Delhi's average AQI reached 370 on Monday, a day after Diwali, nearing the severe category.
DTU and Burari Crossing recorded the worst AQI at 437 each.
Firecrackers contributed to heavy smoke, poor visibility and breathing difficulties across several neighbourhoods.
A thick grey haze hung over Delhi on Monday morning, a day after Diwali, reducing visibility and pushing the city's Air Quality Index (AQI) to 370 -- almost in the 'severe' category.
DTU and Burari Crossing monitoring stations recorded the worst air quality, with an AQI of 437 each. Four other stations recorded 'poor' air quality, while the remaining stations were in the 'very poor' category.
According to the Central Pollution Control Board (CPCB), an AQI between 0 and 50 is classified as 'good', 51 and 100 as 'satisfactory', 101 and 200 as 'moderate', 201 and 300 as 'poor', 301 and 400 as 'very poor', and 401 and 500 as 'severe'.
The air quality remained 'severe' in several areas, including Bawana, Burari, Jahangirpuri, Sonia Vihar, Punjabi Bagh, Okhla and Rohini.
According to the Air Quality Early Warning System, Delhi's air quality is likely to slip into the 'severe' category during the day and remain there over the subsequent few days.
Unfavourable meteorological conditions, vehicular emissions and stubble burning in neighbouring states are likely to contribute to the deterioration.
A ventilation index below 6,000 square metres per second, coupled with an average wind speed of less than 10 kmph, is unfavourable for the effective dispersion of pollutants, according to the Air Quality Early Warning System.
On Diwali night, firecrackers were burst across the capital, despite restrictions on their use. Twenty monitoring stations recorded 'severe' air quality, with Sonia Vihar registering the worst AQI at 463, followed by Jahangirpuri at 450.
The city's average AQI settled at 376 on Sunday. Of the 59 monitoring stations that recorded data at 10 pm on Diwali, 20 were in the 'severe' category, six in the 'poor' category and the remaining 33 in the 'very poor' category.
Firecracker bursting picked up from around 5-6 pm on Sunday, including in Pitampura, Burari, Mayur Vihar and parts of Lutyens' Delhi. As celebrations gathered pace through the evening, smoke accumulated over roads and residential neighbourhoods.
Residents in several areas reported a strong smell of burnt crackers, reduced visibility and difficulty breathing as fireworks continued through the night.
