China's top environmental watchdog on Friday pledged more efforts to curb pollution caused by heavy metal manufacturing, which has been blamed for a growing number of lead poisoning cases in the country.

A child who was diagnosed with excessive lead in his blood receives medical treatment at a hospital in Chenzhou, Hunan province, on March 20.

"In order to protect the environment and guarantee public health, avoiding excessive emissions of heavy metals will be on the top of our agenda this year," Zhou Shengxian, head of the Ministry of Environmental Protection, said at a press conference in Beijing on Friday.

The data released in the first national census on pollution sources in February show heavy metals have become the country's main pollutant, with nearly 900 tons emitted every year.

Heavy metals mainly refer to toxic metals such as lead, mercury and chromium, which can be dangerous to health and the environment in excessive levels. They usually arise from purification of metals in the manufacturing process.

Due to the numerous heavy metal pollution cases last year, more than 92,000 people across the country participated in a nationwide campaign against factories operating illegally by emitting more heavy metal pollution than regulations allow, according to statistics from the ministry.

Of the total 9,123 companies emitting heavy metals, 2,183 of them were found to be harmful to the environment, with officials closing 231 and suspending 641, the statistics showed.

About 1,296 factories causing arsenic emissions were examined and 304 were found to be operating illegally. Some 36 were shut down and 248 were suspended.

In 2009, the frequent cases of heavy metal pollution across the country have raised the alarm for the public and authorities.

At a press conference in January, Zhou told the media that the ministry received 12 cases related to heavy metal pollution in 2009, causing 4,035 people to suffer excessive blood lead levels and 182 with excessive cadmium levels.

Since August 2009, a series of lead poisoning cases occurred in Shaanxi, Hunan and Yunnan provinces. Thousands of children have been diagnosed with excessive lead in their blood.

Investigations later showed those who were poisoned by lead lived near battery factories or smelters.

Excessive lead in the blood can damage brain function, especially for children who are vulnerable to lead poisoning, World Health Organization officials said.

Along with human health, farmland and water are also polluted by heavy metals across the country.

In June 2008, Yangzonghai lake in Yunnan province was found to be polluted with arsenic, affecting water supplies to more than 26,000 people. Investigations found that some companies had released arsenic into the lake.

A study from the Research Center for Eco-Environmental Sciences at the Chinese Academy of Sciences shows about 20 percent of the country's farmland was polluted by heavy metals, causing the crop output to decrease by more than 10 million tons.

Once heavy metals are released into water or land, they are difficult to extract and will have a negative impact on that land in the future, environmental experts said.

In order to fight heavy metal pollution, China will launch a regulation on recycling and dealing with electronic waste in 2011.