Delhi's 24-hour average air quality index (AQI), which measures the concentration of very fine particles know as PM2.5 in the air — particularly harmful pollutants as they're easily inhaled and can settle deep in the lungs — crossed 470 on Friday, per the state-run Central Pollution Control Board.Residents of the Indian capital weren't likely to see much improvement quickly, with weather conditions expected to remain calm and the seasonal crop stubble burning likely to continue."There is no doubt over who has turned Delhi into a gas chamber," Yadav said in a tweet.Punjab's top politician, Bhagwant Mann, defended his administration, saying it only took office half a year ago and calling for a collaborative effort by state and federal authorities to address the problem.The practice has been formally banned by the country's Supreme Court, and farmers were warned they would face fines for violating the decree, but it has served as a weak deterrent."