India's National Air Quality Index (AQI) was launched by the Central Pollution Control Board (CPCB) to turn several pollutant measurements into a single number and a colour-coded category.
The index has six categories: Good (0-50), Satisfactory (51-100), Moderate (101-200), Poor (201-300), Very Poor (301-400) and Severe (401-500).
An AQI is built from sub-indices for individual pollutants, and the overall value is driven by the worst-performing pollutant, not by an average. It also requires a minimum amount of valid data. This is why the Observatory never averages concentrations to produce its own approximate AQI: when the required data are missing, the honest answer is "not available".