Dec. 29: Calcutta is poised to claim the crown of most polluted Indian city from Delhi this winter.

The central pollution control board's Air Quality Index (AQI) shows the city ahead of the capital in this winner-loses-all race, based on the severity of pollution in some locations.

Independent reports generated by a pollution measuring Android app called Plume Air Report suggest that Calcutta's air quality is even worse than some of the world's more lung-unfriendly cities.

According to the Central Pollution Control Board, the city has been experiencing the worst spell of air pollution this year since December 23.

The AQI, calculated after collating data sourced from various government agencies on different kinds of air pollutants, including particulate matter, shows that the average pollution count between December 23 and 27 was 363. A count above 300 is categorised as "very poor".

On December 25, the AQI spiked to 414, which is considered "severe". Pollution at this level can "affect healthy people and seriously impact those with existing diseases". The AQI varied between "very poor" and "poor" over the next two days.

The index is calculated on the basis of the dominant pollutants in an area, which in most parts of India is the concentration of particulate matter. The figures highlighted by Metro are from the pollution monitoring centre at Rabindra Bharati University. Victoria Memorial, the other centre in Calcutta, generated less data.

Pollution researchers believe that the data from Rabindra Bharati is more representative of the city since the monitoring station at Victoria Memorial is housed amid greenery.

On December 25, when Calcutta's AQI at Rabindra Bharati was pegged at 414, the reading at ITO, Delhi, was 390.

State pollution control board data from the Rabindra Bharati monitoring station shows that the average of Particulate Matter (PM) 10 between December 23 and 27, 2015, had been 234 micrograms. The PM10 average for the same period this year is 332 micrograms.

Pollution increases in winter due to the phenomenon of "temperature inversion". The cold traps pollution close to the ground, particularly in the absence of wind, and maximises the effect on those exposed to it.

Data generated independently through the Plume Air Report app shows Calcutta worse off this winter than most cities worldwide. In terms of PM 2.5, the city had the highest pollution levels in the world during the period November 25 to December 24. PM 2.5 is the most potent air pollutant because it can reach deep into the lungs and trigger a range of respiratory and cardiovascular diseases, including lung cancer.

On the days mentioned, Calcutta's PM 2.5 count was an average of 243 micrograms per cubic metre, which is four times above the national permissible limit and 10 times above the World Health Organisation's red line. The Plume Air Report app, which covers most metros in the world, uses pollution data from government and other sources in the public domain to collate PM 2.5 levels.

"Calcutta had the worst average PM 2.5 air pollution among the major metropolises in India and other countries, and the list includes Delhi and Beijing, the global air pollution capitals," said Johnny Clarke of Lifetree, an organisation working on air pollution.