Is Delhi's AQI 'worse' than you think? How international agencies calculate air quality

India has also capped its maximum AQI at 500 and therefore a count bigger than that won’t get reflected, even if the situation is extremely severe.

The residents of Delhi continued to struggle with severely toxic quality of air as the Air Quality Index (AQI) jumped to 494 on November 19.

Data from the Central Pollution Control Board (CPCB) showed that more areas in the national capital registered an AQI of 500. This forced schools and colleges in the national capital region to suspend physical classes and move to online mode

The Supreme Court also swung into action on Monday and asked Delhi-NCR states to immediately set up teams to enforce GRAP 4 restrictions which, it said, will remain in effect even if the AQI is below 450.

Although, there is no disagreement over the severity of the situation, there is one point of contention over what is the actual AQI in Delhi and the surrounding areas. According to IQAir, an international monitoring app, the AQI in Delhi is at a whopping 1,600.

So, what’s the reason behind the difference?

The standards for measuring the air quality varies in every country. Each country has developed its own measuring system, which considers several parameters such as pollutants and difference measuring scales.

The scale of PM 2.5 in India is 60, while in other countries, who have adopted the World Health Organisation standards, it is at 5 or 10.

India has also capped its maximum AQI at 500 and therefore a count bigger than that won’t get reflected, even if the situation is extremely severe.

In India, an AQI between 0 and 50 is considered good, 51 and 100 satisfactory, 101 and 200 moderate, 201 and 300 poor, 301 and 400 very poor, 401 and 450 severe, and above 450 to 500 is severe-plus, according to the CPCB.

However, IQAir has ‘hazardous’ as the highest level of scale and has a scale of over 500.

According to IQAir, the readings are based on the US Environmental Protection Agency (EPA) National Ambient Air Quality Standards (NAAQS).

It’s AirVisual Series air quality monitors measure PM2.5, PM1, PM10, along with carbon dioxide levels. It uses PM2.5, or fine particulate matter, to determine the AQI, according to its website.

These international agencies have installed their own sensors at certain locations across the Delhi NCR region through which they monitor the quality of air.

Should we follow the data provided by CPCB, which has sensors installed at over 40 locations or international agencies? There is no wrong answers here as, despite the difference in the numbers, they all point to the same grim reality of the national capital.