. In principle the end products are carbon dioxide and water. In practice, oxidation of real indoor VOC mixtures is often incomplete at the short contact times available in a room device, and laboratory studies have measured intermediate products such as formaldehyde and acetaldehyde. Some systems use lamps that generate ozone as well. PCO is an active research area; without independent test data for by-products, it is sensible to treat claims cautiously.
UV-C light
Ultraviolet-C light at around 254 nm can inactivate microorganisms given enough intensity and exposure time. It is well established in specific applications, such as upper-room germicidal UV systems in some healthcare settings and irradiation of HVAC cooling coils. In small portable purifiers, air may pass the lamp in a fraction of a second, so the actual microbial dose depends heavily on design. Important distinctions:
- UV-C does nothing for particles or gases — it does not remove dust, pollen, smoke or odors.
- Lamps that emit at 185 nm produce ozone; lamps designed to block that wavelength do not.
- Direct exposure to UV-C is harmful to eyes and skin; lamps must be fully enclosed in consumer devices.
How to check a product before buying
- Identify the technology. Look past brand names such as "plasma", "bi-polar", "hydroxyl" or "nano" and ask what physical process is used.
- Ask for ozone test data. Look for CARB certification (listed on CARB's public database) or a UL 2998 zero-ozone validation.
- Confirm there is an off switch. Prefer devices where any active feature is optional and the mechanical filter works on its own.
- Ask for CADR with the feature on and off. This shows whether the active feature contributes meaningfully to clean air.
- For PCO, plasma or similar, ask whether by-products such as formaldehyde were measured and under what conditions.
Where active technologies fit
This is not a blanket rejection of every electronic technology — well-engineered, independently tested systems have legitimate uses, especially in commercial HVAC. But for a room air purifier used around people every day, the simplest robust approach is mechanical: particle filtration for particles, a sorbent such as activated carbon for gases, enough clean air delivery for the room, and ventilation for CO₂.
How this applies to CEAROX
The CEAROX CX-PRO-580 is a mechanical system: a HEPA H13 particle stage followed by a carbon filter module, moved by an EC fan. Its air cleaning does not rely on ionisation, ozone or photocatalysis.
Key takeaways
- Mechanical filters capture pollutants; some active technologies can add ozone or by-products.
- Avoid ozone generators in occupied rooms, whatever they are called.
- CARB's 0.050 ppm limit and UL 2998's 0.005 ppm threshold are useful ozone checks — neither says how well a device cleans.
- Prefer devices where active features can be switched off and whose CADR comes from mechanical filtration.
Frequently asked questions
Is a small amount of ozone from an air purifier harmful?
Ozone is a respiratory irritant and public-health agencies advise against deliberately adding it to occupied spaces. California limits air cleaner ozone emissions to 0.050 ppm for certification, and the UL 2998 'zero ozone' validation uses a far lower threshold of 0.005 ppm. If a device offers an ionizer or similar mode, you should be able to switch it off.
Do ionizers clean the air?
Ionizers charge particles so they stick to surfaces or to each other. That can reduce airborne concentrations, but the particles are deposited on walls, floors and furniture rather than captured in a filter, and some ionizers produce ozone as a by-product. Mechanical filtration captures and holds particles.
Is UV-C in a portable purifier effective?
UV-C needs sufficient intensity and exposure time to inactivate microorganisms. In a compact portable unit, air passes the lamp in a fraction of a second, so the benefit depends heavily on design and is rarely documented with independent test data. It does nothing for particles or gases.
Sources and further reading
- California Air Resources Board — List of CARB-certified air cleaning devices
- U.S. EPA — Guide to Air Cleaners in the Home
- UL 2998 — Environmental Claim Validation Procedure for Zero Ozone Emissions from Air Cleaners
- U.S. EPA — Ozone Generators that are Sold as Air Cleaners