Athick blanket of smog engulfed the national capital as the air quality in some parts of Delhi deteriorated to "hazardous" category a day after Diwali on Friday. In most areas, the AQI hovered around the 451 (severe) mark.

The air quality at Janpath was recorded in the "hazardous" category early on Friday, with the particulate matter (PM) 2.5 concentration at 655.07, according to news agency ANI. Several people in the national capital reportedly complained of itchy throat and watery eyes.

At several locations in Delhi, firecrackers were burst on Diwali in complete disregard to the government's ban on them, coinciding with a sharp increase in fumes from farm fires. The share of farm fires in Delhi's pollution increased to the season's highest at 25 per cent, PTI reported.

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The neighbouring cities of Faridabad (424), Ghaziabad (442), Gurgaon (423) and Noida (431) also recorded 'severe' air quality.

An AQI between zero and 50 is considered 'good', 51 and 100 'satisfactory', 101 and 200 'moderate', 201 and 300 'poor', 301 and 400 'very poor', and 401 and 500 'severe'.

Experts said the air quality turned severe due to unfavourable meteorological conditions -- calm winds, low temperature and low mixing height -- and a poisonous cocktail of emissions from firecrackers and stubble burning, PTI reported.