In northwest Hui county, part of destitute Gansu province, up to 2,000 people have been tested for lead poisoning, which can cause severe mental retardation in children, according to state media.

"The smoke coming out of that factory has been awful for everyone. The ash and dust covers all of our vegetable and grain cropland," a woman surnamed Wang told AFP by phone.

"The factory has been there for 11 years and it has been like this all along. I think everyone in this area will be diagnosed with lead poisoning."

So far up to 350 people, including 146 children, in the villages of Xinsi and Muba have been shown to have high lead levels in their blood, a county spokesman told AFP by telephone.

With the figure likely to rise once test results for hundreds of others are in, locals said the smelting plant was to blame.

Wang said villagers had held several noisy demonstrations in front of county government offices and once blocked a road to demand the plant's closure.

The government only took action after local villagers were sent to hospitals in Xian, the nearest big city, and diagnosed with high levels of lead in their blood in August, press reports said.

"The smelting plant has been ordered shut down and moved," the county spokesman told AFP.

"Right now the county government is paying all the medical expenses for the local villagers, but we will seek compensation from the factory."

Following nearly 25 years of robust economic growth, China's cities have some of the worst air pollution in the world, while its polluted waterways are already exacerbating existing water shortages in many parts of the nation.

"The children must be treated quickly and effectively because they are in the most danger," said villager Wang. "My boy also has lead poisoning, he is always feverish and shakes. Before, his health was excellent."