Delhi's air quality oscillated between 'very poor' and 'severe' categories on Sunday as wind speed dropped and slowed down dispersion of pollutants, the authorities said.
According to Central Pollution Control Board (CPCB) data, the overall air quality index (AQI) in the city was 399, which falls in the 'very poor' category, and remains just two points below 'severe'.
The air quality on Sunday morning was recorded in the 'severe' category, CPCB data showed.
An AQI between 100 and 200 comes under 'moderate' category, 201 and 300 is considered 'poor', 301 and 400 'very poor', while that between 401 and 500 is 'severe'.
On Sunday, as many as 10 areas recorded 'severe' air quality and 22 'very poor', the CPCB said.
In the National Capital Region, Faridabad, Noida and Ghaziabad recorded 'severe' air quality, while Greater Noida recorded 'very poor' air quality, it said.
The overall PM2.5 level -- fine particulate matters in the air with a diameter of less than 2.5 micrometers -- in Delhi was 332, while the PM10 level was 497, it said.
The Centre-run System of Air Quality and Weather Forecasting (SAFAR) said the overall air quality over Delhi is fluctuating between 'very poor' and 'severe'.
"The dispersion is low. Western disturbances influence may impact Delhi's air quality by introducing the moisture and making the air heavy. The expected fall in temperature and moderate fog is likely to bring down the inversion layer to keep both gaseous and particulate pollutants near the surface.
"Hence, significant recover only by Wednesday if sufficient amount of rain occurs as expected," the SAFAR said.