India, Nov. 2 -- Delhi's air quality deteriorated to the "very poor" category for the first time this season on Monday, with a thick haze enveloping the national capital as pollution levels rose sharply after last week's brief respite brought by rain.

The air quality index (AQI) stood at 307 at 7.05 am on Monday. The city had recorded a 24-hour average AQI of 261 (poor) on Sunday, according to the Central Pollution Control Board's (CPCB) 4 pm bulletin, marking a sharp deterioration from 117 (moderate) a day earlier. On Friday, the AQI was 60, after of heavy rain washed the pollution out of Delhi's air and gave the cleanest October day in four years.

Of Delhi's 59 air quality monitoring stations, data was available for 58. Of these, 36 r...