A huge and thick mass of air carrying particulate matter (PM) of less than 2 micrometre travels to West Bengal under the influence of the strong north-westerly winds, Abhijit Chatterjee, an environmental scientist at Bose Institute here told PTI.
Travelling at a speed of about 20 km per hour, the soot or black carbon takes four days to cover a distance of about 2000 km between northern India and parts of West Bengal, he said.
"Black carbon concentration over Sundarban was found to be around 15 microgram per cubic meter of air which is quite high for a remote and virgin island. In Darjeeling, the average concentration was 3.5 microgram per cubic meter which is also high compared to a high altitude hill station in the Himalayas," he said.
In Darjeeling, the researchers analysed the origin of the soot and found that 43 per cent did not originate locally.
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Most of the pollution enter Bengal between winter to
pre-monsoon months-- November to April.
Being very fine, some even less than 1 micrometre, these particles remain suspended in the air till wind transports them, Chatterjee said.
Globally PM 2.5, both ambient and indoor, is estimated to result in millions of premature deaths - majority of them in developing countries, he said.
As the size of black carbon is very small it is respirable and can directly affect human lungs and cause several cardiovascular disorders.
The polluted air is also affecting the health of the mangrove forests (the Sundarbans) as the soot blocks the pores of the leaves affecting growth, say scientists.
Black carbon also hastens the process of climate change as it leads to the melting of snow when it gets deposited on glaciers, he added.