The Air Quality Index (AQI) is an indicator of air quality developed by government agencies across the globe to communicate to the public how polluted the air is. It was launched keeping in mind the idea of ‘One Number-One Colour-One Description’. There are different scales and methodologies used by different countries. In India, the National Air Quality Index scale classifies Air Quality into six categories (ranging from good to severe):
| AQI | Category |
| 0 – 50 | Good |
| 51 – 100 | Satisfactory |
| 101 – 200 | Moderate |
| 201 – 300 | Poor |
| 301 – 400 | Very Poor |
| 401 – 500 | Severe |
The measurement of air quality is based on the eight pollutants (PM 2.5, PM 10, Nitrogen Dioxide, Sulphur Dioxide, Carbon Monoxide, Ozone, Ammonia and Lead) for which short-term (up to 24-hourly averaging period) National Ambient Air Quality Standards are prescribed and the worst reading in these pollutants represents the AQI for that region. The Sub-indices for individual pollutants at a monitoring location are calculated using its 24-hourly average concentration value (8-hourly in case of CO and O3) and health breakpoint concentration range. The worst sub-index is the AQI for that location. All the eight pollutants may not be monitored at all the locations. Overall AQI is calculated only if data are available for minimum three pollutants out of which one should necessarily be either PM2.5 or PM10. Else, data are considered insufficient for calculating AQI.