Rain triggered by Cyclone Asani substantially improved the air quality in Kolkata on Wednesday as the index remained below 50 (particulate matter 2.5) in most parts, a West Bengal Pollution Control Board official said.
He said the air quality improved by 60 per cent from last week.
Last week the air quality index was hovering between 60-200 (AQI - PM 2.5) which had been the trend for the past one month, the official said.
The rain, he said, has cleansed the air of fine pollutants like nitrogen dioxide and other particulate matters.
"It is a known fact that rain improves air quality and lessens pollution. We hope similar conditions remain," the official said.
On Wednesday, the second day that showers lashed the city, the AQI (air quality index) was recorded at 47 (PM 2.5) at Ballygunge, 32 at Bidhannagar, 37 at Fort William, 44 at Jadavpur and 36 at Rabindra Sarobar air monitoring stations at 2pm, he said.
The AQI was recorded at 70 at the air monitoring station at Rabindra Bharati University in north Kolkata and 87 at Victoria Memorial in the southern part.
From April 7 to May 7 the pollution level across the city was recorded at 90 mg/cubic metre to 150 mg/cubic metre as the finer particulates hung in the air in hot and humid conditions without any rain, the official said.
He said AQI is defined as satisfactory if it is between 51 to 100 causing minor breathing discomfort to sensitive people.
If the AQI is between 0-50, it is defined as good which means minimal impact on people, he added.
Particulate matter 2.5 means pollutants measuring two and one-half microns in width or even lesser.
"With brief but frequent spells of showers since May 10-11, the air quality has improved by 60 per cent. If the same pre-monsoon conditions prevail even after the cyclone goes, the ambient air condition will prevail. The rain is good for sanitisation as well," environmentalist and activist Rabindra Sarobar movement, Somendra Mohan Ghosh said.