GENEVA • India is home to four of the top 10 cities in the world with the worst air pollution, the World Health Organisation said yesterday, citing its latest global pollution survey.

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 the UN agency's 2016 update of its urban air pollution database.

The latest version of the database covers 3,000 towns and cities in 103 countries for the years 2008 to 2015, and ranks urban centres based on levels of ultra-fine particulate matter, such as soot, of less than 2.5 microns (PM2.5) in size and less hazardous PM10 readings.

The dirtiest air was recorded at Zabol in Iran, which recorded a PM2.5 measure of 217. Among the top 10 were the Indian cities of Gwalior, Allahabad, Patna and Raipur. China had two cities in the top 10, Xingtai and Baoding, both in Hebei province near Beijing.

India's capital New Delhi was the survey's eleventh worst city, with an annual mean PM2.5 measurement of 122 micrograms per cubic metre (mcg/m3) of air - far beyond the WHO's air quality guideline of 10 mcg/m3.

Tiny particulate matter can cause lung cancer, strokes and heart disease over the long term, as well as triggering symptoms such as heart attacks. WHO says more than seven million premature deaths occur every year due to air pollution, three million of them due to outdoor air quality.

New Delhi was ranked worst in the WHO's 2014 survey with a PM2.5 reading of 153. It has since tried to curb private car use.

Common causes of air pollution include too many cars, especially diesel-fuelled vehicles, poor 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 shows only 2 per cent of cities in poorer countries have air quality that meets WHO standards.

REUTERS