Delhi's air quality slightly improved with increased wind speed but was recorded in "very poor" category on Friday, authorities said.

The air quality had slipped to "severe" category on Thursday.

According to Central Pollution Control Board (CPCB) data, the overall air quality index (AQI) in the city was 394, which falls in the "very poor" category.

An AQI between 100 and 200 comes under the 'moderate' category, 201 and 300 is considered 'poor', 301 and 400 'very poor', while that between 401 and 500 is 'severe'.

On Friday, as many as 11 areas recorded "severe" air quality and 18 "very poor" air quality, the CPCB said.

In the National Capital Region, Ghaziabad and Greater Noida recorded "severe" air quality, while Faridabad, Gurgaon and 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 252, while the PM10 level was 385, it said.

The Centre-run System of Air Quality and Weather Forecasting (SAFAR) said the overall air quality has improved to "very poor" category from "severe" since morning due to an appreciable increase in wind speed to 3.8 kmph in spite of other adverse meteorological conditions, including dense fog.

"It is likely to improve marginally and slowly but the AQI is expected to remain in very poor range for next three days. An active western disturbance is very likely to cause fairly widespread of moisture intrusion over northern Indian region, including Delhi," the SAFAR said.

"If it results in sufficient amount of rain then rapid improvement (in air quality) to poor is expected. Else pre-shower conditions or little shower only aggravates the situation. Dense to very dense fog will continue at isolated places over Delhi which is not good for air quality as it will resist pollutants to mix vertically," the SAFAR said.