Knowing AQI and whether it accurately represents the worst days

A preset formula that takes into account the concentrations of various contaminants and condenses the result into a single value is used to calculate the AQI.

The concentration of PM2.5 (particles with a diameter of 2.5 micrometers or less) at Jawaharlal Nehru Stadium and Najafgarh stations in Delhi was 549.6 micrograms per cubic meter (µg/m3) for the 24 hours ending at 4 p.m. on November 18. In both cases, the peak of the Air Quality Index (AQI) was 500.

This poses a fascinating query. To what extent does the AQI serve as a reliable indication of pollution?

Because it is intended to capture health hazards (and policy reaction) rather than quantify pollution, the quick answer is that it exhibits a diminishing return. The index has an upper bound since the health emergency peaks at a specific pollution level.

Continue reading for the lengthy response.

The actual amount of contaminants in the air is not represented by the AQI, which is a measure designed to show pollution. "The Air Quality Index is a tool that effectively communicates the state of the air quality to people in easily understood terms." According to the Central Pollution Control Board (CPCB), it converts complicated air quality data of different pollutants into a single number (index value), nomenclature, and color.

As implied by the CPCB's statement, the AQI is determined by applying a preset formula to the concentrations of several contaminants, then reducing the outcome to a single figure. Subindices of the AQI for various pollutant types are provided by the first section of this computation, which involves calculations on the concentrations of various pollutants. Reducing subindices to a single integer is the second, and rather easy, step. The air monitoring station's AQI is the highest subindex. As anticipated, it is the initial phase that causes the AQI to move slowly at greater concentrations before peaking.

In what ways does the computation of subindices understate air deterioration? The accompanying chart, which is a graphical representation of the CPCB's algorithm for turning PM2.5 concentrations into its subindex—which, on most days, also serves as the AQI for Delhi—makes this clear.

According to the chart, the AQI rises by 1.67 for every 1µg/m3 increase in PM2.5 concentration up to 60 micrograms per cubic meter (µg/m3). This occurs in the good and satisfactory categories, or the AQI range of 0-100. The subindex highlights deteriorating air quality when PM2.5 concentrations are between 60 and 120 µg/m3, moving at twice the speed (3.33 units for every 1 µg/m3 rise in concentration). This occurs in the moderately polluted (101-200) and poor (201-300) AQI ranges, which is 101-300. However, the PM2.5 subindex plateaus at concentrations above 120µg/m3 or in the very poor and severe categories, rising by just 0.77 units for every unit rise in concentration. It stops when the PM2.5 concentration hits 380 µg/m3 or an AQI of 500—a further increase does not budge the needle.

Delhi's AQI on November 18 was merely 494, in part because of the AQI flatlining at 500. While several stations had an AQI slightly below 500, resulting in an average somewhat below 500, fifteen stations reached their maximum rating. HT computed the AQI for every day of 2024 up until November 18 without the cap in order to demonstrate the impact that such a cap at 500 produces. We followed some of the same criteria as the CPCB, including selecting stations with at least 16 hours of data and calculating the average for the 24-hour period ending at 4 p.m. Nevertheless, stations without data for at least three pollutants were included (they are excluded by the CPCB). According to this research, there have been 12 days of severe quality so far in 2024, as opposed to eight, and 66 days of extremely poor quality, as opposed to 54. Furthermore, HT's investigation revealed that three out of the twelve severe AQI days (January 14, November 13, and November 18) had an AQI higher than 500. Indeed, the average AQI was just slightly higher than 500 on January 14 and November 13 (506 and 503, respectively). Only on November 18th did this number rise significantly to 653.

Is the official AQI useless in light of this analysis? Not at all. The specific goal of AQI, according to the CPCB study outlining the development of India's AQI, is not only effective communication, but effective communication on health hazards. The good category limit for PM2.5, for instance, is halfway to the national level of 60 µg/m3, whereas the satisfactory category finishes at 60 µg/m3. There is a reason why the AQI plateaus at greater concentrations.According to the 2013 HEI Global Burden of Disease Report, "the next break point for category moderate is kept as 90μg/m3 because the relative risk of Ischemic Heart Disease increases up to 90µg/m3 and then essentially plateaus off," the CPCB paper states. AQI readings are also intended to guide policy because they are associated with the Graded Response Action Plan (GRAP) to pollution.

This indicates that there are drawbacks to using the AQI as a gauge of air pollution, even though its creation may have been significant. Because the AQI travels slowly in the extremely bad and severe range, rapidly deteriorating air would appear to deteriorate slowly in areas like Delhi, where there are only a few months of good and satisfactory days. For this reason, Delhi's sense of concern is not adequately conveyed by AQI as it currently exists. Indeed, the lack of a proactive program to prevent pollution in Delhi and the northern plains as a whole should be considered the greater failure.