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Air purifiers for smog: what to buy and what size

Last reviewed 4 Nov 2026 · DanixSoft

Buy a purifier with a true HEPA filter and a CADR (clean air delivery rate) for smoke that suits your room: a common rule is a smoke CADR, in cubic feet per minute, of at least two-thirds of the room’s floor area in square feet (for ceilings about 8 ft; more for higher ceilings). Run it on a high setting with the door and windows shut, and budget for replacement filters.

CADR calculator for your room

Length (ft)Width (ft)Ceiling (ft)

Minimum smoke CADR: 140 cfm · 238 m³/h

Floor area 168 sq ft; at this CADR the room’s air is cleaned about 5 times an hour. In heavy smog, bigger is better.

What to look for

True HEPA: captures at least 99.97% of 0.3-micron test particles. "HEPA-type" or "HEPA-like" has no such standard.
A published smoke CADR, ideally verified by AHAM; if a seller only quotes "room size", ask for the CADR.
Filter price and how easy filters are to find in Pakistan: you will replace them every few months in smog season.
Noise on the setting you will actually use at night.

What to avoid

Ozone generators and "activated oxygen" purifiers: ozone irritates the lungs (the US EPA advises against them).
Ionisers alone, without a HEPA filter.
A unit far too small for the room: it runs and does little.

Getting the most from it

One clean room: put it where you sleep and keep that door closed.
Leave space around it; do not push it into a corner behind furniture.
Run it continuously on smog days; air quality indoors drops quickly when it stops.

Questions people ask

Do air purifiers work for smog?

Yes, a purifier with a true HEPA filter removes most PM2.5 from the air passing through it. In a closed room of the right size it can bring indoor PM2.5 well below outdoor levels.

What CADR do I need for a 12 × 14 ft bedroom?

The floor area is 168 sq ft, so with the two-thirds rule you need a smoke CADR of at least about 112 cfm (about 190 m³/h) for an 8 ft ceiling. Use the calculator for other sizes and ceiling heights.

Should I buy a purifier with an ioniser?

Only if it also has a true HEPA filter and the ioniser can be switched off. Avoid anything that makes ozone.