However, four other Indian cities were among the seven most polluted cities, the World Health Organization (WHO) said on Thursday, warning that over 80 per cent of the world's city dwellers breathe poor quality air.

According to a new report by WHO based on data collected between 2008 and 2013, New Delhi was the 11th most polluted city, while four other Indian cities - Gwalior (2), Allahabad (3), Patna (6) and Raipur (7) - figured among the seven cities with worst air quality.

The improvement in New Delhi's ranking comes after a string of measures taken by both the Centre and the Delhi government. These included levying of environment cess and implementation of the vehicle-plying rationing or the odd-even scheme to regulate traffic.

In a sample of selected mega-cities with a population above 14 million, New Delhi was, however, the most polluted, followed by Cairo and Bangladesh's capital Dhaka.

New Delhi's air quality was measured by the presence of particulate matter (PM) 2.5 which had an annual average measurement of 122.

Ten other Indian cities were also among the 20 most polluted cities in the world. In WHO's 2014 report, 13 out of 20 most polluted cities were in India. According to the WHO report, Delhi's annual mean PM10 was 229. Zabol in Iran figured as the city with dirtiest air.

Delhi Chief Minister Arvind Kejriwal tweeted, "Latest WHO report - Delhi no more most polluted city. Congrats Delhiites."

Tracking the presence 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 the Southeast Asia.

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.

"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 UN agency's latest air pollution database reveals an overall deterioration of air in the planet's cities, and highlights the growing risk of serious health conditions also including stroke and asthma.