In fact, it now ranks 11th among 3,000 cities in 103 countries in terms of PM 2.5 (fine, particulate pollution) and 25th in terms of PM 10 (coarse pollution particles) levels. This is though a considerable improvement since 2014 when Delhi was ranked the most polluted city in terms of PM 2.5 levels, WHO had monitored only 1600 cities last time. This time 1400 more cities have been included in the database.

Zabol in Iran is the most polluted city in the world according to the database. Gwalior and Allahabad are a close second and third in terms of PM 2.5, which is associated with more serious health impacts than PM 10. Patna and Raipur rank 6th and 7th. Totally, four Indian cities are among the world's ten most polluted cities, 10 out top 20 are also in India. In WHO's 2014 report, 13 out of 20 most polluted cities were in India.

Delhi's annual PM 2.5 mean for 2013 (second half) is 122 micrograms per cubic metres according to WHO's latest report compared to 153 micrograms per cubic metres as per WHO's previous report. Delhi's annual mean is about three times the Indian safe standard and 12 times the WHO standard of 10 micrograms per cubic metres. Chinese cities Xintai and Baoding are at ninth and 10th in the ranking, Beijing ranks far below at 56th. Beijing was at 75th last time. Sinclair in US is the least polluted city with an annual mean of only 2 micrograms per cubic metres.

WHO used data from various government and research organisations for the database, it's based on ground measurements of annual mean concentrations of particulate matter (PM 10 and PM 2.5) and "aims at representing an average for the city or town as a whole, rather than for individual stations. Years of measurements range from 2010 to 2015, unless the latest available data was older," the report said.

Experts said Delhi's efforts to control air pollution may have reflected in the improvement in ranking. Some however also raised an interesting point - WHO has been mainly focusing on particulate matter (PM) but not so much on oxides of nitrogen (NOx) which is a problem in many parts of the West. "To get a more accurate and balanced picture of air quality globally, WHO should have taken NOx in to account too. NOx levels are in high in many parts of Europe too," said an expert.

Global trends since 2008 show air pollution levels have gone up by 8% despite improvements in many cities. WHO in its statement also said urban air pollution levels were lowest in high-income countries, with lower levels most prevalent in Europe, the Americas, and the Western Pacific Region but highest in low-and middle-income countries in eastern Mediterranean and South-East Asia Regions, with annual mean levels often exceeding 5-10 times WHO limits.

"It is crucial for city and national governments to make urban air quality a health and development priority," said WHO's Dr Carlos Dora. "When air quality improves, health costs from air pollution-related diseases shrink, worker productivity expands and life expectancy grows. Reducing air pollution also brings an added climate bonus, which can become a part of countries' commitments to the climate treaty," he added. During the World Health Assembly, between 24-30 May, member states will discuss a road map for an enhanced global response to the adverse health effects of air pollution.

"Delhi has improved. Ahmedabad has stabilized and Patna has worsened. We have seen that Delhi has managed to arrest the declining air quality trend in 2015. Air policy action has started kicking in - with an environment compensation charge on on trucks, action against other sources. We are responding to action but the levels are still very high in the city, it only shows that action has to be sustained to meet clean targets," said Anumita Roy Chowdhury, executive director, Centre for Science and Environment (CSE).

"I am not surprised that four Indian cities are in the top ten list. In 2014, 13 out of top 20 cities were from India. Our fossil fuel consumption has increased since then. In India there are also multiple sources of particulate matter emissions that need to be addressed urgently like industries, thermal power plants, and biomass burning. The environment ministry has recently announced standards for thermal power plants. That's a welcome solution but we need an action plan now and will have to move away from a fossil fuel dependent economy," said Sunil Dahiya, campaigner with Greenpeace India.