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Tuesday, February 7, 2023

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Here’s How To Understand AQI Numbers

If you logged into social media the morning after Diwali to check the air quality index in your city, chances are you’ve seen multiple numbers for the same time and location. Are they all correct? Mint explains how to interpret them:

What is the Air Quality Index (AQI)?

It measures how safe the air around you is to breathe. Organizations reporting AQI measure the density of various pollutants in the air (such as PM2.5, PM10, nitrogen dioxide, ozone, etc.) at various measurement stations, in micrograms per cubic meter. But there is a catch. A certain amount of one pollutant may not be as harmful as the same amount of another pollutant. So the amount of each pollutant 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).

What is the official AQI number?

The commonly used National Air Quality Index (NAQI) from 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 a few hours for a reality check. This year, Delhi’s AQI improved during the day after Diwali. That’s because the city had a few hours of better air on Monday night (real-time AQI ~150) compared to Monday morning (~340), before turning bad again that night (500) when the fireworks started. This means that the average improved as Tuesday progressed, but deteriorated again to reach the worst only Tuesday evening

air meter

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air meter

Is there a real-time air quality number available?

The AQI given by the SAFAR unit of the Ministry of Earth Sciences is real time, which is often mentioned. So don’t get confused if it doesn’t match the CPCB’s AQI. The latter will be significantly lower if there have been phases of better air in the previous 24 hours. For example, on Wednesday morning, Noida’s AQI was 310 according to SAFAR, but only 270 according to CPCB.

Why do I need to know the differences?

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

Are “dangerous” and “serious” the same thing?

The CPCB and SAFAR indices are categorized as 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 dangerous. That’s the difference (it’s not easy to draw parallels). In addition, the apocalyptic AQI of 999 that we have seen in recent years is not possible on the CPCB index, as it limits the AQI to 500, but it does on others.

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