HEPA (High-Efficiency Particulate Air filter)

A dense filter mesh that physically traps fine airborne particles, including PM2.5-sized smog and dust.

A True HEPA filter is a specific, tested standard of filtration — a dense mesh fine enough to capture the overwhelming majority of particles down to roughly 0.3 microns, which covers PM2.5 particulate matter (a major component of smog), pollen, dust and many airborne allergens.

It's a purely physical filtration process — air is forced through the mesh and particles are trapped on it — not a chemical or ionising process. That's why filter replacement matters: a HEPA filter loaded with trapped particles eventually restricts airflow and needs changing, it doesn't "wear out" chemically.

HEPA handles particulates specifically — it does nothing for humidity (see relative humidity) and, on its own, limited odour control, which is why purifiers often pair it with an activated carbon stage.