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If you logged on to social media to check the air quality index in your city the morning after Diwali, chances are you saw multiple numbers floating around for the same time and location. Are all of them correct? Mint explains how to interpret them:

What is the air quality index (AQI)?

It measures how safe the air around you is for breathing. Organizations that report AQI measure the density of various pollutants in the air (such as PM2.5, PM10, nitrogen dioxide, ozone, etc) at different monitoring stations, in micrograms per cubic metre. But there’s a catch. A particular amount of one pollutant may not be as harmful as the same amount of another pollutant. So, each pollutant’s quantity in the air is adjusted to a common scale (say, 0 to 500) that works for all pollutants. Finally, the pollutant with the worst sub-index determines the AQI for that time and location (see chart).

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What is the official AQI number?

The widely-used National Air Quality Index (NAQI) given by the Central Pollution Control Board is a 24-hour average. So, if the NAQI on the morning after Diwali sounds too good to be true, wait for a few hours for a reality check. This year, Delhi’s AQI improved throughout the day after Diwali. That’s because the city had better air (real-time AQI ~150) for a few hours on Monday evening as compared to Monday morning (~340), before turning bad again (500) that night as fireworks began. This means the average improved as Tuesday progressed, but worsened again to reach its worst only by Tuesday night

Air gauge
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Air gauge

Is there a real-time air quality number available?

The AQI given by the Ministry of Earth Sciences’ SAFAR unit is real-time, which is commonly cited. So, don’t get confused if it doesn’t match with the CPCB’s AQI. The latter will be markedly lower if there have been phases of better air in the preceding 24 hours. E.g., on Wednesday morning, Noida’s AQI was 310 as per SAFAR, but only 270 as per CPCB.

Why do I need to know about the differences?

A Delhi BJP spokesperson on Tuesday cited a third index (by the World Air Quality Index Project) to claim an AQI much higher than CPCB’s or SAFAR’s. That index uses a different scale, developed by the US Environmental Protection Agency, which itself admits that NAQI is better suited for Asian dust. Yet, this third index is useful because it allows us to compare cities across the world. If we don’t know the difference, politicians may use convenient numbers (sometimes unintentionally) to underplay or overplay AQI.

Are “hazardous" and “severe" same?

The CPCB and SAFAR indices are categorized into good, satisfactory, moderate, poor, very poor, and severe, with an additional “severe-plus/emergency" label for 500+ in SAFAR. The World Air Quality Index Project uses good, moderate, “unhealthy for sensitive groups", unhealthy, very unhealthy, and hazardous. That’s the difference (it’s not easy to draw parallels). Moreover, the apocalyptic AQI of 999 seen in past years isn’t possible on the CPCB index since it caps the AQI at 500, but is possible on others.

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Tanay Sukumar

Tanay leads Mint's data journalism team. His role involves editing and overseeing the newspaper's diverse data offerings, ranging from deep analytical pieces to bite-sized social media charts.
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