Air purifiers: what HEPA actually means and when one is worth it

Air purifiers sold briskly through every smoke season and pandemic scare - and half the marketing around them is fiction. Here is what the machines genuinely do, in plain definitions.

The definition that matters: HEPA

True HEPA - "high efficiency particulate air", a standard defined by the US Department of Energy - means the filter captures at least 99.97 per cent of particles at 0.3 microns. That size is not a gimmick; it is the hardest particle size to catch, and both larger and smaller particles are trapped even more efficiently. Labels saying "HEPA-type" or "99%" carry no such guarantee - the two-word difference is the whole product. Note what HEPA does not do: it traps particles, not gases - odours and volatile chemicals need an activated-carbon layer, which good units include and cheap ones skip.

Sizing: the spec that decides everything

A purifier is only as good as its fit to the room. The number to check is CADR - clean air delivery rate - which combines filtration and airflow; the practical rule is matching the unit's rated room size to (or above) your actual room, and remembering that claims assume the unit runs for hours with the door closed. One small purifier "for the whole house" purifies the corner it sits in.

When a purifier genuinely earns its plug

Smoke days - wildfire smoke has made this the tier-1 use case, and a HEPA unit in the bedroom is the measured winner there. Pet dander and pollen seasons - it reduces airborne particles; allergy sufferers often notice bedrooms most. Dusty city flats with windows that cannot stay open. It pairs with - not replaces - the boring wins: source control and ventilation - the ventilation basics are here - and for central systems, filtration at the air handler: MERV 13 is the EPA-level recommendation if your system supports it.

The type to never buy

Avoid anything whose selling point is ozone - "energised oxygen", "fresh-air" ionisers that emit it deliberately. Ozone is a lung irritant with no safe indoor level; regulators are unambiguous on this. If the box promises to clean the air by reacting with it, leave it on the shelf.

The honest verdict: one true-HEPA unit, correctly sized, running in the room that matters most (usually the bedroom), is a legitimate buy for smoke, pets and pollen - while the bathroom's moisture problem, like every source problem, belongs to the fan, not the purifier.