A thick haze enveloped Delhi on Friday morning as air quality slipped back to 'very poor' category, recording AQI at 311 by 6 am after two days of mild relief. The AQI at ITO, Narela and Chandni Chowk was recorded at 334, 333 and 350 respectively. The AQI at Indira Gandhi International (IGI) Airport was in 'poor' zone at 290.