Air Quality Index

India’s Air Quality Index (AQI) translates measurements of several air pollutants into one number and a health-related category that people can understand. It converts each available pollutant concentration into a sub-index on a common scale, then uses the highest sub-index as the overall AQI rather than averaging them.

Pollutants and averaging periods

The national AQI framework developed by the Central Pollution Control Board (CPCB) covers eight pollutants. Their measured concentrations must correspond to specified averaging periods before conversion into sub-indices.

• The eight pollutants are particulate matter PM2.5 and PM10, nitrogen dioxide, sulphur dioxide, carbon monoxide, ozone, ammonia and lead.
• Concentrations are assessed using 24-hour averages, except for carbon monoxide and ozone, which use 8-hour averages.
• An overall AQI requires data for at least three pollutants, including either PM2.5 or PM10; measurements of all eight are not compulsory.

From concentrations to the composite index

Each pollutant has its own concentration breakpoints, which separate air-quality categories. These differ because pollutants have different concentration ranges and health effects.

• First, the measured concentration is located between the appropriate lower and upper concentration breakpoints.
• The pollutant sub-index is calculated by linear interpolation: I = [(Ihigh - Ilow)/(Bhigh - Blow)] × (C - Blow) + Ilow, where C is the concentration, B values are its bracketing concentration breakpoints, and I values are the corresponding index breakpoints.
• The overall AQI is the maximum sub-index among the pollutants for which valid data are available.
• Consequently, relatively clean readings for other pollutants cannot dilute the category determined by the worst pollutant; AQI is neither a sum nor an arithmetic mean of concentrations.

Reading the six air-quality categories

The resulting index is communicated through six categories, associated colours and health descriptions. Higher values indicate poorer air quality and greater health concern.

• Good covers 0-50 and indicates minimal health impact.
• Satisfactory covers 51-100 and may cause minor breathing discomfort to sensitive people.
• Moderately polluted covers 101-200 and may cause breathing discomfort to people with lung or heart disease, children and older adults.
• Poor covers 201-300 and may cause breathing discomfort to most people on prolonged exposure.
• Very poor covers 301-400 and may cause respiratory illness on prolonged exposure.
• Severe covers 401-500 and can affect healthy people and seriously affect those with existing diseases.