The city's air quality index became 'severe' on the New Year's Day morning, 10 hours after the state pollution control board's computerised monitors showed it to be 'very poor' at midnight.
A West Bengal Pollution Control Board (WBPCB) official Tuesday said one of the key factors for the change in pollution level within 10 hours is bursting of firecrackers to celebrate the new year.
At midnight when the celebrations began, the air quality index (AQI) read 374 PM 2.5 at the air monitoring station of WBPCB at Rabindra Bharati University, one of the two automated air monitoring stations of WBPCB in the city, a WBPCB official said Tuesday.
The AQI at the university read 400 PM 2.5 at 10 AM Tuesday and remained the same at 11 AM, the official said.
The AQI is an indicator of air pollution caused by three pollutants - NO2, PM 10 and PM 2.5. The index indicates air quality as 'good' for values of 0-100, 'moderate' for 101 -200 and 'poor' for 201-300.
Particles less than 2.5 PM (micrometres) are called PM 2.5. They are approximately 1/30th of the average width of human hair and is generally described as fine particles. PM 10, on the other hand, is particulate matter 10 micrometres or less in diameter, an official explained.
The AQI between 301-399 is called 'very poor' and it causes respiratory illness on prolonged exposure, the WBPCB official said adding that it causes respiratory illness on prolonged exposure.
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The reading at 400 was marked as 'severe', a notch above the very poor and such conditions affect even healthy people, he said.
The AQI at the second computerised air monitoring station of WBPCB in the Victoria Memorial, located in the city's green belt Maidan, read at 365 PM 2.5 at 10 PM on Monday. It was also termed as 'very poor', the official said.
The reading came down to 333 PM 2.5 at 11 AM, but still it remained in the 'very poor' category.
The AQI remained 'very poor' for most of the days in December and November.
In November, the air pollution level was "poor" or "very poor" in 21 of the 30 days which was unprecedented by any standard, Ajay Mittal of the 'Kolkata Clean Air', an environmental NGO, had said.
WBPCB Chairman Kalyan Rudra said the reading of one air monitoring station at certain point of time did not project the entire situation and particles also hung heavily in air due to typical weather situations during the winter.
Environmentalist S M Ghosh said the air pollution in the city reached alarming level since the past few months and installing more automatic air monitoring stations across the city will project how alarming the entire situation has turned into.