What Is AQI and How Is It Calculated in India? (A Complete Guide)

Step outside in Delhi on a November morning, and you can often see the pollution before anyone tells you the number. A grey haze sits over the flyovers, your eyes sting a little, and your phone’s weather widget flashes a number like “412 – Severe” in angry red. That number is the Air Quality Index, or AQI, and for most Indians it has quietly become as routine a check as the day’s temperature.

But very few people actually know what that number means, who calculates it, or why the same pollution level in Delhi and Chennai can sometimes read completely differently on paper. This guide breaks down exactly what AQI is, how India’s Central Pollution Control Board (CPCB) arrives at that single number every day, what the categories from “Good” to “Severe” actually mean for your health, and what typically drives AQI higher in Indian cities.

What Is AQI?

The Air Quality Index (AQI) is a standardised scale used to communicate how polluted the air is at a given time and place, and what that level of pollution means for human health. Instead of asking people to interpret raw pollutant concentrations — say, 92 micrograms of PM2.5 per cubic metre — the AQI converts that technical data into a single, easy-to-understand number, usually between 0 and 500, along with a colour code and a plain-language category such as “Moderate” or “Very Poor.”

Think of it as a report card for the air. Just as a report card compresses marks across several subjects into one overall grade, the AQI compresses concentrations of multiple pollutants into one overall score, so that a school-going child, a delivery rider, or an elderly person with asthma can glance at it and instantly know whether it is safe to step outside.

AQI vs. Pollution vs. PM2.5: Clearing Up the Confusion

People often use “AQI,” “pollution level,” and “PM2.5” interchangeably, but they aren’t quite the same thing. Pollution refers to the actual presence of harmful substances in the air — gases, dust, smoke. PM2.5 is one specific pollutant, a measurement of fine particles in micrograms per cubic metre. AQI, on the other hand, is the translated, standardised score built from PM2.5 and seven other pollutants together, designed specifically to be understood without any technical background.

So when someone says “pollution in Delhi is 400,” what they usually mean is that the AQI reading is 400, which in turn is being driven by a specific pollutant — most often PM2.5 — that has crossed a defined concentration threshold. Knowing this distinction matters because a high AQI caused by ozone (which behaves differently and peaks in daylight) calls for different precautions than one caused by PM2.5 (which tends to be worse at night and in early mornings during winter).

Why Did India Introduce Its Own National AQI?

Before 2014, India measured air quality using a system that tracked only three pollutants and was not particularly intuitive for the general public. As pollution levels in cities like Delhi, Kanpur, and Patna began drawing national and international attention, the Ministry of Environment, Forest and Climate Change, working with the CPCB and technical experts from IIT Kanpur, launched the National Air Quality Index (NAQI) on 17 October 2014 as part of the broader Swachh Bharat push for cleaner public infrastructure and awareness.

The new index expanded monitoring from three pollutants to eight, adopted the “One Number – One Colour – One Description” format used by countries like the United States, and was designed specifically around health breakpoints relevant to the Indian population and climate. Today it operates in more than 400 cities and towns through the CPCB’s Continuous Ambient Air Quality Monitoring Stations (CAAQMS) network, alongside the SAFAR system in metros like Delhi, Mumbai, and Pune.

Which Pollutants Does India’s AQI Actually Measure?

India’s National AQI is built from eight pollutants, though not every monitoring station tracks all eight at all times. Here is what each one represents and where it typically comes from:

• PM2.5 (fine particulate matter): Particles smaller than 2.5 microns — about 30 times thinner than a human hair — produced by vehicle exhaust, diesel generators, construction dust, and crop burning. Because they are small enough to enter the bloodstream through the lungs, PM2.5 is usually the pollutant that decides a city’s overall AQI in North India.
• PM10 (coarse particulate matter): Slightly larger dust and soot particles from construction sites, unpaved roads, and industrial activity.
• Nitrogen Dioxide (NO₂): Released mainly by vehicles and diesel-powered engines, including generator sets, during combustion.
• Sulphur Dioxide (SO₂): Comes largely from burning coal and fuel oil in power plants and some industrial units.
• Carbon Monoxide (CO): A colourless, odourless gas from incomplete combustion, common near heavy traffic corridors.
• Ozone (O₃): Not emitted directly but formed when sunlight reacts with other pollutants, which is why ozone readings tend to spike on hot, sunny afternoons.
• Ammonia (NH₃): Largely linked to agricultural activity and fertiliser use.
• Lead (Pb): Now far less common since leaded petrol was phased out, but still monitored near certain industrial clusters.

