Calcutta, Nov. 18: The city’s smallest but deadliest enemy was today given a name — PM 2.5.

This fine pollutant spewed mainly by automobile exhaust, measuring less than 2.5 micron in diameter and invading the innermost parts of the lungs, made its debut in the national ambient air quality standards announced in the capital.

PM (particulate matter) 2.5 from polluting buses, taxis and autos — that enjoy a free run on the city streets thanks to Buddhadeb Bhattacharjee’s inaction and Mamata Banerjee’s indifference — enter the bronchi and alveoli, causing irreparable damage to lungs and serious health hazards.

“Particulate less than 2.5 micron are mainly responsible for the respiratory diseases rampant in Calcutta as they enter the respiratory tract. Also, the new standard has been announced on World COPD (chronic obstructive pulmonary disease) Day, and this disease should increasingly be linked to fine particulate in the air we breathe,” said A.G. Ghosal, the director of the National Allergy Asthma & Bronchitis Institute off Park Circus.

Children are the most vulnerable to the killer christened PM 2.5. “It can block minor airways of children and cause respiratory problems triggered by tract inflammation and depletion in alveolar oxygen supply,” said pulmonologist Plabon Mukherjee.

Till now, 10 microns or less was the standard index of respirable air pollution known as PM 10. But a pollution control board study carried out in Calcutta stating that around two-third of the city’s respirable particulate matter (RPM) would fall under the PM 2.5 category nudged the central ministry of environment and forests to include the new standard, fixed at 40 micrograms per cubic metre of air.

According to Dipak Chakrabarty, the chief scientist with the state board, the levels of smallest particulate matter were found to be highest in areas like Park Street and Rajabazar during a 2002-04 study conducted in some busy traffic corridors.

“This study, carried out for the first time in the country is vital, as this small and deadly particulate matter has been found to dominate the city air polluted by vehicles,” said a researcher with the projec.

On the basis of the report, the annual level of PM 2.5 in Calcutta can be pegged around 65 micrograms per cubic metre of air, well above the upper limit of 40 set today.

“The Calcutta study was taken into consideration while setting the national standards,” confirmed Anumita Roy Choudhury, an air pollution expert from the Delhi-based Centre for Science and Environment (CSE).

But no new norm can help Calcutta breathe easy if it remains on paper. “What is the point of setting stringent norms when even the earlier norms were flouted with impunity?” said Biswajit Mukherjee, the chief law officer of the state environment department.