New Delhi, Feb. 5 -- A combination of fog and high particulate matter (PM) 2.5 build-up led the Capital's air quality to deteriorate several points into the "very poor" category on Wednesday, ending a three-day streak of "poor" air. However, forecasts say pollution levels are expected to improve, even up to the "moderate" category, in the coming days on the back of increasing wind speeds. On Wednesday, the 24-hour average air quality index (AQI) was recorded at 339 (very poor) at 4pm, according to data from the Central Pollution Control Board (CPCB). The AQI was 272 (poor) at the same time a day before. CPCB classifies AQI between 101 and 200 as "moderate"; 201 and 300 as "poor" , 301 and 400 as "very poor" and beyond 400 as "severe". Centre's Air Quality Early Warning System (EWS) bulletin for Delhi said, "The air quality is likely to be in the 'poor' category on Thursday. The air quality is likely to be in the 'moderate' category from Friday to Saturday." Over the subsequent six days is that the air quality is likely to be in the "moderate" to "poor" category, it said. According to flight tracking website FlightRadar24, at least 500 flight at Delhi's Indira Gandhi International (IGI) airport were delayed as a result, with no cancellations or diversions....