The top official of a northwest China province has pledged to clean up the sky over the provincial capital after it was named China's city with the worst air pollution by a World Health Organization (WHO) survey published in September.

In winter, the most polluted season, Lanzhou is typically shrouded in a haze. [File photo]?

Lanzhou, a heavy industry city situated in the Yellow River valley in the country's arid northwest, ranked the worst among Chinese cities in the WHO's survey tracking air pollution by the levels of airborne particles smaller than 10 micrometers (PM10) measured in 1,086 cities in the world, mostly between 2008-2009.

Lanzhou's annual PM10 average was 150 micrograms per cubic meter, dramatically higher than the WHO-recommended upper limit of 20 micrograms per cubic meter. The Geneva-based organization says PM10s can cause heart disease, lung cancer, asthma, and acute lower respiratory infections.

In winter, the most polluted season, Lanzhou is typically shrouded in a haze that can block sunlight to the extent that the day is sometimes as dark as the night. Residents generally avoid opening windows, otherwise their furniture will soon be coated in thick dust.

Wang Sanyun, the newly-appointed top official of Gansu province, said that his government "is resolved to win the tough and arduous battle against air pollution."

Wang, secretary of the Gansu provincial committee of the Communist Party of China, told provincial officials Thursday that the government will go after factories releasing pollutants, promote clean energy in public transport, build subways and light rails to reduce car exhaust emissions, and replace coal with natural gas to sustain the city's winter heating system.

Due to an urbanization rush, most cities in China face tremendous challenges in keeping air pollution in check. Only one in the 31 Chinese cities included in the WHO survey had PM10 levels under 50 micrograms per cubic meter, while the vast majority of European and North American cities reported PM10 levels under 50 micrograms per cubic meter.

Many Chinese cities have been periodically enveloped in smog this winter, but the official air quality index typically classifies pollution as "light," deepening public frustration over the quality of air people breathe every day.

Minister of Environmental Protection Zhou Shengxian has urged a prompt overhaul of the current air quality monitoring standard to factor PM2.5 - the measure of microscopic airborne particles smaller than 2.5 micrometers - as well as measures of ozone and carbon monoxide into its pollution monitoring system.