Delhiites woke up to a thick haze hanging over the city as the air quality was recorded in the `very poor` category. The overall air quality index (AQI) of Delhi was recorded at 316, according to Central Pollution Control Board (CPCB) data. 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 401 and 500 `severe`. The CPCB said 22 areas of Delhi recorded `very poor` air quality while 13 areas recorded `poor` air quality. The level of PM2.5 -- particles in the air with a diameter of less than 2.5 micrometres -- was recorded at 159 and the PM10 level was recorded at 316, it said. In NCR, Ghaziabad, Faridabad and Noida recorded `very poor` air quality while Gurgaon recorded `poor` air quality, the CPCB data showed. According to the Indian Institute of Tropical Meteorology, a haze has engulfed Delhi and the wind speed and ventilation index are "extremely unfavourable" for dispersion of pollutants. Ventilation index determines how fast pollutants can get dispersed. The ventilation index of around 6,000 sqm/second gets rid of pollutants, but it came down to 1,500 sqm/second on Tuesday in the city. The air quality is very poor and as expected, gradual increasing trend is noticed," the Centre-run System of Air Quality and Weather Forecasting (SAFAR) said. "It is likely to remain very poor with a gradual increasing trend till Wednesday without any major episode and then likely to decline, but (will) continue to remain in very poor (category)," it said.