Under CPCB rules, an AQI value can only be published if data is available for at least three pollutants, and at least one of those three must be PM10 or PM2.5, with a minimum of 16 hours of valid readings in a day. This is why, on some days, a monitoring station may report “No Data” instead of a number.

How Is AQI Calculated in India? (Step by Step)

This is the part most articles skim over, so let’s actually walk through it.

Step 1: Continuous monitoring

Automatic sensors at each monitoring station record the concentration of each pollutant, typically averaged over 24 hours for PM2.5, PM10, SO₂, NO₂, and NH₃, and over 8 hours for CO and O₃ (a 1-hour average is used for ozone at very high concentrations).

Step 2: Converting concentration into a “sub-index”

Raw concentration numbers aren’t directly comparable across pollutants — 90 µg/m³ of PM2.5 is a very different health risk from 90 µg/m³ of NO₂. So each pollutant’s measured concentration is converted into a “sub-index” value on the common 0–500 scale using a piecewise linear formula:

Sub-Index (I_p) = [(I_HI − I_LO) ÷ (BP_HI − BP_LO)] × (C_p − BP_LO) + I_LO

Where:

• C_p = the measured concentration of the pollutant
• BP_HI and BP_LO = the upper and lower breakpoint concentrations that bracket C_p (from the official CPCB breakpoint table below)
• I_HI and I_LO = the AQI values corresponding to those breakpoints

A worked example: Suppose a monitoring station records a 24-hour average PM2.5 concentration of 75 µg/m³. Looking at the breakpoint table, 75 falls between the “Satisfactory” breakpoint (60) and the “Moderately Polluted” breakpoint (90), which correspond to AQI values of 100 and 200 respectively. Plugging this into the formula:

Sub-Index = [(200 − 100) ÷ (90 − 60)] × (75 − 60) + 100 = (100 ÷ 30) × 15 + 100 = 50 + 100 = 150

So a PM2.5 reading of 75 µg/m³ translates to a sub-index of 150, placing it in the “Moderately Polluted” band.

Step 3: Taking the worst sub-index as the final AQI

Once a sub-index is calculated for every pollutant being monitored at that station, the CPCB does not average them. Instead, it takes the highest (worst) sub-index value among all the pollutants and reports that as the overall AQI for the location, along with naming the “prominent pollutant” responsible.

This is a deliberate, health-first design choice. Averaging could mask a dangerous spike in one pollutant by diluting it with safer readings of others. If PM2.5 alone breaches “Severe” levels while every other pollutant is “Good,” the city’s air is still genuinely hazardous to breathe — so the AQI reflects the worst case, not the average case.

Step 4: Aggregating city-level AQI

Most Indian cities have multiple monitoring stations. The city’s headline AQI (the number you see on apps and news reports) is usually derived from the average of the individual station AQIs, or sometimes the worst-performing station, depending on the reporting body.

India’s AQI Categories and What They Mean for Your Health

Once calculated, the AQI number is slotted into one of six categories, each with its own colour and a plain-language description of health impact:

AQI Range	Category	Colour Code	Health Impact
0–50	Good	Green	Minimal impact on health; safe for outdoor activity.
51–100	Satisfactory	Light Green	May cause minor breathing discomfort to sensitive people.
101–200	Moderately Polluted	Yellow	Breathing discomfort to people with lung disease, children, older adults; asthma and heart patients may feel discomfort.
201–300	Poor	Orange	Breathing discomfort on prolonged exposure for most people; discomfort to people with heart disease.
301–400	Very Poor	Red	Respiratory illness on prolonged exposure; pronounced effects for people with lung or heart conditions.
401–500	Severe	Maroon	Affects healthy people and seriously impacts those with existing diseases; can trigger respiratory emergencies.

For context, several Indian cities — Delhi, Ghaziabad, and parts of the National Capital Region among them — regularly cross into the “Very Poor” and “Severe” bands during winter, when cold, still air traps pollutants closer to the ground.

