During the period under study, Calcutta's average air quality index (AQI) - based on the prevalence of the most potent pollutant, PM2.5 - was 260. Delhi's AQI stood at 243.

Experts fear Calcutta's air quality might be worse than what the figures suggest as the PM2.5 count is measured manually in the city. "Environment scientists are unanimous that manual measurement throws up less than accurate figures," an environment expert said.

The AQI figures have been deduced from data collected by the West Bengal Pollution Control Board in Calcutta and the Central Pollution Control Board in Delhi. The comparative analysis casts a cloud on environment minister Sovan Chatterjee's claim that Calcutta's air is better than Delhi's.

Metro studied the two sets of data and found that Calcutta's air - based on the PM2.5 count - has been poorer than Delhi's through most of the month.

On some days, Calcutta was twice as polluted as the national capital, which often vies with Beijing for the crown of the world's most polluted city.

The Calcutta AQI was calculated based on data generated by four stations of the state pollution control board - at Behala Chowrasta, Minto Park, Moulali and Shyambazar. The other stations of the board in the city do not measure PM2.5.

The monthly average - 260 - falls in the category "poor". According to the central board, "poor" air can trigger breathing problems.

Experts feel the trend vindicates their repeated assertions that ill-maintained diesel vehicles are the key contributors to the city's air pollution.

"The role of vehicular emission became clear in a study we had conducted a few years ago. Combined with the atmospheric condition during this time of the year, the air pollution level becomes alarming," said Siddhartha Dutta, professor and environment expert at Jadavpur University.

In Delhi, burning of crop residue around the city and thermal power plants are major contributors to air pollution. "Delhi's lower pollution level compared with Calcutta may be explained by the successful implementation of the anti-pollution action plan," said Anumita Roy Choudhury, of the Delhi-based Centre for Science and Environment.