A lethal combination

A combination of local geography, atmospheric conditions and the province's industrial structure have contributed to the poor air quality, said Sun Yulong-deputy director of the Gansu Environment Protection Department. For example, Lanzhou is situated in a valley of the Yellow River, which restricts the circulation of the air.

Lanzhou is home to a large number of factories, including many involved in petroleum processing, and is frequently hit by dust storms, especially in winter and spring. Gansu saw 44 sand storms last year. The highest concentration of inhalable particles was 8.4 mg per cubic meter, 55 times higher than the national limit for level-two air quality.

"There are many reasons for the air pollution in the five Gansu cities the government has been monitoring, but heavy industry must shoulder most of the blame," Sun said. "Sand storms are seasonal, but factories produce emissions every day. "

By the end of 2012, the number of vehicles on Lanzhou's roads had topped 500,000 and harmful emissions were generated every day in winter by 800 large and 200 small coal-burning boilers. Nearly 80 percent of the factories surrounding the city center were related to petrochemicals and non-ferrous metals.

The metals industry has had a huge effect on the air quality in Baiyin. In the past half-century, the refining of copper and other metals has become a pillar industry in the city. The metals industry contributed greatly to the national economy in the first few decades after the foundation of the People's Republic, but the resultant pollution has created a harsh environment for local residents.

"We never opened the windows of our house. In the copper workshops, workers couldn't wear cotton underwear because it would be eaten away within a few days. Even a kilometer or so away from the factory, there was a strong smell and fumes that irritated the eyes," said Zhao.