The national Capital has no respite from toxic pollution as its air quality turned severe again on Thursday due to inclement weather that includes low wind speed that hampered dispersion of pollutants, authorities said.

The Central Pollution Control Board data recorded the overall air quality index (AQI) at 403, which is categorised under severe. According to CPCB, as many as 22 areas recorded severe pollution, while 13 areas witnessed 'very poor' air quality.

Neighbouring cities of Ghaziabad and Faridabad in the national capital region (NCR) recorded severe air quality while Gurugram and Noida inhaled 'very poor' air quality.

The overall PM2.5 level — fine particulate matter in the air with a diameter of less than 2.5 micrometre — was recorded at 322 and the PM10 level at 485 in Delhi, CPCB said.

The national Capital recorded its second-highest pollution level of the year Sunday with an AQI of 450. The air quality remained 'severe' on Monday and Tuesday. There was a slight drop in pollution level and the air quality moved to the 'very poor' category on Wednesday. It worsened again on Thursday and turned severe.

According to the Indian Institute of Tropical Meteorology, the air quality is likely to improve marginally on Friday.

The ventilation index is marginally favourable for dispersion of pollutants at 4,500 sqm/second, IITM said.

The ventilation index is the speed at which pollutants can disperse. A ventilation index lower than 6,000 sqm/second, with average wind speed less than 10 kmph, is unfavourable for dispersion of pollutants.

According to SAFAR, the overall air quality is likely to improve tomorrow by a few notches but will continue to remain in the very poor category in the next three days.

"Major reason for the predicted increase is a decline in surface wind speed which will disperse pollutants slowly unlike yesterday. Other meteorological factors are also not very favourable," SAFAR said.