Delhi's AQI 494 or 1,600? What's with the difference and which is more accurate

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ET OnlineLast Updated: Nov 19, 2024, 11:54:00 AM IST

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Delhi experienced alarming air pollution levels on Monday. The city's air quality index (AQI) reached 494, the second-highest recorded value. While international platforms reported even higher figures, both measurements indicated hazardous air quality. The discrepancy arises from different AQI scales and calculation methods. Despite the variations, Delhi's air pollution posed significant health risks.

Delhi’s air pollution hit a dangerous peak on Monday, with the city recording an average AQI of 494, according to the Central Pollution Control Board (CPCB). This marked the second-highest AQI since the index’s inception in 2015.

Simultaneously, international platforms like IQAir reported AQI values surpassing 1,600, sparking confusion among residents: Which reading is accurate? Well, technically, both are.

The discrepancy arises because India’s AQI scale is capped at 500, with anything beyond this classified as “severe plus.”

"In our AQI scale, values from 451 to 500 fall in the ‘severe plus' category, which is considered the highest level of air emergency and corresponds to the strictest anti-pollution measures envisaged so far," Dipankar Saha, former head of CPCB's Air Laboratory Division, told TOI.

Conversely, the US-based AQI model used by Swiss company IQAir lacks an upper limit, allowing it to report values exceeding 500.

Another key difference lies in measurement methodologies. IQAir's AQI is sensor-based while the Indian one is analyzer-based, making the latter more accurate. Despite these differences, both scales painted the same grim picture: Delhi’s air was severely hazardous.

The US and Indian Air Quality Indexes (AQIs) use different formulas to calculate air quality. This means the two indexes will produce different scores even with the same data about pollutants in the air, making the two incomparable.

"Analyser-based AQI is considered more accurate. Also, we don't entirely know how the IQAir AQIs are being calculated," Saha said.

Highlighting pollutant concentrations, Delhi’s PM2.5 levels averaged 422.8 micrograms per cubic meter—well above India’s limit of 60 and the World Health Organization’s stricter cap of 15.

On Monday, the Supreme Court noted the fact that the Air Quality Index (AQI) entered into the 'severe plus' category and directed all Delhi-NCR states to immediately set up teams to strictly enforce anti-pollution GRAP 4 restrictions, reiterating that the curbs will stay in place till further orders.

(With TOI inputs)

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