Lalbazar, in central Calcutta where the police headquarters are located, and the neighbourhood of the Howrah Municipal Corporation office, as well as Bator and Bandhaghat in Howrah, have levels of nitrogen dioxide above national norms, Central Pollution Control Board monitoring stations have shown.

While the national air quality standards specify 60 micrograms per cubic metre for residential areas and 80 micrograms per cubic metre for industrial areas, Lalbazar recorded an annual average of 63, Bator recorded 83 and the Howrah Municipal Corporation, 95. The highest concentration was 97, at Bandhaghat, according to a paper by the pollution control board’s scientists that was released in New Delhi on Friday.

Bandhaghat is near Salkia and is located just across the Hooghly from Ahiritola in north Calcutta. Bator is in central Howrah, near Ramrajatala. Both are dotted with small industrial units, such as iron foundries, chemical factories and rolling mills.

The pollution control board monitors air quality at 300 locations in 115 cities and towns across India. It was only at the four sites in Calcutta and Howrah, and one location each in Bangalore and Ludhiana, that nitrogen dioxide levels exceeded the standards. (See Chart)

The paper, based on observations in 2004, said a steep rise in the number of vehicles is pushing up levels of the gas in India. An oxide of nitrogen in vehicle exhausts turns into nitrogen dioxide in the air. Combustion in power plants, heavy industry and microbial activity in the soil can also contribute to its presence in the atmosphere.

Nitrogen dioxide is absorbed along almost the entire respiratory tract and the lungs. Studies have established that it can increase susceptibility to respiratory infections and worsen conditions like asthma. Long-term exposure can contribute to acute or chronic bronchitis.

Earlier this week, European scientists reported a connection between exposure to nitrogen dioxide and total mortality as well as mortality from cardiovascular and respiratory causes. Their 30-city study, published on Tuesday in the European Respiratory Journal, found that the effect of the gas on respiratory mortality was higher in cities with a higher proportion of elderly people.

The pollution control board said nitrogen dioxide levels are “well within limits” in most cities of India and “stable” where governments have introduced stringent norms for vehicles. “It is expected the levels will drop in future with strict enforcement of regulations,” it said.

The importance of this gas — as a marker for air pollution and its potential impact on human health — has triggered an international effort to map it on a global scale using the European Space Agency’s Envisat, the world’s largest environment-monitoring satellite. Last year, instruments on Envisat revealed that the world’s largest amount of nitrogen dioxide was hanging over Beijing and northeastern China.