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How to read India’s new colour-coded Air Quality Index

Most developed countries have a colour-coded AQI to help citizens decide what precautions to take - keep the kids home from school or put off that run - for example.   | Photo Credit: R. RAGHU

The Index is centred around five chief pollutants: Particulate Matter, Ozone, Nitrogen Dioxide and Carbon Monoxide

India’s new Air Quality Index has a nifty website and you could lose a lot of time playing with it. You could also have some major questions, but we’ll get to that in a bit.

Most developed countries have a colour-coded Air Quality Index that helps citizens look up the air quality at a monitoring station near them, and decide what precautions to take based on it – keep the kids home from school, or put off that run, for example. India’s AQI announced on Monday is based on >recommendations by the Indian Institute of Technology, Kanpur.

In India, as in many other countries, the Index is centred around five chief pollutants – Particulate Matter with a diameter less than 10 micrometres (PM10), Particulate Matter with a diameter of less than 2.5 micrometers (PM2.5), ozone (O3), Nitrogen Dioxide (NO2), and Carbon Monoxide (CO). A monitoring station should be able to give you the concentration of a particular pollutant at that moment in time, and its average over a period of time – for CO and O3, the average is taken over eight hours, while for the other three, it is a 24-hour average. The unit of measurement is microgram (or milligram in the case of CO) per cubic meter.

India has set standards for what it thinks are appropriate warnings for a particular level of pollutant. Here’s what the code is:



What does this actually mean? Here’s what the readings of a monitoring station in Dwarka (Delhi) looked like as of 2 pm on Wednesday.

What the Index does is colour code each pollutant’s AQI reading according to the official code, and then assign an overall AQI on the basis of top pollutant’s reading. So since Dwarka has the highest PM 2.5 readings out of all its pollutants, its overall AQI is the PM 2.5 value. If a city has higher CO readings than PM 2.5 at a given time, CO will be the chief pollutant, and its value will be that city’s AQI.

It’s worth remembering that the values here are not the actual concentrations of that pollutant, but the concentration weighted to the scale. I called up Mukesh Sharma, Professor in the Department of Mechanical Engineering at IIT Kanpur, who developed the AQI, to explain it a little for me. Here are the “breakpoints”, meaning the boundary values that tip a pollutant from say the “good” category to “satisfactory”.

This helped me solve one of the puzzles I referred to in the beginning. On Monday when the index came out, I calculated averages for the first week of April, and >found that BTM Layout in south Bengaluru actually had the highest AQI of all locations currently measured. The Karnataka State Pollution Control Board swung into action the next day and >claimed that it had confused the CO and PM 2.5 readings. This is demonstrably false.

CO values never soar into the range of 350+ which BTM Layout’s PM 2.5 values were when I did the story – they rarely cross double digits. Look at BTM Layout’s readings as of 2 pm on Wednesday.

Its current PM 2.5 readings were a huge 500 which would be classified as an emergency in most countries. Its 24-hour PM 2.5 average, of course, were considerably lower.

All of which only highlights the biggest problem with the AQI: it hasn’t been attached to an action plan. If I live in BTM Layout and looked at the website at 2 pm, what was I to do? Should schools shut, cars go off the road, factories shut? We don’t yet know what to do with the AQI, except look it at and panic.

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