Despite the health risks, the high level of pay in the metals industry made it an attractive proposition. During the 1980s and '90s, the average monthly salary of between 80 ($13) and 150 yuan was as much as five times higher than that of a local public servant.

Wang Xiuying, 74, who also worked for a copper company in Baiyin, said new workers from other provinces were even allowed to stay in the city's best hotel during the first few weeks of their transition period. "To work for a metals company was really something to be proud of," she said.

At the time, the amenities in villages were primitive. Few of the rural workers who arrived in Baiyin had used a shower before the age of 16 when they started work, but they soon discovered that the water in the shower rooms at the factories ran all day.

Daily ablutions were a mandatory post-work process, because the acid used during production evaporated and turned into acid mist. It ate away people's clothes and mixed with their sweat, causing severe discomfort and blisters. "On rainy days, even small drops of rain would leave a white stain on your clothes. Sometimes, the raindrops even burned a small hole," said Wang.

She said life in the electrolysis workshop was physically exhausting and risky in terms of health. The workers were supposed to wear masks, but "Most didn't because the temperature in the workshop was too high."

The late 1990s saw changes to China's economic growth model as heavy industry made way for the new energy sector. As the city's largest metals company, Baiyin Nonferrous Metals once employed one out of every 10 residents, but business fell off after 2000.

"My salary was 1,000 yuan in 2002, just half the average monthly income in Baiyin," said Xi Jiang, 42, who works for Baiyin Nonferrous Metals. The low wages and health risks affected the workers socially too. "Ads in the lonely hearts column of the local paper almost always ended with a variation on 'Thanks, but no thanks if you work for Baiyin Nonferrous Metals,'" said Xi.

The outdated production facilities also caused severe environmental problems. On Dec 19, 2002, the concentration of sulfur dioxide in Baiyin's air was measured at 7.6 mg per cubic meter, far outstripping the legal limit of 0.5 mg.

"That was the highest concentration I've experienced in my entire career. It was incredibly high and would definitely damage the respiratory system. The poor air quality could be life threatening, especially for those with asthma," said Chai Fahe, vice-president of the Chinese Research Academy of Environmental Sciences.

The twin pressures of falling profits and air pollution forced the company to change. By the end of 2008, it had invested around 9 billion yuan to update the facilities and build a recycling system. From 2007 to 2012, sulfur dioxide emissions were reduced by approximately 70,000 tons, accounting for 80 percent of the provincial reduction.