🌬️ Air guide

AQI Explained: What Air Quality Numbers Actually Mean in India

6 min readPublished 5 February 2026Updated 1 August 2026

The Indian AQI scale

AQI	Category	What it means for you
0 - 50	Good	Minimal impact. Open the windows.
51 - 100	Satisfactory	Minor breathing discomfort possible for sensitive people.
101 - 200	Moderate	Discomfort for people with asthma, heart or lung conditions.
201 - 300	Poor	Breathing discomfort for most people on prolonged exposure. Purifier indoors, limit outdoor exertion.
301 - 400	Very Poor	Respiratory illness on prolonged exposure. Keep windows shut, run purifiers, avoid outdoor exercise.
401 - 500	Severe	Serious health impact even for healthy people. Minimise all outdoor exposure.

Why PM2.5 is the number to watch

AQI is a composite index built from several pollutants, and in most Indian cities the reported figure is driven by PM2.5 — particulate matter under 2.5 microns.

Size is what makes it dangerous. Larger particles are largely trapped in the nose and upper airway. PM2.5 is small enough to reach the deepest parts of the lungs and to cross into the bloodstream, which is why it is associated with respiratory and cardiovascular effects rather than just irritation.

It is also the fraction a good HEPA purifier handles well, which is why a numeric PM2.5 readout is the most useful feature on an air purifier.

Note

The WHO's annual PM2.5 guideline is 5 µg/m³. Many Indian cities average 10 to 20 times that. Acclimatisation is real — but it is a change in perception, not in physiological effect.

What to target indoors

Below 15 µg/m³ PM2.5 — excellent. Achievable in a closed room with a correctly sized purifier.
15-25 µg/m³ — good. A reasonable everyday target for an Indian home in winter.
25-50 µg/m³ — acceptable, but your purifier is undersized for the conditions or the filter is loading up.
Above 50 µg/m³ indoors — something is wrong: exhausted filter, undersized unit, or significant infiltration from open windows or gaps.
Test it yourself: note the reading, close the room, run on high for 40 minutes and watch the number fall. That is your proof it works.

Practical habits that matter as much as the machine

Keep windows and doors closed when outdoor AQI is above about 150 — ventilation is working against you at that point.
Use your kitchen chimney or exhaust fan every time you cook. Frying can push indoor PM2.5 above 300 µg/m³ on its own.
Do not burn incense, candles or mosquito coils in a closed room you are trying to keep clean.
Run the purifier on high for the first hour after you close up a room, then let auto mode hold it.
Prioritise the bedroom. Eight uninterrupted hours in one closed room is where a purifier delivers most of its value.

Frequently asked questions

What is a safe AQI level?

On India's CPCB scale, 0-50 (Good) and 51-100 (Satisfactory) are broadly safe for most people. Above 200 (Poor), prolonged exposure causes discomfort for most people; above 300 (Very Poor) it risks respiratory illness. Indoors, aim to keep PM2.5 below 25 µg/m³ and ideally below 15.

What does PM2.5 mean?

PM2.5 is particulate matter smaller than 2.5 microns. It matters more than larger particles because it can reach the deepest parts of the lungs and pass into the bloodstream, which is why it is linked to respiratory and cardiovascular effects. It is also the pollutant a HEPA air purifier removes most effectively.

Does an air purifier reduce AQI?

It reduces indoor PM2.5, which is what actually affects you — it has no effect on the outdoor AQI reported by monitoring stations. In a closed room, a correctly sized HEPA purifier can hold indoor PM2.5 below 50 µg/m³ even when outdoor AQI is in the 400s. Verify it with a unit that has a numeric readout.