TOI looks at the four key takeaways in the CPCB data:

Delhi is not India’s pollution capital

Jekyll-and-Hyde cities

Weather and location influence air quality

Differing fuel standards

NEW DELHI: The capital lost a dubious crown on Friday with the country’s pollution watchdog saying it is not India’s most polluted city, perhaps not even the second worst. But Central Pollution Control Board’s (CPCB) data for the September 2015-January 2016 period clearly shows that Delhi’s air is far from healthy.CPCB has published air quality indices (AQI) for 24 cities that help in comparing pollution levels at a glance with a colour code and a numerical value. In India, AQIs are determined based on the concentrations of seven pollutants, including PM2.5 (fine, respirable particles), sulphur dioxide (SO2), nitrogen dioxide (NO2) and carbon monoxide (CO).In January, when Varanasi in UP and Muzaffarpur in Bihar had “severe” AQI values of 409, Delhi scored a “very poor” with 362. Even neighbouring Faridabad was worse with an AQI of 399. Only three cities—Bengaluru, Haldia and Panchkula—had moderate air quality during the period.Although the AQIs will draw attention to the problem of air pollution in smaller cities, experts say the indices are at best indicative because in most cities CPCB has very few monitoring stations, and sometimes these are out of order. For instance, Muzaffarpur, Gaya, Faridabad and Varanasi have only one station each.Centre for Science and Environment’s analysis of Delhi Pollution Control Committee’s (DPCC) real-time monitoring data for December shows the capital had 23 severe air quality days although CPCB says there were none. “We have to factor in that many CPCB stations are in background areas or less polluted locations,” CSE said.In the past five months, Delhi had 13 “severe” air quality days while Muzaffarpur had 37, Varanasi 24, Agra and Faridabad 22 each, and Lucknow 18. In a “severe” AQI episode, most governments declare an air quality emergency as even young and healthy people can develop a variety of symptoms. People are advised to stay indoors. Delhi, however, had more “very poor” air quality days — AQI of 301-400 — than any other city. It had 69 to Kanpur’s 65 and Varanasi’s 43. Of the remaining 71 days, 43 recorded “poor” air quality in Delhi. The city also had as many as 43 days in the poor category. Delhi’s pollution problem seems worrying because while it swings between bad and very bad, the other cities also have moderate air pollution at times.Some cities manage to have very bad and very good air days in the rainy months, so their average does not reflect the severity of pollution. In September, Chandrapur in Maharashtra recorded a peak of 420, which is severe, and a minimum of 39, which is good, lowering its average to 111 or inside the moderate category. Agra also had a minimum of 26 in September, as against a peak of 404.Cities with the worst AQIs are mostly in the Indo-Gangetic plains, indicating that weather and location affect air quality as much as emissions. “There are geographical factors as well as anthropological (human),” said Gufran Beig, project director at System of Air Quality and Weather Forecasting and Research (SAFAR) under the ministry of earth sciences. Winds from the north and south of the plains converge in the zone, increasing the pollution, he explained. “The area is also the second largest alluvial plain, it is very fertile land and so naturally it is the most polluted. It also has more emission sources like industries, vehicles and brick kilns,” he added. The zone also has a concentration of thermal power plants. Dipankar Saha, additional director, air lab at CPCB, said reducing emissions is the only way to counter the zone’s meteorological disadvantage.Trucks and other heavy vehicles sold in many small cities of north India have BS-III engines although BS IV diesel is available. CSE has called for immediate implementation of BS IV norms across the country. CSE researchers say while places like Chandrapur are highly industrialised, motorisation rates are very high in cities like Kanpur and Lucknow due to rapid urbanisation. “This shows air pollution is a national public health crisis,” said Anumita Roy Chowdhury, head of CSE’s ‘Clean Air’ campaign. “Stronger public opinion has provoked action in Delhi but other cities, especially the second rung cities, are victims of policy neglect. We urgently need a national air quality strategy. National air quality standards should be legally binding.”Although the AQIs will draw attention to the problem of air pollution in smaller cities, experts say the indices are at best indicative because in most cities CPCB has very few monitoring stations, and sometimes these are out of orderCPCB has published air quality indices (AQI) for 24 cities that help in comparing pollution levels at a glance with a colour code and a numerical value. In India, AQIs are determined based on the concentrations of 7 pollutants, including PM2.5 (fine, respirable particles), sulphur dioxide (SO2), nitrogen dioxide (NO2) and carbon monoxide (CO).