A layer of haze hung over Delhi and its surrounding areas on Wednesday, with the capital's air quality slipping deeper into the 'very poor' category primarily due to a dip in temperature and calm winds during the night.

The city's Air Quality Index (AQI) was recorded at 372 at 10 am, the highest so far this season. The 24-hour average AQI was 359 on Tuesday, 347 on Monday, 325 on Sunday, 304 on Saturday and 261 on Friday.

Several areas within the city such as Nehru Nagar (402), Sonia Vihar (412), Rohini (403), Wazirpur (422), Bawana (403), Mundka (407), Anand Vihar (422), and New Moti Bagh (435), experienced a drastic deterioration in air quality, plunging into the 'severe' category.

The concentration of PM2.5 -- fine particulate matter which can penetrate deep into the respiratory system when inhaled and trigger respiratory problems -- at these places was six to seven times the safe limit of 60 microgrammes per cubic metre.

The AQI was 280 in neighbouring Ghaziabad, 318 in Faridabad, 254 in Gurugram, 333 in Noida and 372 in Greater Noida. An AQI between zero and 50 is considered good, 51 and 100 satisfactory, 101 and 200 moderate, 201 and 300 poor, 301 and 400 very poor, and 401 and 500 severe.