Four out of five children have tested positive for lead poisoning
Chinese officials have blamed a lead smelting plant for the poisoning of more than 600 children in the province of Shaanxi.
Tests showed 615 of 731 children in two villages in Changqing township had excessive lead levels in their blood.
Lead poisoning can damage the nervous system and cause death in severe cases.
It is unclear if the Dongling plant, which belongs to one of the largest private companies in the province, will face any sanctions.
The smelter, which employs about 2,000 workers, was shut down by the authorities last week.
Han Qinyou, the local environmental protection officer, told state news agency Xinhua that the lead content in the air was extremely high along main roads.
But he said samples of the company's water and soil emissions had "met national standards".
Mr Han said officials were investigating whether the plant alone, or other factors such as eating habits or vehicle exhaust, were to blame.
Zhao Weiping, a deputy Communist Party secretary of the Dongling Group, has apologised to residents and pledged to co-operate with the authorities, Xinhua said.
A total of 166 children have been treated in hospital for lead poisoning, while the others are receiving treatment at home, officials said.