🌬️ Air guide

How Does an Air Purifier Actually Work?

8 min readPublished 4 March 2026Updated 1 August 2026

The whole machine in one sentence

An air purifier is a fan that pushes room air through filter media and returns it to the room. There is no chemistry, no destruction of pollutants and, in a properly designed unit, nothing added to the air. Pollutants end up in the filter, and they leave your home when you throw the filter away.

That plainness is worth stating because a great deal of marketing in this category implies something more exotic is happening. It is not. The engineering that separates a good purifier from a poor one is in how much air it can move against filter resistance, how well the body seals so air cannot bypass the media, and how much genuine filter media is inside.

Stage by stage, what each layer does

Stage	What it catches	Typical life	Why it exists
Pre-filter (washable mesh)	Hair, lint, insects, coarse dust	Wash every 2-4 weeks	Protects the expensive HEPA from being loaded by things a mesh can catch
True HEPA (H13)	PM2.5, PM10, pollen, dander, most bacteria, smoke particles	6-12 months	The core of the machine — the only stage with a testable standard behind it
Activated carbon	Odours, VOCs, formaldehyde, the smell of smoke	4-8 months	Gases pass straight through HEPA; only carbon adsorbs them
Ioniser / plasma (optional)	Charges particles so they clump and settle	n/a	Marginal benefit; can produce ozone. Best left switched off
UV lamp (optional)	Marketed for microbes on the filter surface	1-2 years	Contact time in a domestic purifier is too short to matter much

Note

Ignore the stage count on the box. Four well-built stages outperform ten thin ones, and Indian marketing routinely counts a mesh and a coating as separate 'stages' to reach a bigger number.

Why HEPA catches particles smaller than its pores

A HEPA filter is not a sieve. It is a dense, randomly arranged mat of fibres, and it captures particles by three mechanisms working at different sizes: interception, where a particle following the airflow brushes a fibre and sticks; impaction, where a heavier particle cannot turn with the airflow and hits a fibre head-on; and diffusion, where very small particles move erratically under Brownian motion and collide with fibres almost inevitably.

The consequence is counter-intuitive and worth knowing. Efficiency is lowest at around 0.3 microns — too small to impact reliably, too large to diffuse much — which is precisely why HEPA is rated at that size. Above and below 0.3 microns, a HEPA filter performs better than its rated figure, not worse. The occasional claim that HEPA 'cannot catch small particles' has this exactly backwards.

CADR and air changes: the numbers that decide whether it works

Per-pass efficiency is not the constraint — every true H13 filter is at least 99.95% efficient. The constraint is how much air the machine can process per hour, which is CADR, measured in m³/h.

Divide CADR by your room's volume and you get air changes per hour (ACH). Two ACH is a noticeable improvement, four to five is the target for a polluted city, and above six you are into diminishing returns for the noise you are accepting.

Room	Approx. volume	CADR for 5 ACH
Small bedroom, 100 sq ft	~26 m³	~130 m³/h
Standard bedroom, 150 sq ft	~40 m³	~200 m³/h
Large bedroom, 200 sq ft	~53 m³	~265 m³/h
Living room, 350 sq ft	~93 m³	~465 m³/h
Open plan, 500 sq ft	~133 m³	~665 m³/h, or two units

Tip

These assume a 2.7 m ceiling and a sealed room. Real Indian homes leak, so treat the figure as a floor rather than a target, and buy one class up.

What it cannot do, stated plainly

It cannot clean a whole flat from one room. Purified air does not travel meaningfully through a doorway against normal air movement.
It cannot keep up with an open window on a high-AQI day. Outdoor air arrives faster than any domestic unit can process it.
It does nothing for carbon dioxide. A closed bedroom with two adults can pass 2,000 ppm overnight, and only fresh air fixes that.
It does not remove carbon monoxide. CO is not adsorbed usefully by activated carbon, and a purifier is not a substitute for a CO alarm near a gas appliance.
It does not remove settled dust. The film on your shelf came from air that has already been cleaned.
It does not control humidity. That is a dehumidifier's job, and in Indian monsoon conditions it is a separate purchase.

Frequently asked questions

Do air purifiers really work?

Yes, within defined limits: in a closed room, with a correctly sized unit and a filter that is not exhausted. Independent testing consistently shows well-sized HEPA purifiers reducing indoor PM2.5 by 50-90%. They cannot clean a room with the windows open, cover a whole flat from one corner, or remove gases without a carbon stage.

What is inside an air purifier?

A fan and a stack of filters: a washable pre-filter for hair and coarse dust, a true HEPA filter for fine particles, an activated carbon bed for odours and VOCs, and sometimes an optional ioniser or UV lamp. That is the whole machine — everything else is casing, sensors and controls.

How long does an air purifier take to clean a room?

In a closed, correctly sized room, expect a substantial drop in PM2.5 within 20-40 minutes and a stable low reading within one to two hours. If levels are not falling, check for an open door or window, an exhausted filter, an undersized unit, or an ongoing indoor source such as cooking or incense.

Should I keep windows open or closed when using an air purifier?

Closed, whenever outdoor air is worse than indoor air — which in most Indian cities is most of the time in winter. Ventilate briefly at the least-polluted part of the day, usually early afternoon, then close up and let the purifier bring levels back down.

Do air purifiers use a lot of electricity?

No. Most domestic units draw 15-60 W depending on fan speed, which is roughly ₹80-250 a month running continuously. The filters cost considerably more than the electricity, and running the unit continuously on auto is both cheaper and more effective than short bursts on high.