The Official CPCB Breakpoint Table

Here are the concentration ranges CPCB uses to assign each pollutant’s sub-index. Values are in micrograms per cubic metre (µg/m³) unless noted, with CO in milligrams per cubic metre (mg/m³):

Pollutant (Averaging Period)	Good (0–50)	Satisfactory (51–100)	Moderate (101–200)	Poor (201–300)	Very Poor (301–400)	Severe (401–500)
PM2.5 (24-hr)	0–30	31–60	61–90	91–120	121–250	250+
PM10 (24-hr)	0–50	51–100	101–250	251–350	351–430	430+
NO₂ (24-hr)	0–40	41–80	81–180	181–280	281–400	400+
SO₂ (24-hr)	0–40	41–80	81–380	381–800	801–1600	1600+
CO (8-hr, mg/m³)	0–1.0	1.1–2.0	2.1–10	10.1–17	17.1–34	34+
O₃ (8-hr)	0–50	51–100	101–168	169–208	209–748*	748+*
NH₃ (24-hr)	0–200	201–400	401–800	801–1200	1201–1800	1800+
Pb (24-hr)	0–0.5	0.51–1.0	1.1–2.0	2.1–3.0	3.1–3.5	3.5+

*Ozone’s “Very Poor” and “Severe” bands are measured on a 1-hour average instead of 8-hour, since ozone can spike sharply for short periods.

How India’s AQI Compares to Global Standards

India is not the only country with its own air quality index — the United States EPA, China, and the World Health Organization (WHO) all maintain their own scales, and the breakpoints differ meaningfully between them. India’s thresholds were deliberately calibrated using Indian epidemiological data and typical baseline pollution levels, which tend to run higher than in Europe or North America. This is a practical choice: if India adopted the stricter US breakpoints directly, most Indian cities would register “Severe” air quality on a near-permanent basis, which would make the index far less useful for day-to-day decision-making.

That said, the WHO’s updated 2021 air quality guidelines recommend an annual mean PM2.5 exposure of no more than 5 µg/m³ — a level that very few Indian cities currently meet even on their best days. This gap between WHO’s health-based guideline and India’s operational AQI breakpoints is intentional: CPCB’s scale is built to reflect achievable, monitorable milestones for a country still building out its industrial and transport emission-control infrastructure, while the WHO figure represents a long-term aspirational health target. For context on how consistently Indian metros compare against each other, our roundup of the most polluted cities in India tracks year-round trends rather than single-day spikes.

Common Myths About AQI in India

A few misconceptions come up repeatedly whenever AQI makes headlines, so it’s worth addressing them directly.

• “AQI only matters in winter.” Ozone-driven AQI spikes are common in peak summer afternoons in cities like Chennai and Ahmedabad, even though PM2.5-driven winter spikes get more media attention in North India.
• “A closed window keeps pollution out completely.” PM2.5 particles are small enough to seep through gaps around doors and windows over several hours; closed windows help, but they aren’t a complete barrier without air purification.
• “AQI readings from every app are identical.” Because different platforms sometimes pull from different sensor networks or apply slightly different aggregation methods across a city’s stations, small variations between apps for the same location are normal and don’t necessarily indicate an error.
• “A rainy day always means clean air.” Rain does help settle particulate matter, but if a city’s underlying emission sources — traffic, construction, industrial activity — remain unchanged, AQI can rebound quickly once the rain stops and the ground dries.

Who Actually Publishes AQI Data in India?

Three bodies form the backbone of India’s air quality reporting:

• CPCB (Central Pollution Control Board): Runs the official National AQI on the CPCB AQI portal, sourcing data from the CAAQMS network and State Pollution Control Boards.
• SAFAR (System of Air Quality and Weather Forecasting And Research): Developed by the Ministry of Earth Sciences, SAFAR additionally forecasts air quality up to three days in advance for Delhi, Mumbai, Pune, and Ahmedabad.
• SAMEER App: CPCB’s official mobile app, giving city and station-wise AQI readings, historical trends, and health advisories in real time.

Independent global platforms such as IQAir and aqicn.org also mirror much of this government data, sometimes alongside their own sensor networks, which is why AQI figures for the same city can occasionally differ slightly between apps.

What Actually Pushes AQI Higher in Indian Cities?

AQI doesn’t spike randomly — it responds to fairly predictable sources of pollution that compound during certain seasons and locations:

• Vehicular emissions: Dense traffic, ageing vehicle fleets, and idling at congested junctions are a major contributor in almost every large Indian city.
• Diesel generator (DG) sets: Backup diesel generators, widely used across residential societies, commercial complexes, and industrial units during power cuts, emit significant PM2.5 and NOx if not fitted with proper emission-control equipment. This is precisely why the CPCB has made Retrofit Emission Control Devices (RECD) mandatory for older DG sets in several states.
• Construction and road dust: Unpaved roads, ongoing construction, and debris left uncovered contribute heavily to PM10 levels.
• Crop residue (stubble) burning: Seasonal stubble burning in Punjab and Haryana sends smoke drifting into the NCR, which is one reason Delhi’s AQI worsens sharply every winter.
• Industrial emissions: Factories and power plants without adequate pollution-control systems release SO₂, NOx, and particulate matter.
• Weather and geography: Cold, calm winter air traps pollutants close to the ground (an effect called temperature inversion), which is why AQI in North India is consistently worse from late October through January than in the monsoon or summer months.

