Delhi pollution: What is AQI ?

Delhi residents can experience a sudden increase in pollution levels during winter, visible as a dense smog layer over the city, resulting in breathing problems and itching in the eyes as immediate consequences and many other health conditions in the long term. Air quality, one of the essential components of Sustainable Development Goals, is deteriorating owing to population increase and rapid economic growth.  To execute policy measures against this deterioration, the government needed a scientific framework, which necessitated the formation of a National Air Quality Index (AQI).
The main objective of the index, as explained by Central Pollution Control Board, is  “to adopt/develop an Air Quality Index (AQI) based on national air quality standards, health impacts and monitoring program which represent perceivable air quality for the general public in easy to understand terms and assist in data interpretation and decision-making processes related to pollution mitigation measures”. Apart from this, the specifics of which index were to target are:
  1. Inform the public regarding the overall status of air quality through a summation parameter that is easy to understand.
  2. Inform citizens about the associated health impacts of air pollution exposure.
  3. Rank cities/towns for prioritizing actions based on a measure of AQI.

An AQI is an overall scheme that transforms weighted values of individual air pollution-related parameters (SO2, CO, visibility, etc.) into a single number or set of numbers, which can communicate detailed and complex scientific information in a simple and precise way. These individual parameters (pollutants) are 12 in number as notified by CPCB.
  1. Carbon monoxide (CO)
  2. Nitrogen dioxide (NO2)
  3. Sulfur dioxide (SO2)
  4. Particulate matter (PM) of less than 2.5 microns size (PM2.5)
  5. PM of less than 10 microns size (PM10)
  6. Ozone (O3)
  7. Lead (Pb)
  8. Ammonia (NH3)
  9. Benzo(a)Pyrene (BaP)
  10. Benzene (C6H6)
  11. Arsenic (As)
  12. Nickel (Ni)
While PM10, PM2.5, NO2, SO2, NH3, and Pb have 24-hourly as well annual average standards, Ni, As, benzo(a)pyrene, and benzene have only annual standards, and CO and O3 have short-term standards (01 and 08 hourly average). To calculate AQI, each pollutant's concentration is converted into sub-indices  and all these sub-indices are aggregated according to their contribution to pollution to get an overall AQI, which is compartmentalized into different categories with a color scheme related to it for easy recognition among the public, as shown in the following table:

A breakpoint is the concentration value at which the index jump to a different category, like breakpoints for CO in a moderate category is 2.1 to 10, which means if the CO concentration goes below 2.1mg/m3  the index will slip into satisfactory and if goes over 10mg/m3  it will jump into poor category. CPCB may review these breakpoints every three years after accounting for new research findings on air pollution exposure and health effects. Each pollution level has health effects attached to it.

Out of all the pollutants, particulate matter (PM) dominates the air of Delhi and contributes maximum to the pollution. Particulate Matter(PM): These are small solid or liquid particles with, a diameter less than 10 micrometers called PM10 and a diameter less than 2.5 micrometers called PM2.5. PM2.5 is more harmful as it can go into the lungs and affect our health, it could cause lung cancer also. The vehicle exhaust, road dust, and stubble burning in the neighboring state of Haryana, Punjab, and UP is the main reason for the increase in the concentration of PM10 and PM2.5 in Delhi.


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