Three air quality monitoring stations in Calcutta reported “poor” air on the Republic Day morning and three others reported “moderate” air.

In the twin city Howrah, one station recorded “very poor” air quality, two recorded “poor” air quality and two others recorded “moderate” air quality.

The foul air on Sunday was a matter of concern since many people enjoyed the holiday outdoors. The Maidan-Park Street-Rabindra Sadan-Victoria Memorial area was teeming with people, as were many other popular destinations.

The three stations in Calcutta that reported “poor” air quality were located at Ballygunge, Jadavpur and Rabindra Bharati University (BT Road campus).

The three stations that reported “moderate” air were located at Bidhannagar, Rabindra Sarobar and Victoria Memorial.

According to the National Air Quality Index, prepared by the Central Pollution Control Board, “poor” air can cause “breathing discomfort to most people on prolonged exposure”.

“Moderate” air can cause “breathing discomfort to people with lungs, asthma and heart diseases”.

There was haze in many parts of the city in the morning.

A scientist at the state pollution control board said negligible wind speed and cool nights and mornings were helping the pollutants remain trapped near the surface.

Cool air during winter does not rise as much as warm air during summer. Also, weak winds during winter come in the way of the dispersion of the air pollutants, which are mostly released from cars, buses, industrial units and construction sites, among others.

The Kolkata Municipal Corporation (KMC) has been sprinkling water on arterial roads to prevent the resuspension of dust. But many construction sites across the city are still uncovered and the dust emanating from them is adding to the pollutant load of the air.

Across the Hooghly, in Howrah, the station at Padmapukur reported “very poor” air.

The National Air Quality Index says “very poor” air can lead to “respiratory illness on prolonged exposure”.

In Howrah, the stations at Dasnagar and Ghusuri reported “poor” air and the ones at Belur Math and the botanical garden in Shibpur reported “moderate” air.

“Public infrastructure work in Howrah is contributing to the air pollution there,” said a scientist at the state pollution control board.