Experts described the trend as a revelation, associating the deaths with rising air pollution, caused particularly by motor vehicles in Calcutta and surrounding areas.

Environmentalist Subhas Datta said: "I will call it the silent act of mass killing by the state."

Despite the growing pollution, the state government has remained unmoved.

The report by the central bureau of health investigation under the Union health ministry says that of the 18,702 deaths from 2009 to 2014 from acute respiratory infection in the country, 3,821, or over 20 per cent, occurred in Bengal. Males accounted for around 65 per cent of the deaths and females for the rest.

Anumita Roy Choudhury, an air pollution expert, said the trend was consistent with the Centre for Science and Environment (CSE) finding that Calcuttans breathe in more poison than their counterparts anywhere else in the country.

Delhi, known as the country's air pollution capital, reported 1,338 deaths during the period. Uttar Pradesh, Andhra Pradesh and Odisha keep Bengal company as runners-up. "If the city does not reduce its air pollution quickly, the vulnerability will be even greater in the future," warned Roy Choudhury, who works with the CSE.

Calcutta also tops the lung cancer list, as reported in The Telegraph earlier.

State health department sources warned that the numbers, alarming as they are, could be worse.

"The report excludes data from many Calcutta-based government hospitals as they are yet to send them. We expect to start getting that data soon," said a health official.

The official also pointed out that the number represented data only from government hospitals, and not private institutions.

Junior health minister Shashi Panja said: "I will not make any comment till I know the details, but I guess the environment department is looking into this."

BiswaRanjanSatpathi, the DHS, too declined to comment without seeing the data. "We have to see whether other states are providing actual data or not," he said.

That could well be true. Besides, the numbers do not take into account the quality of hospital care available in each state - and it is no secret that Bengal is not exactly a leader in that area. Many patients travel to other states for treatment.

In spite of all this, however, pulmonologist Alok Gopal Ghoshal, director of the National Allergy, Asthma and Bronchitis Institute, said: "This (number) is a huge revelation for us....Apart from triggering a range of diseases from asthma to cancer, acute respiratory infection can directly kill a person. There is no doubt that most of these deaths have been occurring in urban areas triggered by extremely high air pollution."

ArunavaMajumdar, a public health expert formerly associated with the All India Institute of Hygiene and Public Health, said the unusually high levels of fine and ultrafine particulates - PM 10 and PM 2.5 - from diesel exhaust were primarily responsible for deaths from respiratory infection.

"Although there have been deaths from indoor air pollution as well, the bulk are definitely due to the high level of fine particulates," Majumdar said.

He warned that even the high number the report had revealed was likely far less than the actual number of deaths as many went unrecorded or were passed off as caused by other diseases.