The air quality of the national capital was in the "very poor" category on Sunday morning with the AQI clocking 398 at 9 am, the Central Pollution Control Board (CPCB) data showed.
Delhiites woke up to a cold morning as the minimum temperature settled at 5 degrees Celsius, two notches below normal, according to the India Meteorological Department (IMD).
Meanwhile, the AQI of Delhi's neighbouring Faridabad stood at 332, Ghaziabad at 368, Greater Noida at 314 and Noida at 367, all in the "very poor" category.
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".
Weatherman has predicted mainly clear sky with shallow to moderate fog in the morning and the maximum temperature may reach up to 21 degrees Celsius.
The relative humidity was 94 per cent at 8.30 am.
The IMD has forecast cold to severe cold wave conditions in northwest India till January 3.
A "severe" cold wave is when the minimum temperature dips to 2 degrees Celsius or the departure from normal is more than 6.4 degrees Celsius.