Air Quality Index (AQI): Definition, Scale, Core Pollutants, Health Risks and India’s Framework

The Air Quality Index (AQI) is a standardised numerical scale used by governments and environmental agencies to communicate how clean or polluted the outdoor air is and what health effects might be a concern. Rather than presenting raw pollutant concentration data, which most people cannot meaningfully interpret, the AQI translates complex atmospheric measurements into a single, easy-to-understand number accompanied by a colour code and a health category.

Think of it as a report card for the air you breathe. Just as a score of 90 on an exam means something immediately, an AQI of 90 tells you, at a glance, that the air is ‘Satisfactory’ and unlikely to cause harm to most people. The AQI is not a universal standard; different countries maintain their own versions, but the underlying logic is consistent: the higher the number, the worse the air quality and the greater the risk to human health.

Table of Contents

• How the AQI Scale Works
• Core Pollutants Measured
• AQI in the Indian Context

How the AQI Scale Works

The AQI scale typically runs from 0 to 500 and is divided into six broad categories, each mapped to a colour for quick visual reference.

AQI Range	Category	Health Implication
0-50	Good	Air quality is satisfactory; little or no risk
51-100	Satisfactory / Moderate	Acceptable; sensitive individuals may experience minor symptoms
101-200	Moderately Polluted	Sensitive groups: children, the elderly and those with respiratory issues may feel the effects
201-300	Poor	Everyone may begin to experience health effects; sensitive groups face a serious risk
301-400	Very Poor	Health alert: significant effects on the general population
401-500	Severe / Hazardous	Emergency conditions; the entire population is likely to be affected

The AQI for any given location is calculated by measuring the concentrations of key pollutants, converting each into a sub-index using a standardised formula and then reporting the highest sub-index value as the overall AQI. This ‘dominant pollutant’ approach ensures that even if only one pollutant is dangerously elevated, the AQI reflects that danger accurately.

Core Pollutants Measured

The AQI is not based on a single substance. Several pollutants are tracked because each poses distinct health risks.

Particulate Matter (PM2.5 and PM10)

PM2.5 refers to fine particles smaller than 2.5 micrometres in diameter; PM10 covers slightly larger particles up to 10 micrometres. PM2.5 is especially dangerous because it penetrates deep into lung tissue and enters the bloodstream, contributing to cardiovascular and respiratory diseases. Sources include vehicle exhaust, construction dust, burning of biomass and industrial emissions.

Nitrogen Dioxide (NO₂)

Primarily produced by motor vehicles and thermal power plants, NO₂ irritates the lungs and lowers resistance to respiratory infections. Long-term exposure is linked to the development of asthma.

Sulphur Dioxide (SO₂)

Emitted largely by coal combustion and industrial smelting, SO₂ reacts in the atmosphere to form fine sulphate particles. Short-term exposure causes bronchospasm; long-term exposure contributes to chronic lung disease.

Ozone (O₃)

Ground-level ozone is not emitted directly but forms when pollutants like NO₂ and volatile organic compounds react in sunlight. It damages lung tissue, reduces lung function and aggravates asthma.

Carbon Monoxide (CO)

An odourless gas produced by incomplete combustion, CO reduces the blood’s ability to carry oxygen, posing serious risks even at moderate concentrations.

Ammonia (NH₃)

Included in India’s AQI framework (unlike many others), ammonia comes chiefly from agricultural activities and fertiliser use. It contributes to the formation of fine particulate matter.

AQI in the Indian Context

India’s National Air Quality Index was launched by the Central Pollution Control Board (CPCB) in 2014 as part of the ‘Swachh Bharat’ initiative, with Prime Minister Narendra Modi formally announcing it. India’s AQI monitors eight pollutants, PM2.5, PM10, NO₂, SO₂, O₃, CO, NH₃ and lead (Pb), a broader set than most other national frameworks. India faces some of the world’s most severe air pollution challenges. Several Indian cities, Delhi, Patna, Kanpur and Faridabad among them, routinely feature in global rankings of most-polluted cities. Delhi, in particular, experiences hazardous AQI levels during winter months when cold, stagnant air traps pollutants from crop residue burning in neighbouring Punjab and Haryana, vehicular emissions, construction dust and industrial output.

The CPCB’s Sameer app and website provide real-time AQI data from monitoring stations across the country. Under the National Clean Air Programme (NCAP) launched in 2019, India has set a target of reducing particulate pollution by 40% in 131 non-attainment cities by 2026. State-level action plans, odd-even vehicle schemes, restrictions on firecrackers and stricter industrial emission norms are among the tools being deployed. Despite progress in monitoring infrastructure, significant gaps remain. Rural areas and smaller towns often lack monitoring stations, leading to blind spots in national data. Experts also point to the need for source apportionment studies, scientifically identifying which sectors contribute most to local pollution, so that policy interventions can be better targeted.