The centre carried out the study last week and released the report today. A copy of the report is with The Telegraph.

Although the overall ultrafine particulate matter (PM 2.5) is higher in Delhi, a person in Calcutta breathes in more poisonous air because of higher exposure, especially at high-density traffic points, the report says.

Experts say PM 2.5 is the most potent pollutant as it can reach the deep crevices of lungs and trigger a host of diseases, including cancer.

"We have found that people in the city are exposed to alarming levels of PM 2.5 - actually two to three times more than the ambient level," said Anumita Roy Choudhury of CSE.

"Winters can be disastrous considering the fact that the ambient ultrafine particulate level in the city is already above the national permissible limit during this time."

Roy Choudhury headed a seven-member team, which carried out the study.

Ujjal Mukherjee, the chief scientist of the state pollution control board, said the PM 2.5 and PM 10 levels were higher than the permissible limit in the city from October to March.

CSE monitored the exposure to PM 2.5 in sensitive areas, hospitals, schools, residential areas and the area adjacent to Nabanna.

The CSE study shows that almost all parts of the city fall in the category of "poor to severe" in terms of particulate pollution.

"We found the air pollution in the SSKM Hospital, Dhakurialake, New Market and airport areas to be severe. Residential areas, schools and the Nabanna area fall in the poor or very poor category," Roy Choudhury said.

Although Delhi's ambient PM 2.5 level is "generally higher than Calcutta", a person in the city, especially near busy traffic zones, inhales more toxic air because of the compactness of the city and the high number of diesel-run vehicles, she said.

The study points out that PM 10 pollution levels has jumped 61 per cent between 2010 and 2013. The level of nitrogen oxide, another major polluter, is two times more than the permissible limit.

Both fine particulate and nitrogen oxide are emitted by diesel-run vehicles and Calcutta is considered to be the diesel capital of the world.

"Over 99 per cent commercial vehicles run on diesel in the city," the study has said.

Children are the most vulnerable to vehicular pollution and they end up with respiratory diseases, according to the study.

ManasRanjan Roy of Chittaranjan National Cancer Institute, associated with the CSE study, said: "We found that even a baby in the mother's womb is affected by the city's toxic air.

"The chances of women giving birth to stillborn babies go up by nearly five times if they are exposed to air pollution. Chances of spontaneous abortion increase by nearly three times."