Understanding AQL and if it precisely depicts worst days

PM2.5 (particles of diameter 2.5 micrometres or less) concentration for 24 hours ending 4pm on November 18 was 549.6 microgram per cubic metre (ug/m3) and 676 ug/m3 at Jawaharlal Nehru Stadium and N ajafgarh stations in Delhi respectively. The Air Quality Index (AQI) in both peaked at 500.

- Abhishek Jha

This raises an interesting question. How good an indicator is AQI when it comes to measuring pollution?

The short answer is that it shows a diminishing return because it is designed to capture health risks (and policy response) rather than measure pollution. Because the health emergency peaks at a given level of pollution, the index has an upper bound.

For the long answer, read on.

AQI is an index meant to represent pollution, but is not the actual level of pollutants in the air. "Air Quality Index is a tool for effective communication of air quality status to people in terms, which are easy to understand. It transforms complex air quality data of various pollutants into a single number (index value), nomenclature and colour," the Central Pollution Control Board (CPCB) says.

As can be expected from CPCB's description, AQI is calculated by a predefined formula on the concentrations of different pollutants and then condensing the result to a single number. The first part of this calculation maths on concentrations of different pollutants - gives subindices of AQI for different kinds of pollutants.

The second part- condensing subindices to one number -is relatively simple. The highest subindex is declared the AQI for an air monitoring station. As expected, it is the first part that leads to AQI moving slowly for higher concentrations and then hitting a peak.