High pollution levels were recorded in some cities in South and East India on Thursday afternoon, with air quality ranging between “poor” and “very poor” in cities such as Chennai, Bengaluru, and Kolkata. While the air quality in Hyderabad was “moderate”, it was “poor” in Amaravati and Visakhapatnam in Andhra Pradesh.

On Thursday, according to the Central Pollution Control Board, four out of five stations in Kolkata recorded an Air Quality Index between 205 and 249 at 3 pm. Three out of four stations in Tamil Nadu’s capital Chennai noted index readings between 214 and 314. In Karnataka’s capital Bengaluru, the Air Quality Index at the monitoring station at BTM Layout was 330, while the pollution levels at two other stations fluctuated between “poor” and “moderate”.

An Air Quality Index between 0 and 50 is considered “good”, 51-100 is “satisfactory”, 101-200 “moderate”, 201-300 “poor”, 301-400 “very poor” and 401-500 “severe”. A figure above 400 poses a risk to people with respiratory ailments and can affect even those with healthy lungs.

In Tamil Nadu, multiple stations recorded a marked increase in pollution, with Chennai’s Velachery area recording an AQI of 96 on Sunday, and 228 the following day. The Air Quality Index has not gone below 200 since Monday.

Till Sunday, air quality was “satisfactory” in Bengaluru, but a significant rise in pollutants was recorded as the week proceeded. The BTM Layout monitoring station recorded an Air Quality Index reading of 73 (satisfactory) on Sunday. It rose to 347 (very poor) on Thursday. Several other stations also recorded poor air quality levels after Sunday. The air quality in Kolkata remained “moderate” till Monday at 135. The following day, the index shot up to 226. It was 247 at 3 pm on Thursday, according to data from the city’s Ballygunge station.

Poor air quality at this time of the year is common in cities in North India, especially the National Capital Region. According to a Central Pollution Control Board official, the sudden spike in pollution in these cities can be attributed to north-westerly winds carrying the smog over North India to the rest of India. A cyclonic depression gaining strength over the Bay of Bengal was also said to have contributed to this drift, Hindustan Times reported.

An unidentified India Meteorological Department scientist claimed that the cyclonic storm, which is likely to make landfall on Friday, has affected the winds. “In fact, strong inward winds from… [the Bay of Bengal] have pushed the winds carrying the smoke towards south from eastern India,” he added.

The official at the pollution monitoring agency said local factors also contributed to the increase in pollution levels. The situation is expected to improve in a few days with the advent of fresh winds. Heavy rain caused by the cyclonic storm should help alleviate the problem, the official added.

“The Atmospheric Dispersion Modeling System [used to calculate concentrations of pollutants] shows that the plumes of the smoke have reached southern India,” Raj Bhagat Palanichamy, a remote sensing analyst with the World Resources Institute, told Hindustan Times.

Meanwhile, authorities in Kolkata have taken measures to bring down the rising pollution. “We are taking several steps to contain air pollution in Kolkata, like sprinkling water at construction sites, stopping movement of over 15-year-old commercial vehicles in the cities of Kolkata and Howrah and banning coal-based ovens in roadside food stalls,” a state Pollution Control Board official told PTI.

Crop burning is considered one of the main reasons for the rising level of air pollution in the Delhi-NCR region and other northern states at this time of the year, though it is exacerbated by firecrackers burst around Diwali. Prime Minister Narendra Modi on Wednesday asked the Ministry of Agriculture to distribute equipment to farmers of Punjab, Haryana and Uttar Pradesh on a priority basis to tackle stubble burning.

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