Delhi, Gwalior and Raipur are among the world’s most polluted cities due to the high content of dust and smoke in the air which in turn caused serious health problems, says the World Health Organisation (WHO).

The latest air quality data released by the world’s apex health agency on Wednesday, show more than 40 per cent of 124 Indian cities surveyed, as highly polluted. Gwalior and Raipur are leading the chart followed by Delhi, Lucknow, Kanpur, Firozabad and Agra. Southern India is relatively cleaner.

Even though the WHO list – prepared on the basis of data from 1,600 cities around the world – is not an exhaustive one, it gives a clear peek into the urban air quality worldwide.

“Too many urban centres are so enveloped in dirty air that their skylines are invisible. This air is dangerous to breathe,” said Flavia Bustreo, WHO assistant director general.

The agency describes air pollution as one of the leading causes of premature death.

Last month, WHO released new estimates suggesting outdoor air pollution as the reason for the death of 3.7 million people under 60 years in 2012. It counts indoor and outdoor air pollution among the largest health risks.

India is one of the countries which is trying to reduce indoor air pollution by 2025 to bring down the number of deaths from non-communicable diseases.

Fresh data from the health agency suggests outdoor pollution is equally harmful. About half of the urban population is exposed to air pollution that is at least 2.5 times higher than the levels WHO recommends — putting them at additional risk of serious, long-term health problems.

Air pollution is measured in terms of presence of coarse and fine dust in the atmosphere.

There are two types of dust particles – the big ones (PM-10 or particulate matter having a diametre of 10 micron) and the fine ones (PM-2.5) or particles with a diametre of 2.5 micron or less, which are considered the best indicator of health risks from air pollution.

The WHO recommended annual limits for PM-10 and PM-2.5 are 20 and 10 microns respectively. The levels are way above the mark in all Indian cities.

“Every Indian city shown in the chart is above the WHO limit. It shows air quality in Indian cities is extremely harmful for public health,” Anumita Roychoudhury, executive director at the Centre for Science and Environment, here told Deccan Herald.

Hasan in Karnataka, Bongaigaon in Assam and Madurai in Tamil Nadu are the three cleanest cities, though their PM-10 levels are double the WHO benchmark. But they appear clean when compared with 52 other Indian cities, all of which have PM-10 level more than 100.