How Does an Air Purifier Work? Filters, Airflow and Sensors Explained

An air purifier works by pulling room air through a series of filters with a fan and blowing the cleaned air back out. A pre-filter stops hair and large dust, a HEPA filter traps fine particles such as PM2.5, and activated carbon adsorbs some odours and gases. Run it in a closed room and the same air passes through again and again, so the particle level falls with every cycle.

The Basic Idea: Fan Plus Filters

There is no chemistry trick at the heart of a good purifier. It is a fan and a stack of filters in a sealed box. The fan creates suction, room air is drawn in through the intake, it passes each filter layer in turn, and the outlet returns it to the room. Here is the path a parcel of air takes:

1. Intake: the fan draws room air in through the grille.
2. Pre-filter: a mesh catches hair, lint and visible dust.
3. HEPA filter: dense, pleated fibre media traps fine and very fine particles.
4. Activated carbon: a porous layer holds on to some odour and gas molecules.
5. Outlet: cleaner air leaves the top or front and mixes back into the room.
6. Repeat: that air is drawn in again, many times an hour.

The order matters. Coarse layers go first so they protect the finer, costlier ones from clogging early. Sealing matters too: any air that leaks around the edge of a filter skips it entirely.

Stage 1: The Pre-Filter

The pre-filter is a simple mesh. It catches what you can see: pet hair, human hair, fluff from clothes and the grey dust that settles on shelves. On its own it does almost nothing for PM2.5, but it keeps the HEPA layer from filling up with large debris. In dusty Indian homes it is the part that needs the most frequent attention; most pre-filters can be vacuumed or washed. Clean it whenever it looks grey or the purifier reminds you.

Stage 2: How HEPA Catches Fine Particles

HEPA is not a sieve with tiny holes. It is a mat of fine fibres laid in random directions, with gaps much larger than many of the particles it stops. Particles are caught in three ways, all ordinary physics:

• Impaction: larger, heavier particles cannot follow the air as it bends around a fibre, so they crash into it and stick.
• Interception: mid-sized particles follow the airflow but pass close enough to a fibre to touch it and stay attached.
• Diffusion: the smallest particles are knocked about by air molecules and wander in a zigzag, which makes them very likely to hit a fibre.

Big particles are caught well by impaction and the tiniest by diffusion. The hardest size sits in between, roughly around 0.1 to 0.3 microns, where neither effect is strong. That is why HEPA grades are tested at the most penetrating particle size: the rating is a worst case, and particles larger or smaller are captured even more easily. An H13 filter captures at least 99.95% of particles at that size in a single pass. For how grades compare, see the HEPA vs ioniser vs UV comparison.

Stage 3: Activated Carbon for Smells and Gases

Gas molecules are far too small for HEPA to hold. Activated carbon works differently: it is charcoal treated to be full of microscopic pores, which gives it a very large internal surface. Some gas and odour molecules stick to that surface, a process called adsorption. It helps with cooking smells, some volatile organic compounds and stale odours. It fills up over time, and it is not a fix for every gas.

What CADR Measures

Clean Air Delivery Rate (CADR) is the volume of particle-free air a purifier delivers per hour, in m³/h. It combines two things: how much air the fan moves, and how much of the pollution the filter removes from that air. A purifier with a superb filter but a weak fan has a low CADR; so does a strong fan pushing air through a leaky or poor filter.

CADR is what turns a box into a result. Divide it by your room’s volume and you get air changes per hour (ACH), the number of times the room’s air passes through the filter each hour. For Indian winters, four to five ACH in a bedroom is a good aim. The air purifier size guide explains the arithmetic, and the calculator does it for you.

Sensors, Auto Mode and Why the Room Must Be Closed

Many purifiers carry an air-quality sensor. A laser particle sensor shines light through a small air sample and counts the flashes scattered by particles, sorting them by size into readings such as PM1.0, PM2.5 and PM10. In Auto mode, the purifier reads this every few seconds and speeds the fan up when the number rises (cooking, incense, a door opened to smog) and slows down once the room is clean. Sensors measure particle size, not what particles are, so no home sensor can tell pollen from dust.

The room should be closed because a purifier only cleans the air it can recirculate. With a window open, outdoor air keeps arriving and the machine ends up cleaning the street. On good-air days, ventilate first and then close up; on bad-air days, keep doors and windows shut while it runs. The where to place an air purifier in an Indian home guide covers fans and ACs.

What an Air Purifier Does Not Do

• It does not remove carbon monoxide. Neither HEPA nor carbon deals with it. A CO alarm and proper ventilation of any combustion source are what protect you.
• It does not add oxygen. It cleans the air already in the room. A stuffy room needs fresh outdoor air.
• It does not change humidity. That is a humidifier’s or dehumidifier’s job.
• It does not clean surfaces. Dust that has already settled on the floor and sofa needs a cloth and a vacuum.

Our guide on what air purifiers cannot remove goes into each limit in more detail.

A Worked Example: Boon Airvana

Boon Airvana follows the same principles, with a few design choices worth explaining:

• Dual suction: air is drawn in from both sides, through two filters, rather than through one intake.
• Four stages: a washable pre-filter mesh, H13 HEPA (99.95% of particles in a single pass), activated carbon for gases and odours, and an anti-bacterial coating on the filter.
• CADR 380 m³/h: about 484 sq ft at a 3 m ceiling at roughly 2.8 air changes an hour, and more than nine air changes an hour in a 12 × 12 ft bedroom.
• AtmoScan sensor: PM1.0, PM2.5 and PM10 (laser-based), plus TVOC, estimated CO₂, NO₂, temperature and humidity, read every second. Auto mode follows the real-time AQI.

Its fresh-air alert is a good illustration of the limits above: when estimated CO₂ rises, the purifier cannot fix it, so it tells you the room needs outdoor air. Airvana is ₹14,900 at the time of writing.

Frequently Asked Questions

How does an air purifier work in simple words?

A fan pulls room air into the purifier, pushes it through a pre-filter for large dust, a HEPA filter for fine particles and an activated carbon layer for some smells and gases, and then blows the cleaner air back out. In a closed room the same air is cleaned many times an hour, so the pollution level keeps falling.

Does a HEPA filter work like a sieve?

No. HEPA is a mat of fine fibres with gaps larger than many particles it catches. Big particles crash into fibres, mid-sized ones brush against them and stick, and the smallest drift randomly until they hit one. The hardest size to catch is around 0.1 to 0.3 microns, which is why HEPA grades are tested at that worst case.

What is CADR in an air purifier?

CADR, or Clean Air Delivery Rate, is how many cubic metres of clean air a purifier delivers each hour. It combines fan airflow and filter efficiency. Divide CADR by your room’s volume to get air changes per hour. For Indian winters, four to five air changes an hour in a bedroom is a sensible target.

Do air purifiers need the windows closed?

Yes, while they run. A purifier cleans the air it can recirculate, so an open window keeps bringing in fresh pollution and the purifier never catches up. On good-air days, open windows to ventilate first, then close them and run the purifier. On smoggy days, keep doors and windows shut.

Does an air purifier add oxygen to the room?

No. An air purifier removes particles and, with activated carbon, some odours and gases. It does not create oxygen, remove carbon dioxide or remove carbon monoxide. If a closed room feels stuffy, it needs fresh outdoor air, which you get by opening a window when the outdoor air quality allows.

How does Auto mode on an air purifier work?

In Auto mode, a built-in particle sensor reads the air every few seconds. When PM2.5 or other readings rise, for example during cooking or when incense is lit, the fan speeds up. When the room is clean again, it slows down. This saves electricity and filter life compared with running on high all the time.