Ozone in a home air purifier comes from one component, and it is never the filter.
A HEPA filter cannot produce a gas. It is a pleated mat of glass fibre with a fan behind it, and nothing in that arrangement generates anything.
Ionisers are a different matter. They clean by charging the air itself, and charged air makes a small quantity of ozone as a by-product.
That is the whole distinction, and the box rarely spells it out.
Ozone is oxygen carrying a third atom, and the third atom is the problem
Ordinary oxygen is Oโ. Ozone is Oโ, and the extra atom is loosely attached.
It breaks away readily and reacts with whatever is nearest. Thirty kilometres up, that is useful chemistry.
At head height in a sitting room, the nearest thing is the lining of your airway.
The UK Health Security Agency treats ozone as a respiratory irritant. Low-level symptoms are unremarkable enough to be missed entirely.
A dry throat. A cough. A chest that feels tight by the evening.
In general, people with asthma or COPD notice sooner than everyone else.
Ioniser, plasma and active oxygen are one family under several names
An ioniser releases electrons into passing air so that particles pick up a charge, clump together and drop out of circulation.
Marketing has never settled on one word for it. Plasma, plasmacluster, bipolar ionisation, active oxygen and nanoe are versions of the same idea.
What they share is an electrical discharge in air, and every electrical discharge in air produces some ozone.
Usually the quantity is trivial. "Usually" is carrying real weight in that sentence, which is why certification exists at all.
A separate product class makes ozone deliberately. Ozone generators are sold for flood restoration and smoke damage, and they are not domestic appliances.
Bottom line: An ozone generator is a room-treatment tool rather than an air purifier, and it is designed to run in an empty room that is ventilated before anyone returns.
Two numbers separate a certified machine from an unlabelled one
The stricter of the two is UL 2998, the Zero Ozone Emissions Validation. A device passes if it emits under 5 parts per billion.
The looser one comes from the California Air Resources Board, which caps an air cleaner at 50 ppb before it may be sold in that state.
Ozone emission limits for air cleaners, in parts per billion, on a scale running to 60.
Neither is a British rule. Both are American, and both travel with the product because manufacturers build one model for every market.
So a purifier on a UK listing may carry a CARB or UL mark it was never obliged to earn here โ or carry nothing at all.
Britain sets no ozone limit for air cleaners, so any figure you see is one an American regulator asked for.
What to look for on a British listing, in order
Start with the simplest test. If the machine cleans by filter alone, there is no ozone question to answer.
If it has an ioniser, look for UL 2998, Zero Ozone Emissions Validated or CARB certified in the specification list.
If none of those appear and the ioniser cannot be switched off, treat the silence as information.
If it can be switched off, switch it off. On a machine with a genuine CADR figure, ionisation contributes very little to the result.
You lose almost nothing, and you remove the only part of the appliance capable of producing a gas.
The three machines we rate on filtration alone are in Best air purifiers in 2026: 3 picks and who each one is for.
Read the UKHSA guidance on air pollutionOfficial UK Government โ
Outdoor ozone remains the larger exposure for most households, and it peaks on still, sunny afternoons in summer.
On those days the sensible move is to keep the windows shut after lunch and let the filter do the work.
