Explainer: Why does the same part of Delhi show different Air Quality readings at the same time?

On November 3, a week after Diwali, Delhi saw its air quality deteriorate to emergency levels. News channels and Twitter users enthusiastically displayed app screenshots depicting doomsday-type air quality index values – some of them at variance with each other. Some users said their apps showed that the AQI was 1,500-plus. Others anxiously claimed that though the pollution levels were much higher, display meters were stuck at 999 because they could not show four digits.

All the while, India’s National Air Quality Index, which is maintained by the Central Pollution Control Board, stayed only in “severe” zone at around 490 for the National Capital Region.

Who, then, was accurate? Was there an error or sensationalism in media reporting or were government-run agencies playing down the situation? The short answer: everyone is correct (and pollution levels are indeed apocalyptic) but they are each displaying numbers from a different source.

Three indices

The three indices most often used by the media when it reports on air quality are the Central Pollution Control Board’s National Air Quality Index, the Ministry of Earth Sciences’ SAFAR, or System of Air Quality and Weather Forecasting And Research, and the index by the World Air Quality Index Project, a global nonprofit that covers 1,000 cities in the world.

Each source has its own scale, computing method, monitoring stations and ways to determine severity levels. That is why one area could have multiple values for Air Quality Index at the same time, as shown below.

But these sources have a few things in common.

They all take into account concentration levels of various pollutants – such as PM10 (particulate matter with a diameter of less than 10 microns), PM2.5, ozone and nitrogen dioxide – at a given place and time. The concentration – a measure of how much of the substance is present in the air – is measured in micrograms per cubic metre (µg/m³). But is 250 µg/m³ of PM10 as dangerous as 250 µg/m³ of PM2.5? No.

To get around this, these agencies convert each concentration level into a “pollutant sub-index” to make them comparable to each other. For the Central Pollution Control Board’s index, a PM10 level of 250 µg/m³ corresponds to a sub-index of 200, while the same concentration of PM2.5 means a sub-index of 400 – much more harmful.

In the final step, each agency picks the pollutant with the worst sub-index, and announces that figure as the Air Quality Index of that place at the given time. For example, if the PM10 sub-index in Dwarka’s Sector 8 is 299 and the PM2.5 sub-index is 385, as in the screenshot below, then the AQI of that place is reported as 385.

This method is used by all three agencies.

Different time scales

However, though SAFAR and Central Pollution Control Board use the same scale and sub-indices, they typically report different figures. For instance, on October 28, the morning after Diwali, SAFAR reported an AQI of 463 (“severe” category) at 10 am in Delhi, while CPCB’s index was still at 345 (“very poor” category).

This is because they use different timeframes. The Central Pollution Control Board uses a 24-hour average data, while SAFAR reports a real-time figure on its website and mobile app. So, on the post-Diwali morning, the pollution control board’s AQI rose as the day progressed as it took into account a longer part of that day. By 4 pm on October 28, the Central Pollution Control Board’s AQI had worsened to 368 for Delhi.

Another reason why the Central Pollution Control Board and SAFAR differ in cases of extreme pollution is that the pollution control board caps pollutant sub-indices at 500. Even if a pollutant sub-index goes up to 800, the pollution control board will not only record it as 500, but also use 500 when calculating the 24-hour average. As a result, there is no way the CPCB index can ever cross 500 – in fact, a pollution control board figure of 499 is apocalyptic because it would mean there had been no period in the previous 24 hours when the AQI had dropped.

SAFAR, on the other hand, refers to 500+ levels as “severe-plus” or “emergency”.

Here’s how the Central Pollution Control Board and SAFAR categorise air pollution levels.

World Air Quality Index Project

Confused about the difference between “hazardous” air quality and “severe” air quality? The answer lies in the third index, maintained by World Air Quality Index Project. This index, like SAFAR’s, is real-time but has a different method to compute and categorise the sub-indices.

While the Central Pollution Control Board and SAFAR compute sub-indices using a similar method, this third index uses the scale developed by the United States Environmental Protection Agency, even though it says that India’s national index, with its higher readings, is “more adapted to Asian dust”. To put it simply through an example, a PM10 concentration level of 400 µg/m³ corresponds to a sub-index of 362 for CPCB and SAFAR, but 264 on the US scale.

Here’s an illustration by World Air Quality Index Project: (the first scale is the US scale, the second the National Air Quality Index scale, and the bottom axis shows the PM10 concentration levels)

Since the index value is so different, so are the severity zones.

This air quality index is capped at 999.

Sources of AQI

Finally, let’s look at the sources each air quality index uses.

The Central Pollution Control Board’s National Air Quality Index uses readings at 37 stations in Delhi – 24 maintained by the Delhi Pollution Control Committee, seven by the India Meteorological Department, and six of its own.

SAFAR’s readings are based on nine stations in Delhi and one each in Noida and Gurugram. The agency also gives AQI levels for Pune, Mumbai and Ahmedabad. Both SAFAR and CPCB give the overall air quality for the city as an average of all their stations.

The World Air Quality Index Project uses readings from stations of Delhi Pollution Control Committee, and does not give a city average.

Here’s what to watch out for when making sense of an AQI figure: the reporting agency, the timeframe and the kind of scale. If you are looking at the Central Pollution Control Board’s figure, be aware that it is a 24-hour average and in extreme pollution cases, may be playing down figures significantly because of its 500 cap. To know the real-time number, use SAFAR or World Air Quality Index Project, but be cognisant of their different scales.

Don’t let media reports comparing apples with oranges fool you into believing the world is suddenly over – even though it may well be.