The latest urban outdoor air quality database showed six of the 10 most polluted cities in the world are in India, the World Health Organisation (WHO) said in a report on Thursday. However, it turns out that Delhi, which was recently in the headlines for its poor air quality, is not the most polluted city in the country after all.

The WHO tested the air quality of 3,000 cities and towns across 103 countries this year, and found that Gwalior in Madhya Pradesh was the most polluted city in India. It ranked second in the list of the world's 10 most polluted cities.

Zabol in Iran has been found to be the most polluted city in the world, with the average annual PM2.5 (in µg/m3) level at 217. The air quality in New Delhi has improved since last year as the PM2.5 level has come down to 122 this year as compared to 153 in 2014, Reuters reported.

The Aam Aadmi Party-government in New Delhi introduced a phase-wise car-rationing plan last year to tackle rising air pollution in the national capital. The plan, which entailed running cars with odd-numbered registration plates on odd dates and cars with even-numbered registration plates on even dates, was implemented for 15 days each during the two phases since January this year.

A panel appointed to study the impact of the scheme was quoted by Indo-Asian News Service as saying that it was "successful."

India has the highest representation even when the list of the most polluted cities in the world is extended to 30. India has 16 of the 30 most polluted cities in the world, according to a Guardian report.

The WHO report led to a demand for "stringent, time-bound" action to tackle the pollution in the cities. "The government has to give up the fossil fuel-based energy and look for cleaner forms of energy, such as renewables," another Guardian report quoted Greenpeace India campaigner Sunil Dahiya as saying.

"Pollution levels have increased since the last WHO report in 2014, and the government must take it seriously. Right now, no one is really accountable for air pollution in the government. They need to make a systematic, comprehensive, and time-bound plan on a national and regional level," he said.

Top 10 most polluted cities in the world
City Country Annual average of PM 2.5 (in µg/m3)
Zabol Iran 217
Gwalior India 176
Allahabad India 170
Patna India 156
Raipur India 152
Bamenda Cameroon 132
Xingtai China 128
Baoding China 126
Delhi India 122
Ludhiana India 122

Pollution worldwide

The air pollution in 795 cities in 67 countries has increased 8 percent in the past five years with 80 percent "people living in urban areas that monitor air pollution are exposed to air quality levels that exceed WHO limits," the organisation's report on air pollution trends between 2008 and 2013 said.

"According to the latest urban air quality database, 98 percent of cities in low- and middle-income countries with more than 100,000 inhabitants do not meet WHO air quality guidelines. However, in high-income countries, that percentage decreases to 56 percent," the WHO report said.

Only two cities in low-income countries reportedly have air quality under the WHO limits. 

Poor air quality increases the risk of stroke, heart disease, lung cancer, and chronic and acute respiratory diseases like asthma. "Urban air pollution continues to rise at an alarming rate, wreaking havoc on human health," Dr Maria Neira, director of the WHO's Department of Public Health, Environmental and Social Determinants of Health, said.