Air Quality Index (AQI) Framework

The Air Quality Index (AQI) is a single number, from 0 to 500, that tells people how clean or polluted the air is on a given day. Instead of giving many separate readings for different pollutants, it turns them into one easy number, one colour and one word, such as "Good" or "Very Poor". The higher the number, the dirtier the air and the greater the health risk. India's National AQI was launched in 2014 and is run by the Central Pollution Control Board (CPCB).

Why does it exist?

Air pollution data is very technical: micrograms per cubic metre of PM2.5, parts per billion of ozone, and so on. An ordinary person cannot tell from these numbers whether to send children to school or go for a morning walk. The AQI solves this, like a traffic signal. Green means safe; red or maroon means danger. It also gives governments a common trigger point for action, such as GRAP in Delhi-NCR.

Where did it come from?

• India's air quality standards, called the National Ambient Air Quality Standards (NAAQS), were revised in 2009 to cover 12 pollutants. These standards are set under the Air (Prevention and Control of Pollution) Act, 1981.
• The CPCB asked IIT Kanpur and an expert group to design an index suited to India.
• The National AQI was launched on 17 October 2014 by the Ministry of Environment, Forest and Climate Change. Its motto was "One Number, One Colour, One Description".
• Before this, India mostly reported raw pollutant values, which few people understood.

Which pollutants does it use?

The AQI covers eight pollutants:

1. PM10 (particles smaller than 10 micrometres)
2. PM2.5 (particles smaller than 2.5 micrometres)
3. NO2 (nitrogen dioxide, mainly from vehicles)
4. SO2 (sulphur dioxide, mainly from coal burning)
5. CO (carbon monoxide)
6. O3 (ground-level ozone)
7. NH3 (ammonia)
8. Pb (lead)

How is it calculated, step by step?

1. Monitoring stations measure each pollutant's concentration.
2. Readings are averaged: 24 hours for most pollutants, but 8 hours for CO and O3.
3. Each pollutant's average is converted into its own sub-index on the 0-500 scale, using fixed breakpoints linked to health effects.
4. The highest sub-index becomes the AQI for that station. The pollutant behind it is called the prominent pollutant. In Delhi in winter, it is usually PM2.5 or PM10.
5. An AQI is reported only if data is available for at least three pollutants, one of which must be PM2.5 or PM10. At least 16 hours of data are needed to calculate a sub-index.
6. A city's AQI is the average of its stations. The CPCB publishes a daily AQI bulletin for cities across India.

A simple picture: if your school report card were graded by your worst subject only, one bad mark would decide the overall grade. The AQI works the same way. The worst pollutant decides the AQI, because that is the one most likely to harm health.

The six AQI categories

What do the numbers mean for PM?

For the two most important pollutants, the 24-hour concentration ranges (in micrograms per cubic metre) for each category are:

• PM2.5: Good 0-30; Satisfactory 31-60; Moderate 61-90; Poor 91-120; Very poor 121-250; Severe above 250.
• PM10: Good 0-50; Satisfactory 51-100; Moderate 101-250; Poor 251-350; Very poor 351-430; Severe above 430.

Notice that the top of the "Satisfactory" band for PM2.5 (60) equals the Indian 24-hour standard. So an AQI above 100 means the air fails India's own standard.

Standards behind the index

The NAAQS limits for particulate matter are:

• PM2.5: 40 micrograms per cubic metre (annual average), 60 (24-hour).
• PM10: 60 (annual), 100 (24-hour).

The World Health Organization's 2021 Air Quality Guidelines are much stricter: PM2.5 at 5 (annual) and 15 (24-hour); PM10 at 15 (annual) and 45 (24-hour). India's annual PM2.5 standard is eight times the WHO guideline. WHO guidelines are advisory; each country sets its own legal standards.

India's position and examples

• The AQI is the trigger for GRAP in Delhi-NCR, enforced by the CAQM.
• The National Clean Air Programme (NCAP), launched in 2019, aims to cut particulate pollution in over 130 cities and uses AQI and PM data to track progress.
• The SAFAR system (System of Air Quality and Weather Forecasting and Research) of the Indian Institute of Tropical Meteorology, Pune, gives air quality forecasts for some big cities.
• Apps like Sameer (CPCB) show live AQI for cities.

Why does Delhi's AQI jump in winter?

Several factors come together from October to January:

• Temperature inversion: a layer of warm air sits over cold air near the ground, trapping pollution like a lid on a pot.
• Low wind speed means pollutants are not blown away.
• Stubble burning in Punjab and Haryana adds smoke, especially in October-November.
• Firecrackers around Diwali, plus vehicle exhaust, dust from roads and construction, and industry add to the load.
• The Indo-Gangetic plain is surrounded by the Himalayas to the north, which limits the escape of polluted air.

Commonly confused concepts

• AQI vs NAAQS: NAAQS are the legal limits for each pollutant. The AQI is a communication tool that converts readings into one number based on these limits and health effects.
• Indian AQI vs US AQI: Both use a 0-500 scale, but the breakpoints differ. The US scale is stricter for PM2.5, so the same air can show a higher number on a US-based app than on the CPCB scale.
• PM2.5 vs PM10: PM10 includes all particles up to 10 micrometres; PM2.5 is the finer part, which goes deeper into the lungs and even the blood. PM2.5 is generally more harmful.
• AQI vs GRAP: The AQI measures; GRAP acts. GRAP stages are triggered by AQI levels.
• Ground-level ozone vs stratospheric ozone: Ozone high in the sky (ozone layer) protects us from UV rays. Ozone near the ground is a pollutant formed when NO2 and other gases react in sunlight.

Issues, criticism and the way forward

• Too few monitors: Many smaller cities and rural areas have few or no monitoring stations, so their AQI may not reflect reality.
• Data gaps: Stations sometimes go offline, especially during high pollution periods, raising questions about data reliability.
• Lenient standards: Critics say India's NAAQS are far weaker than WHO guidelines, so "Satisfactory" air in India may still be harmful. Experts have called for revising the 2009 standards.
• Worst-pollutant method: The AQI shows only the worst pollutant, not the combined effect of breathing many pollutants together.
• Reactive action: Using AQI to trigger emergency steps like GRAP treats symptoms. Experts suggest year-round action on sources: cleaner fuels, public transport, dust control, crop residue management and industrial emission limits, with regional planning across the whole airshed.

Concepts to Know

• Micrometre: One-thousandth of a millimetre. A human hair is about 50-70 micrometres thick, so PM2.5 is around 20 to 30 times thinner.
• Microgram per cubic metre: The weight (in millionths of a gram) of a pollutant in one cubic metre of air. It is the usual unit for particulate pollution.
• Sub-index: A score from 0 to 500 for one pollutant. The AQI is the highest of these.
• Temperature inversion: When warm air sits above cooler air near the ground, stopping air from rising and trapping pollution close to the surface.
• Airshed: An area where air and pollution move together because of geography and weather, regardless of state borders. Delhi-NCR and the wider Indo-Gangetic plain form one airshed.