South Asian cities among the most polluted in the world

Outdoor air pollution causes an estimated 1.34 million premature deaths a year, said WHO. Investments to lower pollution levels quickly pay off owing to lower disease rates and, therefore, lower healthcare costs, it said.

The list, which relies on country-reported data over several years, measures the levels of airborne particles smaller than 10 micrometres – so-called PM10s – for almost 1,100 cities.

WHO recommends an upper limit of 20 micrograms for PM10s, which can cause serious respiratory problems in humans. They are mostly sulphur dioxide and nitrogen dioxide from power plants, vehicle exhausts and industry.

The Iranian city of Ahvaz had the distinction of the highest measured level of airborne particles smaller than 10 micrometres, with an annual average of 372 micrograms of PM10s per cubic metre.

The study found that the Indian capital of New Delhi had an annual average PM10s density of 198 micrograms/m³. The annual mean for all India came to 109 micrograms/m³.

The annual mean for Pakistan came to 198 micrograms/m³.

The fourth most polluted cities in the world were Ludhiana and Quetta with 251 micrograms/m³.

WHO said the reasons for the high levels varied but that often rapid industrialisation and the use of poor quality fuels for transport and electricity generation are to blame.

At the other end of the list are cities in Canada and the US, which benefit from lower population density, favourable climates and stricter air pollution regulation.

Yukon territory's capital, Whitehorse, had a yearly average of just 3 micrograms of PM10s per cubic metre, while Santa Fe, New Mexico, measured 6 micrograms/m³.

Thimphu was the cleanest city in South Asia with just 18 micrograms/m³ of PM10s.

Exposure to particulate matter with an aerodynamic diameter of 10 micrometres (PM10) or less in 1081 cities, 2003-2010/ Photo credit: WHO