The national capital’s air quality index (AQI) slipped into a very poor category on Thursday after improving marginally the previous day. Air Quality Index was at 366 in ITO, 309 in RK Puram, 313 in Anand Vihar, and 339 in Wazirpur, all four in 'very poor' category on Thursday morning. An AQI between 301 and 400 is considered to be very poor. On Wednesday, the national capital’s AQI was 276 that signified a poor category. Stubble burning in Punjab and Haryana is worsening the Capital's air. Between September 25 and October 12, at least 233 cases of stubble burning were reported in Haryana, while hundreds of fires are happening in fields of Punjab.