Thursday, February 6th, 2025

AQI 494 and even 1600! What do these two readings of poisonous air in Delhi mean?

New Delhi: The air quality in Delhi was so bad on Monday that two different reports showed different levels of pollution. According to the Central Pollution Control Board (CPCB), Delhi’s average air quality index (AQI) was recorded at 494, the second worst figure since 2015. On the other hand, international app IQAir reported Delhi’s AQI to be more than 1,600. Why is there this difference? Could both reports be correct?

Different reports, but both correct

Actually, both the reports are correct. India’s AQI scale is limited to a maximum of 500, whereas in reality this figure could be much higher. The amount of different pollutants present in the air is converted into AQI through a formula, which gives us an idea of ​​the quality of the air at that time.

Indian scale stops at 500

Dipankar Saha, former head of CPCB’s Air Laboratory Division, said, ‘On our AQI scale, values ​​of 451 to 500 fall in the ‘above severe’ category, which is considered the highest level of air emergency. . In this situation, the strictest steps are taken to stop pollution.


Experts say that when this air quality index was being created, it was decided that it would not be practical to indicate an AQI of more than 500, because beyond a limit the adverse effects on health remain almost the same. .

However, on days like Monday, when the 24-hour average AQI (at 5 pm) reached a maximum of 500 at at least 15 stations in the city, it probably justifies showing AQI scores above 500. This may be helpful in understanding the severity of Delhi’s air crisis.

No limits in American model

IQAir’s air quality index is based on the American model, created by the Environmental Protection Agency there. Values ​​on this scale represent six different levels of air quality, ranging from ‘good’ to ‘hazardous’.

Like the Indian index, the US AQI is also generally considered on a scale of zero to 500. But there is a difference here. The highest level on the US scale is ‘dangerous’, representing any value of 301 and above. This means there is no upper limit here and values ​​greater than 500 can also be shown.

Different sources of AQI in India and America

Since different formulas are used to calculate Indian and US AQI, both will give different scores even for the same data of pollutants. In short, both scales cannot be compared directly.

On Monday, when the Indian AQI was around 490, IQAir’s index was showing values ​​above 1,000. However, both the reports were showing that the air quality was at very dangerous levels.

According to Dipankar Saha, IQAir’s AQI is sensor based, while Indian’s AQI is analyzer based. he/she told, ‘Analyst based AQI is considered more accurate. Also, we do not know exactly how IQAir AQI is calculated.

The issue of PM10 and PM2.5 also

Another pollution-related data that is often confused with AQI is the concentration of pollutants like PM10 and PM2.5. As the name suggests, it is the amount of a pollutant present per cubic meter of air, measured in micrograms. There is no upper limit to this figure. For example, the 24-hour average concentration of PM2.5 in Delhi rose to 422.8 micrograms per cubic meter at 8 pm on Monday, from 305.5 at 10 pm the previous day.

According to Indian standards, the safe limit of PM2.5 is 60 micrograms per cubic metre, while the World Health Organization (WHO) limit is 15 micrograms per cubic metre.

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