New Delhi was ranked worst in 2014 with a PM2.5 reading of 153.
Geneva - Gwalior, Allahabad, Patna, Raipur among top 10 with bad air quality
India is home to four of the five cities in the world with the worst air pollution, the World Health Organisation said on Thursday.
But while WHO experts acknowledge India faces a 'huge challenge', many countries are so bad that they have no monitoring system and cannot be included in its ranking.
The dirtiest air was recorded at Zabol in Iran, which suffers from months of dust storms in the summer, and which clocked a so-called PM2.5 measure of 217. The next four were all Indian: Gwalior, Allahabad, Patna and Raipur.
India's capital New Delhi was the survey's ninth worst city, measured by the amount of particulate matter under 2.5 micrograms found in every cubic metre of air, with an annual average PM2.5 measurement of 122.
Tiny particulate matter can cause lung cancer, strokes and heart disease over the long term, as well as triggering symptoms such as heart attacks that kill more rapidly. The WHO says more than 7 million premature deaths occur every year due to air pollution, 3 million of them due to outdoor air quality.
New Delhi was ranked worst in 2014 with a PM2.5 reading of 153. It has since tried to tackle its toxic air by limiting the use of private cars on the road for short periods.
Maria Neira, head of public health, environmental and social determinants of health at the WHO, praised India's government for developing a national plan to deal with the problem when others have been unable to.
"Probably some of the worst cities that are the most polluted ones in the world are not included in our list, just because they are so bad that they do not even have a good system of monitoring of air quality, so it's unfair to compare or give a rank," she said.
Common causes of air pollution include too many cars, especially diesel-fuelled vehicles, the heating and cooling of big buildings, waste management, agriculture and the use of coal or diesel generators for power. On average, pollution levels worsened by 8 per cent between 2008 and 2013, although most cities in rich countries improved the state of their air over the same period.
The WHO data, a survey of 3,000 urban areas, shows only 2 per cent of cities in poorer countries have air quality that meets WHO standards, while 44 per cent of richer cities do.
Urban residents in poor countries are by far the worst affected, WHO said, noting that nearly every city (98 per cent) in low- and middle-income countries has air which fails to meet the UN body's standards.
That number falls to 56 per cent of cities in wealthier countries. "Urban air pollution continues to rise at an alarming rate, wreaking havoc on human health," Maria Neira, the head of WHO's department of public health and environment, said in a statement.
The report, which focused on outdoor rather than household air, compared data collected from 795 cities in 67 countries between 2008 and 2013.
Tracking the prevalence of harmful pollutants like sulfate and black carbon, WHO found that air quality was generally improving in richer regions like Europe and North America, but worsening in developing regions, notably the Middle East and southeast Asia.
A sample of European data showed that Rome had slightly worse air than Berlin, followed by London and Madrid.
Carlos Dora, coordinator at WHO's public health and environment department, pointed to several key factors that determine the quality of a city's air.
First was transportation, Dora said, noting that cities which succeed in reducing vehicle traffic while promoting walking, cycling and mass public transport inevitably see their air quality improve.
Energy inefficiency - especially with respect to heating and cooling buildings - is a major cause of dirty air, along with the widespread use of diesel generators as a replacement for cleaner electricity sources, Dora added.
Another crucial factor, especially in developing countries, is waste management, with the smoke generated by burning garbage ranking among the top pollutants. - The WHO database has almost doubled in size since 2014, and the trend towards more transparency translated into more action to deal with the problem, Neira said.
However, there was still very sparse data on Africa, she said.