Is Delhi's AQI 500 or 1500? Here is why you find conflicting readings

Confused by Delhi's AQI readings of 500 and 1,500? Different scales and methods explain the gap. Here's why both figures capture a harsh reality

Delhi’s air pollution reached alarming levels on Monday, with the Central Pollution Control Board (CPCB) recording an average Air Quality Index (AQI) of 494, just shy of the maximum 500 on India’s AQI scale. This marks the second-highest AQI since the index was introduced in 2015.

 

However, platforms like IQAir, a Swiss air quality monitoring company, reported AQI levels above 1,600, leaving residents questioning the accuracy of the numbers. Surprisingly, both are accurate— but for different reasons.

 

India vs international AQI scales  

 

India’s AQI scale is capped at 500. Any reading beyond this is grouped under the “severe plus” category, indicating a public health emergency. 

 

 

“Values from 451 to 500 are already in the ‘severe plus’ category, which triggers the strictest anti-pollution measures,” explained Dipankar Saha, former head of CPCB’s Air Laboratory Division, in a statement to The Times of India.

 

On the other hand, international systems like IQAir do not have an upper limit. This allows them to display AQI values far beyond 500, creating confusion, but also showcasing a grave reality.   

 

Different methodologies 

 

The difference also stems from differing measurement techniques. India’s AQI relies on analyser-based systems, which experts consider more precise. IQAir’s AQI, in contrast, is derived from sensor-based systems, which, while informative, may vary in accuracy. “Analyser-based AQI is more accurate. Also, IQAir’s calculation methods are not entirely clear,” added Saha.

 

Despite these differences, both systems highlight the same harsh truth- Delhi’s air quality has entered a hazardous territory.  Life-threatening air quality in Delhi-NCR: What can citizens, govt do now?

Pollution by the numbers  

 

Delhi’s PM2.5 levels—a key pollutant—averaged an alarming 422.8 micrograms per cubic metre on Monday. This is nearly seven times India’s permissible limit of 60 and a staggering 28 times higher than the World Health Organization (WHO)’s recommended level of 15.

 

Supreme Court’s intervention  

 

As the air quality deteriorated, the Supreme Court of India ordered Delhi and neighbouring states in the National Capital Region (NCR) to enforce the most stringent measures under Graded Response Action Plan (GRAP) Stage-IV. These restrictions will remain until further notice to mitigate the toxic air.