The air quality in most parts of Calcutta deteriorated to 'very poor', while it turned 'severe' in the northern parts of the metropolis on Friday, a West Bengal Pollution Control Board (PCB) official said.

The automated air monitoring station at Rabindra Bharati University (RBU) on B T Road in north Calcutta recorded the air quality index (AQI) of 408 PM 2.5 at 3 pm, which is categorised as 'severe'.

"The AQI recorded at RBU during the day is the worst this year. It was caused mostly due to wintry conditions with particulate matters hanging in the air and a large number of vehicles hitting the roads," he said.

AQI between 401 and 500 is designated as 'severe' and affects healthy people and seriously impacts those with existing diseases, causing respiratory difficulties, environmentalist S M Ghosh said.

"It is caused by the plying of a large number of diesel-run commercial vehicles, use of coal-fired chulha (clay stove) by roadside eateries and garbage burning at dumping yards," he said.

The air quality turned 'very poor' (301-400) in other parts of the city. It can aggravate asthma and other respiratory conditions.

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

"The PCB is taking steps like banning coal-fired chulha and old commercial vehicles in the city. The board is also using water sprinkler vehicles to prevent the accumulation of dust in the air," the official added.