In neighbouring Gurugram in Haryana, air quality was recorded in "very poor" category.
Delhi and neighbouring areas today continued to remain under a thick blanket of haze as air quality in the region continued to deteriorate further, largely due to farm fires in neighbouring Haryana and Punjab. The Delhi government has already banned use of firecrackers in view of the persistently high pollution and smog levels that experts believe put more people at risk from the coronavirus.
The national capital's air quality index (AQI) stood at 397 - which falls in the "very poor" category - at 9 am, news agency PTI reported.
Several areas in the national capital have recorded levels of PM 2.5 particulate -- the most dangerous for human health - above the 400-mark. The worst air quality (507) was recorded in Bawana in outer Delhi, while Wazirpur, at 272, was one of the least polluted, as per the readings.
On Thursday, the 24-hour average AQI was 450, the worst levels since November last year, with farm fires accounting for 42 per cent of its pollution, the maximum this season so far.
Experts said unfavourable meteorological conditions - calm winds and low temperatures - and smoke from farm fires in neighbouring states pushed the air quality index to the "severe" zone on Thursday, the first time since January.
The air quality in Noida in Uttar Pradesh, at 610, was the worst among the neighbouring cities that border the national capital.
Neighbouring Gurgaon in Haryana was shrouded in a toxic haze and visibility dropped due to cooler temperatures and lower wind speeds that let deadly pollutants hang in the air. Bursting of firecrackers in the satellite town near Delhi has been a major factor in the spike in pollution levels.
"People have started bursting firecrackers before the festival of Diwali, we are having breathing problem because of it," said a local in Gurgaon.
"Pollution is increasing day by day, yesterday the situation was really serious, and people are burning garbage and bursting firecrackers," said another local.
The Gurgaon district administration has now designated eight spots for bursting of firecrackers on November 14, the day of Diwali.
According to the Ministry of Earth Sciences' air quality monitor, SAFAR, the farm fire count in Punjab, Haryana, Uttar Pradesh, Uttarakhand, and neighbouring areas have increased significantly to 4,135, the highest this season so far.
SAFAR said the boundary layer wind direction is northwesterly - favourable for the transport of pollutants from farm fires.
"The share of stubble burning in Delhi''s PM2.5 pollution was estimated at 42 percent for Thursday," it said.
An AQI between 0-50 is marked good, 51-100 is satisfactory, 101-200 is moderate, 201-300 is poor, 301-400 is very poor and 401-500 is considered severe. According to experts, the severe category affects the health of people and seriously impacts those with existing diseases.
With inputs from agencies