Delhi no more the most polluted city in the world
Delhi is not the most polluted city in the world any more, according to the World Health Organisation's urban air quality database.
It ranks 11th among 3,000 cities in PM 2.5 pollution levels.
The 2014 WHO report had said that Delhi was the most polluted in the world.
Delhi's efforts to control air pollution may have resulted in the improvement in ranking, said experts.
Methodology of WHO survey
The WHO database compares 795 cities across 67 countries for levels of small and fine particulate matter during the five-year period, 2008-2013.
They include pollutants such as sulfates, nitrates and black carbon.
PM2.5 are very fine particles that can penetrate deep into the respiratory system and can be directly linked to stroke, heart disease and respiratory disorders, while PM10 are coarse, slightly larger particles.
4 Indian cities in top 5
As per the WHO study, the dirtiest air quality was recorded at Zabol in Iran, due to months of dust storms, which clocked a PM 2.5 (Fine Particulate Matter) measure of 217.
The next four were all Indian cities: Gwalior, Allahabad, Patna and Raipur.
10 out of the top 20 most polluted cities are in India while in 2014, this number was 13.
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Pollution kills 3 million people a year
Outdoor air pollution causes more than 3 million deaths a year - more than malaria and HIV/Aids - and is now the single biggest killer in the world. The toll is expected to double as urban populations increase and car numbers approach 2 billion by 2050.
How other countries fared in the survey
For PM 2.5, China has improved its air quality since 2011 and now has only five cities in the top 30.
Nine other countries, including Pakistan and Iran, have one city each in the worst 30.
For PM 10, Nigeria, Saudi Arabia and Pakistan each have two cities in the top 10.
Air pollution levels were much lower for cities in developed countries.
In major cities, New Delhi still most polluted
In a sample of selected mega-cities with a population above 14 million, New Delhi was the most polluted, followed by Cairo and Bangladesh's capital Dhaka.