According to data collected by the WHO from nearly 1,100 cities across 91 countries, elevated level of fine particle pollution, which could cause heart diseases, lung cancer, asthma and acute lower reparatory infections, are common across many urban areas, with some cities registering pollution levels 15 times as much as the WHO guidelines.

For both developed and developing countries, the biggest contributors to urban outdoor air pollution include motor transport, small-scale manufacturers and other industries, burning of biomass and coal for cooking and heating, as well as coal-fired power plants, Xinhua reported.

Residential wood and coal burning for space heating is said to be an important contributor to air pollution, especially in rural areas during colder months, the WHO said in its air quality report published Monday.