India, Nov. 3 -- Delhi's air quality deteriorated sharply for the fourth consecutive day on Monday, pushing the city into the "very poor" category for the first time this season and prompting the Commission for Air Quality Management (CAQM) to invoke Stage 2 of the Graded Response Action Plan (GRAP) across the National Capital Region (NCR).

The city was enveloped in a thick haze less than a week before Diwali, when firecracker emissions could further worsen pollution levels.

Delhi recorded an air quality index (AQI) of 324 at 4pm on Monday, down 63 points from Sunday's 261, but a staggering 264-point deterioration from Friday, when the AQI was 60 in the "satisfactory" category.

Monday's reading was the worst in 271 days, since February...