Ionizer vs HEPA Air Purifier: Filtration Beats Charge

An ioniser emits a stream of negatively charged ions. Airborne particles pick up that charge, repel each other, and are attracted to any earthed or oppositely charged surface — your walls, your floor, your curtains, your television screen. They agglomerate and fall out of the air.

If you measure the air with a particle counter, this works: the airborne count drops. But nothing has been removed from your home. The dust is now on surfaces, where it will be resuspended the next time someone walks across the room, makes the bed or switches on a fan. A HEPA filter, by contrast, has the particles physically in it, and they leave the building when you change the filter.

Producing ions at high voltage can split oxygen molecules and form ozone. How much depends entirely on the design — a well-engineered low-emission ioniser produces very little, and a cheap high-voltage one can produce enough to smell.

Ozone at ground level is a respiratory irritant and a criteria air pollutant in its own right. It aggravates asthma and reduces lung function, and the concentrations that do so are lower than most people assume. Both the US EPA and CPCB treat ozone as something to reduce, not to introduce indoors.

Distinguish an ioniser, which produces ozone incidentally, from an ozone generator, which produces it deliberately and in quantity. These are sold for odour removal, mould treatment and 'shock sanitising', sometimes labelled as air purifiers.

They should never be operated in occupied spaces. Professional use exists — an empty, sealed room, treated and then ventilated for hours before re-entry — but that is a remediation procedure, not a domestic appliance. If a listing offers an 'ozone air purifier' for a bedroom, that is the whole review.

Most mainstream purifiers in India — Philips, Xiaomi, Coway, Sharp, Daikin — include an ioniser, plasmacluster or nanoe stage as a switchable extra alongside a real HEPA filter. These are generally low-emission designs from manufacturers who take the regulatory exposure seriously.

The practical advice is still to leave it off. The HEPA stage is doing essentially all of the useful work, the ioniser's incremental benefit is small and contested, and switching it off removes any ozone question entirely. The one thing not to do is buy a purifier for its ionising claims while paying less attention to its HEPA grade and CADR, which are the numbers that determine whether it works.