National Capital’s air quality deteriorated on Thursday due to low wind speed and was marked under the ‘severe category’ on the National Ambient Air Quality Index (NAAQI).  According to Central Pollution Control Board data, the overall air quality index (AQI) in the city was 434, which falls in the severe category.

The air quality was severe till Sunday but with improved wind speed of 20 km/hr, it significantly improved and was recorded in the ‘poor’ category until Tuesday. On Wednesday, the air quality again deteriorated as the wind speed dropped and was recorded in the very poor category.

“On Thursday, as many as 29 areas recorded ‘severe’ air quality and three ‘very poor’ air quality”, CPCB said.

In the National Capital Region, Ghaziabad, Faridabad, Noida and Greater Noida recorded ‘severe’ air quality while Gurugram 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 365, while the PM10 level was 540, it said.

The Centre-run System of Air Quality and Weather Forecasting (SAFAR) said the overall AQI over Delhi was recorded in the severe category.

“Other weather conditions remain more or less same except that wind speed decreased from 5.3kmph to 2.5kmph. Since the speed drop is substantial, it has slowed down the dispersion significantly,” SAFAR said.

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’.