A thick layer of smog enveloped Delhi on Tuesday morning as the city's air quality deteriorated to the 'very poor' category with 27 monitoring stations falling in the red zone, the Central Pollution Control Board said.

The city's air quality index (AQI) was recorded at 317 at 9 am on Tuesday, it said.

This deterioration in air quality led to prompting enforcement of Stage II of the Graded Response Action Plan (GRAP) in the national capital, which enforces a ban on the use of coal and firewood, including tandoors in hotels, restaurants, and open eateries, as well as diesel generator sets, except for emergency and essential services.

Several areas including Mundka, Bawana, Wazirpur, Jahangirpuri, Anand Vihar, Alipur, Ashok Vihar, Aya Nagar, Burari, Dwarka, Mandir Marg, dropped to the 'very poor' category.

Meanwhile, as the city grapples with pollution, Delhi recorded a minimum temperature of 20.4 degrees Celsius, two notches above normal.

Humidity was at 93 percent at 8.30 am, according to the India Meteorological Department (IMD).

The weather office has predicted clear sky for the day, with the maximum temperature likely to settle around 35 degrees Celsius.

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'.

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