For a broader look at pollutant categories and sources, see our detailed explainer on which types of pollution include CFCs and smog, and practical measures in how we can control air pollution.

Diesel Generators, RECDs, and Their Link to AQI

Because power reliability remains inconsistent across much of India, diesel generator sets are still a practical necessity for hospitals, data centres, malls, factories, and residential towers. But an uncontrolled DG set can be a disproportionately large source of PM2.5 and NOx for its size, especially older units running without modern emission controls.

To address this, CPCB and various State Pollution Control Boards, along with the Delhi-NCR’s Graded Response Action Plan (GRAP), now require Retrofit Emission Control Devices (RECDs) on diesel generators above certain capacities. These devices are designed to cut particulate and NOx emissions significantly without needing to replace the entire generator. If you run DG sets and want to understand compliance requirements, our guides on GRAP rules for DG sets in Delhi-NCR, the benefits of installing an RECD, and how to verify a CPCB-tested RECD are useful starting points, alongside the current DG set emission regulations applicable across India.

How to Check AQI in Your City

Checking today’s AQI takes less than a minute:

1. Visit the CPCB’s National AQI dashboard for official, station-wise data.
2. Download the SAMEER app (available on Android and iOS) for real-time city and locality-level readings.
3. Check the SAFAR-India portal if you’re in Delhi, Mumbai, Pune, or Ahmedabad for short-term forecasts, not just current readings.
4. Use widely available third-party apps and websites, which often layer in satellite data and additional sensors for hyper-local estimates.

Protecting Yourself When AQI Is High

On days when your city’s AQI crosses into “Poor” or worse, a few precautions genuinely help:

• Limit outdoor exercise and strenuous activity, particularly during morning and evening peak-traffic hours when pollutant concentrations near roads are highest.
• Use N95 or N99 masks rated for particulate filtration when stepping outdoors is unavoidable — ordinary cloth or surgical masks do little against PM2.5.
• Run air purifiers with HEPA filters indoors, especially in bedrooms, and keep windows closed during the worst pollution hours.
• Keep an eye on AQI forecasts (not just the current reading) so you can plan errands, school runs, or travel around cleaner air windows.
• People with asthma, COPD, or heart conditions should keep prescribed inhalers or medication accessible and consult their doctor if symptoms worsen on high-AQI days.

Frequently Asked Questions

What is a “good” AQI number in India?

An AQI between 0 and 50 is classified as “Good,” meaning air quality poses minimal or no health risk. Readings up to 100 (“Satisfactory”) are still considered broadly acceptable for most people.

Why does Delhi’s AQI shoot up every winter?

A combination of falling temperatures, low wind speed, temperature inversion trapping pollutants close to the ground, and seasonal stubble burning in neighbouring states causes Delhi’s AQI to spike sharply between late October and January.

Is India’s AQI the same as the US AQI?

No. While both use a 0–500 scale with similar colour coding, the underlying breakpoint concentrations and category names differ. A reading of 150 in India’s index does not correspond exactly to 150 on the US EPA index, since the health breakpoints are calibrated differently.

Which pollutant usually drives AQI the highest in Indian cities?

In most North Indian cities, PM2.5 is typically the “prominent pollutant” responsible for the reported AQI, especially in winter. Ozone can dominate on hot, sunny summer afternoons in some cities.

Can AQI be calculated with data from just one pollutant?

No. CPCB requires valid data for at least three pollutants, with at least one being PM10 or PM2.5, and a minimum of 16 hours of readings in a day, before an AQI value can be officially published for a station.

Final Thoughts

The AQI number flashing on your phone is not an arbitrary figure — it’s the output of a carefully designed, health-first calculation that converts raw pollutant concentrations into something every citizen can act on. Understanding how it works, which pollutants drive it, and what typically causes it to spike in Indian cities makes it far easier to protect your family, plan your day, and push for genuine improvements — whether that’s supporting cleaner public transport, backing stricter industrial norms, or simply ensuring the diesel generator in your building complies with current emission-control requirements.

For more on how India is tackling air pollution at its source, explore our guides on controlling air pollution and maintaining your RECD-equipped diesel generator for long-term compliance.