The air quality in most parts of Kolkata deteriorated to ''very poor' while it turned ''severe'' in the northern parts of the city on Friday (December 4).

The automated air monitoring station at Rabindra Bharati University (RBU) on BT Road in north Kolkata recorded the air quality index (AQI) of 408 PM2.5 at 3 pm on Friday.

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A West Bengal Pollution Control Board official said, “The AQI recorded at RBU during the day is the worst this year. It was caused mostly due to cold conditions with particulate matters hanging in the air and a large number of vehicles hitting the roads.”

Environmentalist SM Ghosh said, “AQI between 401 and 500 is designated as ''severe'' and affects healthy people and seriously impacts those with existing diseases, causing respiratory difficulties.”

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However, the air quality turned ''very poor'' (301-400) in other parts of the city.

Ballygunge recorded AQI of 309, Bidhan Nagar 320, Fort William 324, Victoria Memorial 314 and Jadavpur 303.