Air Purifiers: How They Work, What Actually Matters, What to Buy

• Pre-filter — a washable mesh that catches hair, lint and large dust. Cleaning it monthly is the single highest-value maintenance task, because it protects the expensive filter behind it.
• True HEPA (H13) filter — pleated glass-fibre media that captures at least 99.95% of particles at the most-penetrating size, around 0.3 microns. This is what removes PM2.5, PM10, pollen, dust-mite debris and most bacteria.
• Activated carbon — a bed of porous carbon that adsorbs gases and odours: vehicle exhaust, cooking smells, smoke, VOCs from new furniture. HEPA does nothing for gases, so carbon depth is what stops a room smelling of diesel.
• Optional final stages — UV lamps, ionisers or photocatalytic layers. Treat these as supplementary. A well-sealed HEPA plus a thick carbon bed does the real work.

Manufacturer coverage figures are measured in a sealed chamber of exactly that size with no ongoing pollution source. Your bedroom has a door that opens, a window that leaks and a city outside, so treat the claim as an optimistic ceiling.

Work from CADR instead. Multiply your room's floor area in square metres by five for a baseline, then apply a pollution multiplier: about 1.6× at AQI 100–200, 2.2× at 200–300, and 3× above 300. A 15 m² bedroom in Delhi therefore wants roughly 225 m³/h, not the 75 m³/h a simple reading of the rule would suggest.

• It cannot clean a room with the windows open, or an open-plan space from one corner.
• It cannot remove gases or odours without a real activated-carbon stage.
• It cannot reduce the outdoor AQI reported by monitoring stations — only your indoor PM2.5, which is what affects you.
• It cannot remove dust mites living in mattresses and pillows. That needs allergen-proof covers and hot washing.
• It cannot fix a damp wall. Purifiers capture mould spores; they do not stop mould growing.
• It is not a substitute for prescribed asthma medication.