Four air quality monitoring stations in Calcutta and Bidhannagar recorded “poor” air on Wednesday.

Three stations in Howrah recorded “very poor” air.

The residents of the twin cities separated by the Hooghly continue to breathe foul air with the advancing winter.

The three stations in Howrah that recorded “very poor” air are in Dasnagar, Ghusuri and Padmapukur. The ones in Calcutta that recorded “poor” air are in Ballygunge, Jadavpur, Victoria Memorial and Bidhannagar.

According to the National Air Quality Index, prepared by the Central Pollution Control Board (CPCB), “very poor” air can lead to “respiratory illness on prolonged exposure” and “poor” air can cause “breathing discomfort to most people on prolonged exposure”.

Asked about “respiratory illness”, Saurabh Maji, a pulmonologist and intensivist at the RN Tagore International Institute of Cardiac Sciences, said: “There will be breathing problems and some people may experience choking. Some may experience irritation in the eyes and throat. Those with chronic lung diseases will experience an aggravation of their illnesses.”

Recent research, Maji said, has shown that prolonged exposure to foul air can increase the risks of heart attack.

An infectious disease specialist said a drop in air quality weakens the body’s natural defence and makes people vulnerable to infections.

Calcutta mayor Firhad Hakim had on Monday chaired a meeting of officials of the Kolkata and Howrah Municipal Corporations to discuss the measures required to reduce air pollution.

Sources in the state pollution control board (PCB) said Howrah’s air quality has been worse than Calcutta’s.

“We are sprinkling water on roads, including the narrower ones, to prevent resuspension of dust. We are also focusing on preventing waste burning and asking construction sites to prevent dust from spreading,” said Bandana Pokhriyal, the commissioner of the Howrah civic body.

KMC sources said they, too, are sprinkling water on roads across the city.

Scientists say sprinkling water on roads is an important step in fighting air pollution but a host of long-lasting measures need to be taken to improve the air quality. Increased use of public transport, forcing industrial units to shift to cleaner fuel and proper disposal of solid waste are among the measures the scientists have flagged.

A study in 2017-18 found that vehicles accounted for 25 per cent of PM2.5, toxic particulates found in the air, in Calcutta’s air during winter, secondary aerosols contributed 32 per cent, wood combustion 15 per cent and coal burning 9 per cent.

In Howrah, vehicles accounted for 31 per cent of PM2.5, secondary aerosols 27 per cent, and wood and coal combustion 11 per cent each.