Hundreds of people angry about the lead poisoning of their children attacked a metal smelting plant suspected of causing the problem.
About 100 police officers were sent to Shaanxi province's Changqing township after villagers broke into the smelting plant Monday.
The protesters broke down the fence surrounding a special plant railway. Police say they are in control of the scene, but at least 10 trucks were damaged.
The attack comes after medical tests revealed at least 615 children in two nearby villages have excessive levels of lead in their blood. These children live near the Dongling Lead and Zinc Smelting Company plant.
Lead poisoning occurs when the body absorbs lead, either by eating it or breathing it in. Dr. Marie Shieh of Beijing United Family Hospital says children are more vulnerable to lead poisoning than adults.
"They absorb the lead more easily in their digestive tract, and they have gaps in their blood-brain barrier that allows the lead to be deposited in their brain," said Shieh.
Some of the children tested in Shaanxi have blood lead levels of more than 500 milligrams per liter. Any level above 200 is considered hazardous. Such high levels can lead to severe problems of the brain and nervous system.
So far 166 children were hospitalized and their medical expenses will be covered by the county government. The rest will be treated at home.
Shieh says even with treatment the effects of lead poisoning can last a lifetime.
"The problem with lead is the neurological symptoms are irreversible," said Shieh. "That means even if these kids are treated they're going to have perhaps symptoms throughout their lifetime, because the lead doesn't go away very quickly."
Local authorities ordered the smelting plant to close on August 6, about two weeks after the first lead poisoning case was reported.
Environmental protection officials in the area say the lead content in the air near the plant was more than six times higher than monitoring sites about 350 meters away. Still, they say the plant's groundwater, surface water, soil and waste discharge all meet the national standards.
The Dongling Group is one of the biggest private companies in Shaanxi province. Polluting industries are frequently placed in poorer areas of China to develop the local economy.
Before the plant opened, the county government promised to relocate those living near the plant. But only a quarter of the families had been moved by the time the poisoning cases were revealed.
As its economy has grown over the past three decades, China has suffered a series of industrial pollution incidents that have fouled the air, water supplies and soil. On many occasions, such incidents have led to large protests by people harmed by the pollution.