Delhi's air quality for the first time this season deteriorated to 'very poor' category with several areas in the national capital nearing towards severe levels of pollution, according to authorities. The overall Air Quality Index (AQI) of Delhi was recorded at 309, according to a data of the Centre-run System of Air Quality Forecasting and Research. 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 that between 401 and 500 is categorised as 'severe'. On Wednesday, Anand Vihar recorded an AQI of 380, Dwarka Sector 8 registered AQI of 376, ITO recorded 295 and Jahangirpuri recorded an AQI of 349, Rohini an AQI of 353, according to the Central Pollution Control Board (CPCB) data. The PM10 level (presence of particles with diameter less than 10 micrometres) in Delhi stood at 287 and the PM2.5 level was recorded at 131, according to the data. This is the first time this season that the air quality deteriorated to 'very poor' category. Further deterioration of air quality is forecast in the coming days with PM10 to reach 330 and PM2.5 to 151. An official with the CPCB said a number of factors are responsible for deteriorating air quality, including vehicular pollution, construction activities and meteorological factors. Among meteorological factors the main reason is the drop and change of wind speed which is now flowing from the stubble burning areas, another official said. The deterioration of air quality comes days after implementation of an emergency plan to combat air pollution. The GRAP, notified by the Ministry of Environment, Forest and Climate Change in 2012, prescribes a set of measures to curb air pollution based on the CPCB daily Air Quality Index. The emergency plan gets implemented if 'very poor' or 'severe' air pollution levels are recorded for 48 hours, but given the extent of winter pollution witnessed by the national capital, the plan has been implemented as the air quality slipped to the 'poor' category, the official said. In addition, the CPCB has also deployed 41 teams to monitor the implementation of measures taken to combat air pollution.