What the AQI Number on Your Phone Actually Measures
An AQI of 180 sounds alarming, but what is it counting? Here’s how India’s Air Quality Index is worked out, what its colours mean, and why different apps show different numbers for the same air.
Daily Life Buzz Editorial 5 min readExplainer
General information, not medical advice. If you have a health condition, ask your doctor what is right for you.
In this article5 sections
On a winter morning in Delhi, Lucknow or Patna, the weather app on your phone might show an AQI of 320 in dark red. In Bengaluru on the same day it might show 70 in yellow. We all understand that a bigger number is worse. But what is the number actually counting?
The answer is less obvious than it looks. The AQI isn’t a direct measurement of anything. It’s a score, built from several measurements, designed to tell you quickly how the air might affect your health.
The short answer
India’s National Air Quality Index was launched on 17 October 2014 and is published by the Central Pollution Control Board (CPCB). Monitoring stations measure the amounts of up to eight pollutants in the air. Each pollutant’s measurement is converted into a score on the same 0 to 500 scale, called a sub-index.
The AQI you see is simply the highest of those sub-indices. In other words, it tells you about the worst pollutant at that moment, not an average of all of them.
The eight pollutants
The Indian AQI can include these pollutants:
| Pollutant | What it is | Averaged over |
|---|---|---|
| PM2.5 | Very fine particles, 2.5 micrometres across or smaller | 24 hours |
| PM10 | Coarser particles, up to 10 micrometres | 24 hours |
| Nitrogen dioxide (NO2) | Mainly from vehicle exhausts and power plants | 24 hours |
| Sulphur dioxide (SO2) | Mainly from burning coal and some fuels | 24 hours |
| Ozone (O3) | Forms when sunlight reacts with other pollutants | 8 hours |
| Carbon monoxide (CO) | From incomplete burning of fuel | 8 hours |
| Ammonia (NH3) | From fertilisers, waste and some industry | 24 hours |
| Lead (Pb) | A toxic metal, now much lower since leaded petrol was phased out | 24 hours |
For an overall AQI to be calculated, a station needs data for at least three pollutants, and one of them must be PM2.5 or PM10. In most Indian cities, especially in winter, fine particles are the pollutant that sets the number.
The AQI tells you about the worst pollutant at that moment, not an average of all of them.
What the colours mean
The 0 to 500 scale is split into six bands, each with a colour and a short health message from the CPCB:
| AQI | Category | What it means |
|---|---|---|
| 0–50 | Good | Minimal impact |
| 51–100 | Satisfactory | Minor breathing discomfort for sensitive people |
| 101–200 | Moderately polluted | Breathing discomfort for people with lung or heart conditions, children and older adults |
| 201–300 | Poor | Breathing discomfort for most people on prolonged exposure |
| 301–400 | Very poor | Respiratory illness on prolonged exposure |
| 401–500 | Severe | Affects healthy people and seriously affects those with existing diseases |
To give a sense of scale, for PM2.5 the “Good” band covers 24-hour averages of 0 to 30 micrograms per cubic metre, and “Satisfactory” covers 31 to 60. Anything above 250 micrograms counts as “Severe”.
Why your apps disagree
If you’ve ever seen one app say 150 and another say 250 for the same city, there are three common reasons.
1. Different scales
Many international weather apps use the US AQI, developed by the American Environmental Protection Agency. It also runs from 0 to 500, but its bands are much stricter for fine particles. A 24-hour PM2.5 level of 60 micrograms per cubic metre sits at the top of “Satisfactory” on India’s scale, but falls in the “Unhealthy” band on the American one.
Neither is wrong. They simply draw the lines in different places. The World Health Organization’s guidelines are stricter still: its 2021 guideline for PM2.5 is 15 micrograms per cubic metre over 24 hours.
2. Different time periods
The official Indian AQI uses averages over 24 hours, or 8 hours for some pollutants. Some apps show the latest hourly reading instead, which can jump up and down much more.
3. Different stations
Pollution can vary a lot between neighbourhoods. An app may be using the nearest government station, a private sensor, or a model that estimates air quality between stations.
For the official Indian figure, check the CPCB’s AQI portal or its Sameer app.
Using the AQI in daily life
The AQI is most useful as a quick guide for planning your day:
- Above 200, consider moving outdoor exercise indoors or to a cleaner time of day, especially for children, older people and anyone with asthma or heart conditions.
- Above 300, limit time outdoors where you can, and keep windows closed during the worst hours.
- Check the trend, not just the number. Air is often worse early in the morning and late at night in winter, when cold air traps pollution near the ground.
- Ask your doctor if you have a lung or heart condition and want specific advice, such as on masks or medicines.
Frequently asked questions
Is an AQI of 100 safe?
On India’s scale, 51 to 100 is “Satisfactory”, meaning most people won’t notice effects, but sensitive people may feel minor discomfort. International health guidelines are stricter, so lower is always better.
Why is the AQI worse in winter in north India?
Cooler, calmer air in winter traps pollution close to the ground instead of letting it rise and spread. Smoke from farm fires, vehicles, industry and household fuel all add to it. We explain this in detail in why north India chokes on smog every winter.
Does an air purifier lower the AQI?
It can reduce the fine particles inside a closed room, but it doesn’t change the outdoor AQI shown on your phone. Look for one with a HEPA filter sized for your room.
Which AQI should I trust?
For official decisions in India, such as school closures or restrictions on construction, authorities use the CPCB’s National AQI. If your app shows a different number, check which scale and which station it is using.
Sources
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