​AQI Over 500, Air Quality Deteriorates To ‘Severe’ As Crop Fires Surge Over Past Two Days | N18VA day after the air quality in the national capital plummeted to severe levels, it continues to deteriorate amid unfavourable weather conditions. The night temperatures have dropped and calm surface winds prevail, creating conducive conditions for the high level of pollutants to remain suspended near the surface for long hours and make the air toxic.Delhi reported an Air Quality Index (AQI) of 427 on Thursday evening for the first time this winter, which surged past 500 (Very Severe) on Friday morning as per the air quality early monitoring system. It remained around 550 for most part of the day with dangerous levels of PM2.5 and PM10 in the air triggering a health warning.