Myth vs fact · Air
The claim: “One air purifier can cover a whole flat”
Can one air purifier clean a whole flat?
The ruling
False. CADR is measured in a sealed test chamber under ANSI/AHAM AC-1, and it describes what a unit does to the volume of air it is circulating. Air does not migrate meaningfully between rooms with the doors closed, and with the doors open the unit is trying to clean the whole flat's volume against continuous infiltration from outside. Coverage figures printed on boxes are typically derived at two air changes per hour, which is inadequate for Indian conditions even in the single room the unit is standing in.
How this was checked
Checked against a published standard
AC-1 measures CADR in a 28.5 m³ sealed chamber. Applying that figure to a multi-room flat requires an assumption about air mixing that does not hold with doors closed, and an assumption about infiltration that does not hold with them open.
- 1ANSI/AHAM AC-1 uses a standardised sealed chamber. The resulting CADR is a property measured under those conditions.
- 2Converting CADR to a coverage area requires choosing an air change rate. Two per hour is common in marketing and produces a coverage figure roughly 2.5 times larger than five per hour would.
- 3Air exchange between rooms with closed doors is small — dominated by the under-door gap. A unit in the living room is not cleaning a closed bedroom in any useful sense.
- 4With interior doors open, the effective volume is the whole flat, so the same CADR now delivers a fraction of the air change rate it did in one room.
- 5Infiltration from outside continues regardless, and in a leaky flat it can refill the space as fast as the unit cleans it.
What is true in the claim
Every durable myth has a grain of truth, and pretending otherwise teaches nobody anything.
What it means for your purchase
- Size for the room, not the flat: room volume in m³ × 4 to 5 air changes per hour = the CADR you need.
- Designate one clean room, usually the bedroom, and put your best unit there with the door closed. That is where you spend the longest continuous period.
- Ignore the square-footage figure on the box and work from CADR against your own room volume.
- Seal the largest infiltration paths before buying more capacity — draught excluders are cheaper than a second purifier.
What this page does not establish
- Open-plan spaces behave differently and a single unit may genuinely serve them, though rarely as well as the coverage claim suggests.
- Ducted HVAC systems with central filtration are a different architecture entirely and this argument does not apply to them.
Frequently asked questions
How many air purifiers do I need for a 2BHK?
Should I keep the door open or closed when running an air purifier?
Sources
Every limit and standard quoted on this page comes from one of the documents below. Where a figure is not in one of these, we have not published it.
ANSI/AHAM AC-1: Method for Measuring Performance of Portable Household Electric Room Air Cleaners
Association of Home Appliance Manufacturers · checked 2026-08-01
WHO Global Air Quality Guidelines
World Health Organization · published 2021-09-22 · checked 2026-08-01
These pages check claims against published standards, the underlying physics, or our own testing where it exists — and each page says which. We do not emit ClaimReview structured data, because that markup is reserved for accredited fact-checking organisations and we are a commercial publisher. Last reviewed 2026-08-08.