According to the study, some of the worst polluted cities of the world are in India. The study, released this week but based on 2010 data, estimates that, globally, 2.7-3.4 million preterm births may be associated with exposure to PM 2.5. South Asia, with 1.6 million preterm births, is the worst hit, the study added

The review by 48 scientists and experts from 16 international institutions has warned that air pollution has adverse health effects on people even when the concentration of pollutants meets WHO standards.

At the same time, it said reducing air pollution will lead to massive health benefits. The study, called 'The Lancet Countdown: Tracking Progress on Health and Climate Change' — a review of significant scientific studies — refers to the recent International Energy Agency (IEA) report which had collated PM 2.5 exposure in lower, middle and upper income countries.

In the report, Patna and Delhi figure among the most polluted cities, both having an annual PM 2.5 concentration of more than 120 micrograms per cubic metres — about 12 times the WHO guideline. The WHO annual PM 2.5 guideline is 10 micrograms per cubic metres. This IEA report had sourced data from the World Health Organisation (WHO's) urban air quality database of 2016.

"An estimated 18,000 people die every day because of exposure to ambient and household air pollution, making it the world's largest single environmental health risk. Ambient air pollution is particularly pertinent in urban areas, but it also affects non-urban populations," the Lancet Countdown states.

"About 80% of people living in urban areas around the world are exposed to air pollution levels in excess of WHO guidelines. This number rises to 98% for urban populations in low-income and middle-income countries," the countdown states.

The report also adds that household air pollution that results from use of solid fuels and other biomass contributes to around 4.3 million deaths annually, which are related to pneumonia, stroke, lung cancer, heart disease, and chronic obstructive pulmonary disease (COPD).

The Lancet Countdown will report annually in The Lancet journal. The institutions involved in the project includes bodies such as University College London, Tsinghua University and the Centre for Climate & Security among others. They have collaborated with the World Health Organization (WHO) and the World Meteorological Organisation (WMO) too. The report underlines that air pollution and climate change are related and addressing air pollution will have climate change benefits too.