While some are seeing a rising trend and some, interestingly, are also showing a decreasing trend in PM 10 (coarse pollution particles) levels, the study also shows that most cities saw an improvement in 2015 compared to previous years. The CPCB concludes in its report that improvement could be linked to implementation of stricter vehicle norms and better fuel quality.

The report was compiled by the CPCB after the NGT in its February 9 order directed it to file an "analysis report" on pollution levels in all major cities identified by the tribunal. The report is also one of the documents on the basis of wh- ich the NGT is spearheading the efforts to reduce pollution in other major cities while an air pollution case pertaining to the Capital is being heard in the Supreme Court.

The NGT asked for more air pollution data from states on Wednesday with details of major sources so that it can come up with effective solutions. The report, which compiles pollution levels from manual monitoring stations in 15 cities, shows that PM 10 levels seem to be falling in Mumbai, Pune, Ludhiana, Amritsar, Chennai while in Lucknow they are stabilising.

There are no clear trends in Bengaluru, Jalandhar, Allahabad, Kanpur and Kolkata, but PM levels are seen increasing in Hyderabad and Varanasi. "The reason for decrease in PM 10 levels may be implementation of stricter vehicle norms (BS IV)," the study claims. "The fluctuating trends in some cities could be because of vehicles, diesel gen sets, small scale industries, biomass incineration, resuspension of traffic dust and others," it said. Sulphur dioxide (SO2) levels were found to be meeting safe standards in nearly all cities. Most cities also met the nitrogen dioxide (NO2) standards except Patna, Pune, Amritsar and Kolkata in some years.

Experts, however, said the report is inadequate as it doesn't correlate PM 10 levels with vehicle numbers, establishment of new industries, introduction of cleaner fuels and other interventions. They also pointed that PM 10 is not an effective indicator for pollution caused by combustion sources like vehicles or waste burning. The CPCB should have provided with PM 2.5 data (fine, respirable pollution particles) to give a clearer picture.

BS IV was introduced in NCR, Mumbai, Kolkata, Chennai, Bengaluru, Hyderabad, Ahmedabad, Pune, Surat, Kanpur, Lucknow, Sholapur, Jamshedpur and Agra in 2010. The gradual PM 10 rise in Varanasi could be because it is still on BS III but experts said improvements in Ludhiana, Amritsar or Nagpur cannot be due to better fuel norms.

"The report doesn't tell us how changes in PM 10 levels are correlating with interventions in these cities. Manual monitoring highly underestimates real air pollution levels. Its high time CPCB provided real time monitoring with details of PM 2.5 levels, Benzene, Polyaromatic Hydrocarbons (PAHs), secondary particulates like sulphates and nitrates," said Anumita Roy Chowdhury, head of Centre for Science and Environment (CSE's) clean air campaign.