Delhi's air quality significantly worsened on Friday, December 26, reaching an Air Quality Index (AQI) of 566, categorized as 'hazardous'. This sharp decline occurred shortly after authorities revoked Stage IV of the Graded Response Action Plan (GRAP) due to earlier improvements. The deterioration from a 'poor' AQI of 234 on Thursday led to renewed concerns about dense fog and poor visibility, potentially impacting flights and travel.