How India’s Air Quality Index Is Calculated (CPCB Explained Simply)

Why a single AQI number exists

Air is a mixture of many pollutants. A laboratory report that lists raw micrograms per cubic metre for PM2.5, PM10, NO2, SO2, CO, and ozone is accurate, but most people cannot compare those units under time pressure. India’s National Air Quality Index (AQI) was designed so that a parent, a school administrator, or a delivery rider can glance at one number and understand whether outdoor exposure is low risk, concerning, or unsafe.

Track India AQI surfaces that same public measurement system in a cleaner interface. Understanding how the number is built helps you avoid two common mistakes: treating AQI as a pure “weather feel” score, and assuming every station in a city always tells the same story.

The six pollutants behind India’s AQI

The Central Pollution Control Board (CPCB) framework for the National AQI focuses on pollutants that are widely monitored and linked to health impacts. On most public dashboards you will see some combination of the following:

• PM2.5 — fine particles that can reach deep into the lungs and bloodstream
• PM10 — coarser dust that mainly irritates the upper respiratory tract
• Nitrogen dioxide (NO2) — strongly linked to traffic exhaust and combustion
• Sulphur dioxide (SO2) — often elevated near industrial fuel use and some power sources
• Carbon monoxide (CO) — incomplete combustion, including vehicles and generators
• Ozone (O3) — ground-level ozone formed when sunlight reacts with precursor gases

Sub-indices: each pollutant gets its own score first

AQI is not a simple average of all pollutants. First, each pollutant concentration is converted into a sub-index using official breakpoints. Those breakpoints map concentration ranges to AQI bands such as Good (0–50), Satisfactory (51–100), Moderate (101–200), Poor (201–300), Very Poor (301–400), and Severe (401–500).

Because the conversion curves differ by pollutant, a PM2.5 reading and an ozone reading that look “similar” in raw units can land in completely different AQI bands. That is intentional: health risk is not linear across pollutants, and the index is meant to reflect risk categories rather than laboratory aesthetics.

The highest sub-index becomes the overall AQI

After sub-indices are calculated, the overall AQI for a location is usually the maximum sub-index among the pollutants with valid data. If PM2.5 maps to 280 and ozone maps to 120, the headline AQI is 280 (Poor), not an average of the two. This “worst pollutant wins” rule exists so that a severe spike in one toxin is never diluted by cleaner readings in another.

That rule also explains why the dominant pollutant label on a dashboard matters. On a winter morning in north India, PM2.5 often drives the index. On a hot, still summer afternoon in a traffic corridor, ozone can take the lead even when particulate matter is only moderate.

What “station AQI” really means

AQI is reported from monitoring stations, not from every street corner. A station near a busy intersection can read higher than one inside a green campus a few kilometres away. When Track India AQI shows multiple stations for a city, treat the highest current reading as a caution signal and the station list as local context—not as proof that every neighbourhood is identical.

Missing data is another practical detail. If a sensor is offline, or if only a subset of pollutants is reported, the AQI still reflects the available valid measurements. Absence of a reading is not the same as clean air.

How to use AQI for daily decisions

Use the number as a planning tool, not a panic button. On Satisfactory days, most healthy adults can keep normal outdoor routines while sensitive people watch for mild irritation. From Moderate upward, shorten outdoor workouts, prefer filtered indoor air for children and elders, and check whether PM2.5 or gases are driving the risk.

Pair AQI with time of day. Many Indian cities show morning inversion spikes in winter and afternoon ozone rises in summer. Checking once at 7 a.m. and once at 4 p.m. is more useful than checking once a week. For a full action matrix by AQI band, see our Health Guide; for city-specific seasonal patterns, open a published city guide.

Limits of the index (and of this site)

AQI compresses complex chemistry into a communication tool. It does not replace clinical advice, emergency alerts, or regulator orders under graded response plans. Indoor air can also be worse than outdoor AQI if cooking smoke, incense, or poor ventilation dominate a room.

Track India AQI is an independent interface to public monitoring data. We process and present records for usability, publish original explainers, and keep methodology transparent—but we are not CPCB, and station networks remain the authoritative measurement source.