Howrah in West Bengal topped the list of the most polluted cities in India.

Its air quality was much worse than Delhi and other cities in the National Capital Region, which have seen an improvement in the situation over the last few days because of rain and wind.

According to the daily AQI (Air Quality Index) bulletin of the Central Pollution Control Board, the 24-hour average AQI value of Howrah on Wednesday was 285.

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On the AQI scale, a value of 0-50 is considered good quality air; 51-100 satisfactory; 101-200 moderate; 201-300 poor; 301-400 very poor and 401-500 severe. 

On May 1, during the COVID-19 lockdown, both Kolkata and Howrah registered AQI values of less than 50.

According to West Bengal Pollution Control Board (WBPCB) data, the level of PM10 (coarse pollutants that are seven times finer than the human hair) at Ghusuri in Howrah shot up to 852 micrograms per metre cube around 6 am and to 915 micrograms per metre cube around 10 am on Wednesday. The PM10 should not ideally go above 100 micrograms per metre cube.

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The level of PM2.5 (finer particles that can reach up to the lungs) also shot up three to four in some places during the night, primarily because of temperature inversion in winter that traps pollutants close to the surface.

Most of the monitoring stations in Kolkata recorded PM10 two times above the safe limits.