What's in my air
Ozone — a regulated pollutant that some purifiers deliberately produce
Ozone is a reactive form of oxygen that is a regulated ambient air pollutant, and which some air-cleaning devices generate deliberately or as a by-product of their operation.
Limits and standards
India's NAAQS are ambient (outdoor) standards, and there is no statutory Indian indoor air quality standard for homes. WHO's guideline values are health-based targets rather than regulatory limits, and are consistently stricter.
| Standard | Limit | What it means |
|---|---|---|
| India NAAQS — 8-hour average | 100 µg/m³ | — |
| India NAAQS — 1-hour average | 180 µg/m³ | — |
| WHO 2021 guideline — peak season 8-hour | 60 µg/m³ | — |
| CARB air cleaner regulation | Under 0.050 ppm ozone emission | Approximately 100 µg/m³. Certification is required for air cleaners sold in California and is widely used as an independent quality signal elsewhere. |
Where it comes from
Ozone generators
Devices that produce ozone deliberately, sold for odour removal and 'sanitising'. Often marketed with terms that avoid the word ozone.
Ionisers and plasma devices
Corona discharge produces ozone as an unavoidable by-product. The quantity varies enormously between designs, which is why the certification matters.
Some UV and photocatalytic units
UV lamps emitting at 185 nm generate ozone from oxygen. Lamps specified at 254 nm only do not.
Outdoor photochemical smog
Ozone forms outdoors from NOx and VOCs in sunlight, peaking on hot sunny afternoons, and infiltrates indoors.
Laser printers and photocopiers
A minor contributor in offices with older equipment.
Why it matters
- Ozone is a respiratory irritant, and WHO's guideline values reflect evidence of effects on lung function and respiratory symptoms.
- The concentration required to have a meaningful sanitising effect on surfaces or odours is considerably above what is acceptable to breathe. A device cannot be both effective as a sanitiser and safe to run in an occupied room.
- Ozone reacts with terpenes — limonene from citrus cleaners, pinene from pine products, and compounds released by air fresheners — to form formaldehyde and ultrafine secondary particles indoors.
- So an ozone device in a home with normal cleaning products is not a neutral choice; it is producing new pollutants from existing ones.
Note
Can you detect it yourself?
Consumer air quality monitors are genuinely useful for tracking change and poor at absolute measurement. Knowing which of the two you are looking at prevents most of the disappointment in this category.
| Question | Answer |
|---|---|
| Can you sense it? | A distinctive sharp smell, often described as being like the air after a thunderstorm or near an electrical arc. Detectable by most people at low concentration, though the sense fatigues quickly with continued exposure — so the smell fades while the exposure does not. |
| What a consumer monitor shows | Consumer ozone sensors exist but are uncommon in general air quality monitors and are frequently inaccurate. The more practical approach is to avoid ozone-emitting devices rather than measure their output. |
| Field test | Not practical domestically. Rely on device certification instead. |
What removes it
Effectiveness ratings below describe the technology working correctly, within its service life, on water or air that suits it. “Depends” means there is a condition in the caveat that decides the outcome — read it before buying.
- Not buying an ozone-emitting deviceEffective
The whole treatment for indoor ozone in most homes is to remove the source. There is no reason to have one.
- Activated carbonEffective
Carbon decomposes ozone catalytically and effectively. This is why a purifier with a substantial carbon stage also reduces infiltrated outdoor ozone.
- Ventilation timingPartial
Outdoor ozone peaks on hot sunny afternoons. Ventilating in the early morning rather than mid-afternoon reduces what comes in.
- CARB or equivalent certificationEffective
Not a removal technology but the practical purchasing control — it caps emission at under 0.050 ppm.
What does not work
Each of these is commonly assumed — or actively marketed — to help with ozone. None of them do.
HEPA filtration
A gas. HEPA does nothing to it, in either direction.
Running an ozone generator 'only when nobody is home'
Ozone reaction products — formaldehyde and ultrafine particles — persist long after the ozone itself has decayed, and they are what you return to. The reaction products, not the ozone, are the durable problem.
Source control that beats filtration
For most indoor pollutants the highest-leverage intervention is not a device. These are the changes worth making before, or instead of, buying anything.
- Do not buy ozone generators, however they are described. 'Activated oxygen', 'energised oxygen' and 'ozone therapy' all mean ozone.
- Check for CARB certification, or a published ozone emission figure, before buying anything with an ioniser, plasma or photocatalytic function.
- If your purifier has an ioniser mode, leave it off. It adds little to particle removal and may add ozone.
- Ventilate in the morning rather than the afternoon on hot sunny days, when outdoor ozone peaks.
Why 'activated oxygen' is a marketing word
Ozone has a poor reputation with regulators, so devices that produce it are frequently sold under other names: activated oxygen, energised oxygen, super-oxygen, trioxygen. All of these are ozone.
The argument offered is usually that ozone is naturally occurring — which is true, and irrelevant. Ozone occurs naturally in the stratosphere and is regulated at ground level precisely because it is harmful where people breathe.
The second argument is that it sanitises. This is also partly true, at concentrations far above what is acceptable in occupied space, which is why professional ozone shock treatment of a building requires the building to be empty and then thoroughly ventilated before re-entry. That is a fundamentally different thing from a device running in your bedroom.
Important
Frequently asked questions
Are ozone air purifiers safe?
Do ionisers produce ozone?
Is 'activated oxygen' the same as ozone?
Does activated carbon remove ozone?
Sources
Every limit and standard quoted on this page comes from one of the documents below. Where a figure is not in one of these, we have not published it.
National Ambient Air Quality Standards (NAAQS), notification dated 18 November 2009
Central Pollution Control Board, Ministry of Environment, Forest and Climate Change · published 2009-11-18 · checked 2026-08-01
India's legally applicable ambient air standards. They are ambient (outdoor) standards — India has no equivalent statutory indoor air quality standard for homes.
World Health Organization · published 2021-09-22 · checked 2026-08-01
The 2021 revision tightened most values substantially. WHO guideline values are health-based targets, not regulatory limits, and are considerably stricter than India's NAAQS.
California Air Resources Board · checked 2026-08-01
CARB certification requires air cleaners sold in California to emit under 0.050 ppm ozone. It is the most widely referenced independent check on whether a device adds ozone to indoor air.
This page states what published standards say and what the physics of each technology allows. It does not state what is in your supply — no page can. Where the two questions meet is a test result, and every entry in this library says which test to ask for.