Was Delhi AQI’s 494 or 1600? The difference in pollution readings explained

What’s Delhi’s Air Quality Index right now?

That’s the question some are asking.

On Tuesday, Delhi remain choked with thick, grey haze with the pollution levels of 488 AQI putting it in the ‘severe plus’ category.

This, a day after Delhi recorded its second-worst air quality in six years on Monday.

While the Central Pollution Control Board (CPCB) pegged the AQI at 494 on Monday, IQAir, an international monitoring app, showed that the AQI was at an eye-popping level of 1,600.

But what happened? And why is there a discrepancy between the two?

Let’s take a closer look:

What happened?

Delhi’s air quality entered the ‘severe plus’ category on Sunday.

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The Commission for Air Quality Management (CAQM) announced stricter pollution control measures for the Delhi-NCR effective from 8 am on Monday.

This, as the AQI touched 441 at 4 pm and rising to 457 by 7 pm due to unfavourable weather conditions.

Delhi then implemented tighter pollution control measures under Stage IV of the Graded Response Action Plan (GRAP) the next morning.

These included a ban on the entry of trucks except for those carrying essential items or using clean fuel (LNG/CNG/BS-VI diesel/electric), a ban on construction and demolition activities and school closures.

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All construction activities, including highways, roads, flyovers, power lines, pipelines and other public projects, are suspended.

First implemented in 2017, the GRAP is a set of anti-air pollution measures followed in the capital and its vicinity.

But it didn’t help.

In fact, on Monday, Delhi’s pollution levels hit an alarming high.

The CPCB said it recorded an average Air Quality Index (AQI) of 494.

This is the second highest-AQI recorded since the index was first introduced in 2015.

As per New Indian Express, of the 38 functional air quality monitoring stations in Delhi, the worst pollution levels were reported in Dwarka and Najafgarh on Monday with an AQI of 500.

As per India Today, the concentration of particulate matter in the National Capital was nearly 40 per cent more than on the night of Diwali.

The average PM2.5 level was 354 micrograms per cubic metre — over 20 times the standard prescribed by the WHO.

However, according to IQAir, the international monitoring app, Delhi’s AQI was at a scarcely believable level of 1,600.

Why is there a discrepancy between the two?

This is because each country and each agency has its own way of measuring pollution.

India, for instance, caps its AQI level at 500, as per Business Standard.

Any reading around this level is put under the ‘severe plus’ category – which indicates a public health emergency.

The CPCB classifies the air quality in the Delhi-NCR under four different groups

Stage 1: ‘poor’’ (AQI 201-300)
Stage 2: ‘very poor’ (AQI 301-400)
Stage 3: ‘severe’ (AQI 401-450)
Stage 4: ‘severe plus’ (AQI above 450).

“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, told The Times of India.

As per NDTV, IQAir, a Swiss company, follows the US-based AQI scale.

This has been developed by the US’ Environment Protection Agency.

This AQI scale has no upper level, as per Business Standard.

On this scale, anything pollution level over an AQI of 500 is considered ‘hazardous’ to human health.

Some say India’s way of measuring AQI – which uses analyser-based systems – is more accurate than international systems who rely on sensors.

“Analyser-based AQI is more accurate. Also, IQAir’s calculation methods are not entirely clear,” Saha told the newspaper.

However, regardless of which system one would prefer, there’s no getting away from the fact that Delhi’s air quality is of grave concern for all concerned.

Supreme Court slams Delhi govt

The Supreme Court on Monday slammed the Delhi government over delay in implementation of stricter anti-pollution measures under GRAP-4.

It said it will not allow scaling down of the preventive measures without its prior permission.

“The moment the AQI reaches between 300 and 400, stage 4 has to be invoked. How can you take risk in these matters by delaying applicability of stage 4 of GRAP,” the bench told the counsel.

It told the state government that the court wants to know what steps it has taken to curb the alarming rise of pollution level.

“We won’t allow scaling down of preventive measures under stage 4 even if AQI goes below 450. Stage 4 will continue till court permits,” the bench said, adding it will hear the matter in detail at the end of the day’s work.