Heavy industry in Tangshan, Hebei province, has been hit by efforts to control air pollution, with the value of its output dropping by 35.3 billion yuan ($5.5 billion) in the past two years, according to the Ministry of Environmental Protection.

That accounts for about 5.7 percent of the city's GDP in 2014, according to Tangshan's bureau of statistics.

Under pressure to curb pollution, the city has shut down 2,382 companies involved in the iron, steel, cement and coking industries.