New Delhi’s air quality dropped to the “severe” category on Thursday – at around 9pm – for the first time this season as the Capital celebrated Diwali with people flouting a ban on firecrackers.

While New Delhi’s air quality index (AQI) was recorded at 382, which is “very poor”, according to a 4pm bulletin released by the Central Pollution Control Board (CPCB), calm wind conditions and contribution from stubble burning, which touched a season high of 25% on Thursday, further deteriorated the air quality.

By evening, emissions from firecrackers began to be added to New Delhi’s air as the city’s PM 2.5 concentration levels spiked to over 33 times of what is deemed the safe limit.

CPCB data showed the 24-hour average AQI touched “severe” at 9pm, gradually rising each hour as a smoky haze enveloped the Capital.

The AQI was 404 at 9pm, and then it rose to an average of 422 by midnight, 428 by 2am, 444 by 6am, 446 by 7am, and 451 by 8am on Friday.

Also Read: Delhi’s air quality recorded in ‘hazardous’ category in some places after Diwali

The CPCB uses a 24-hour rolling average to ascertain the air quality index of the last 24 hours.

The CPCB classifies an AQI of 51-100 as “satisfactory”, 101-200 as “moderate”, 201-300 as “poor”, 301-400 as “very poor” and above 401 as “severe”.

Agencies had forecast New Delhi’s air to reach the higher end of the “very poor” category on Diwali, even without firecrackers, as the wind direction changed to northwesterly in the last 24 hours.

It meant contribution from stubble burning, which dropped to just 8% on Wednesday, was up at a season-high share of 25% on Thursday.

The System of Air Quality and Weather Forecasting And Research (SAFAR), run by the ministry of earth sciences, had said in its Diwali forecast that if even 50% of the firecracker load from 2019 was burst this year, the air quality would plunge to “severe” category and would remain in that range on Friday as well.

As per SAFAR’s model, the share of stubble burning is expected to touch 35% on Friday and 40% on Saturday.

“The extremely calm wind conditions in Delhi, combined with a 25% stubble share (from 2,293 fire counts) are two major factors today,” said SAFAR in its forecast on Thursday. “Relief is expected only from the evening of Saturday, but the AQI will still fluctuate within the ‘very poor’ range.”

Ashok Vihar in northwest Delhi recorded the highest hourly PM 2.5 concentration in the last 24 hours, touching 1,984 micrograms per cubic metre at midnight on Friday – 33 times the national standard of 60 micrograms and nearly 397 times the new World Health Organization standard of 5 micrograms.

As per the fire department, around 152 fire-related calls were received on Diwali till midnight, after which, about 36 more calls came in.

New Delhi also saw the season’s first shallow fog spell on Thursday morning, which helps trap pollutants, along with a layer of haze, combining to form “smog” on Friday morning.

On Thursday, visibility at both Palam and Safdarjung had dropped to 500 metres – dropping a further 200 metres on Friday morning.

RK Jenamani, a scientist at the Indian Meteorological department (IMD) said, “Fog conditions this morning (Friday) have intensified in Delhi NCR… with visibility levels of 200-500 metres reported at its two airports – Safdarjung and Palam till 5.30am. The IGI Palam airport recorded a drop in visibility towards the city side to 200 metres.”

The IMD said wind speeds are expected to pick up only from Saturday and may reach 40kmph.