Amid the air quality depleting again and thick layer of smog blanketing the national capital, the Delhi International Airport Limited (DIAL) has issued a passenger advisory due to low visibility.The Delhi Airport tweeted, “Low Visibility Procedures are in progress at Delhi Airport. All flight operations are presently normal. Passengers are requested to contact the airline concerned for updated flight information.”After two days of remaining in 'poor' category, Delhi's air quality entered 'very poor' category today with an overall Air Quality Index (AQI) of 303 at 6:30 am. The Centre's air quality panel may take a call on revoking curbs in place in Delhi-NCR under stage 3 (severe) of the Graded Response Action Plan (GRAP), PTI reported.The Delhi government had on Monday decided to reopen primary classes from November 9 and revoke the order asking 50 per cent of its staff to work from home in view of the improvement in the city's air quality over the last few days.The 24-hour average Air Quality Index (AQI) in the national capital stood at 295 on Thursday. It was 260 on Wednesday, 372 on Tuesday, 354 on Monday, 339 on Sunday and 381 on Saturday.An AQI between 201 and 300 is considered 'poor', 301 and 400 'very poor', and 401 and 500 'severe'.