The air quality in Delhi slipped into 'very poor' category again on Tuesday after two days of being in 'poor' category. As of 6 am, the national capital's Air Quality Index (AQI) stood at 331, according to Central Pollution Control Board (CPCB). Bawana, Anand Vihar, Wazirpur and Burari registered AQI at 387, 381, 362 and 361 respectively.