The overall air quality in the national capital was in the "very poor" category on Thursday despite thunderstorm and rains in several parts of the city, authorities said.
They said the air quality is expected improve on Friday.
According to data provided by the Central Pollution Control Board (CPCB), the overall air quality index (AQI) of Delhi was at 335.
An AQI between 0 and 50 is considered "good", 51 and 100 "satisfactory", 101 and 200 "moderate", 201 and 300 "poor", 301 and 400 "very poor", and between 401 and 500 "severe".
The CPCB said 29 areas in the national capital recorded 'very poor' air quality, while in three areas it was 'severe'.
The level of PM2.5 (particles in the air with a diameter of less than 2.5 micrometres) was recorded at 163 and the PM10 level was at 261, it said.
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In the National Capital Region (NCR), Ghaziabad, Greater Noida and Noida recorded 'very poor' air quality while Gurgaon had 'poor' air quality.
"Thundershowers with winds associated with the westerly disturbances occurred in many places over Delhi and surrounding areas. Wet deposition washout and high winds will improve air quality further to 'moderate' by late tomorrow. Air quality will further improve and will be in moderate category till February 17," the Centre-run System of Air Quality and Weather Forecasting (SAFAR) said.