Local sources of pollution, rather than farm fires, were primarily to blame for Delhi’s deteriorating air quality ahead of Diwali this year, according to a report by the non-profit organisation Centre for Science and Environment on Thursday.

Delhi’s air quality deteriorated from “poor” to the “very poor” category of the air quality index even though stubble burning in the neighbouring states of Punjab and Haryana was only responsible for 4.4% of the pollution load in the capital, the analysis said.

An air quality index value between 201 and 300 is considered “poor”, while an index value between 301 and 400 is considered “very poor”. Air quality in the “very poor” category can lead to respiratory illness on prolonged exposure.

The Centre for Science and Environment analysed PM2.5 concentration trends from September 15 to October 28 and compared the data with that of previous years.

PM2.5 refers to tiny airborne particles about 30 times smaller than the width of a human hair and can easily be breathed into the lungs and the bloodstream.

The organisation found that vehicular emissions contributed to more than half of the pollution emanating from local sources in Delhi. This was followed by residential burning at 13%, industry at 11%, construction at 7%, energy at 6%, waste burning at 5% and road dust and other sources at 4% each.

From September 15 to October 28, there were 16 days when Delhi’s air quality was either “poor” or “very poor”, up from 13 days each in the past two years.

On the other hand, the number of “good” air quality days fell from 11 in 2022 to just two days this year. An air quality index between zero and 50 is considered “good”.

The analysis showed that traffic congestion was the highest during the weekend just before Diwali, on October 26 and October 27. During this time, nitrogen dioxide levels spiked, peaking at 75 micrograms per cubic meter on the evening of October 27.

The study said that vehicles caught in traffic or idling can spew emissions several times higher than while driving.

‘Can’t hide behind smokescreen anymore’

Anumita Roychowdhury, executive director, research and advocacy at the Centre for Science and Environment, noted that for most of the period under study, the contribution of farm fires to pollution remained in the range of 1% to 3%. The contribution of stubble burning only rose to between 8% and 16% on two days, she said.

“With the overall number of farm fire counts declining, the city cannot hide behind its smokescreen anymore,” Roychowdhury said. “This demands very stringent advanced action at scale and with speed to minimise the local pollution in Delhi and its surrounding region.”

The non-profit organisation published its findings even as many parts of Delhi remained covered in smog on Thursday, on the eve of Diwali.

Air quality deteriorates sharply in the winter months in Delhi, which has often been ranked as the world’s most polluted capital. Other than traffic, falling temperatures, decreased wind speeds and emissions from industries and coal-fired plants are factors that contribute to the uptick in air pollution.

The burning of crop stubble in neighbouring states is believed to compound the problem on a few days during the winter months.

Also read: Why some Delhi residents leave the city during Diwali