"These results are crazy," said Kate Lin Pui-yi, a campaigner with the group, which released its report yesterday. "We humans so far have not fully grasped the harmful effects of pesticides, but there is evidence that a mixture of pesticides is much more harmful [than a single chemical]."

Pesticides have caused deaths on the mainland, and studies show some accumulate in the body over time, leading to hormone and reproductive problems and learning difficulties.

The worst contaminated sample was flower bought from a branch of Tongrentang in Beijing, which had 39 types of pesticide. Residue levels in some samples were hundreds of times above the European Union's food safety limits, the Greenpeace report said. Hong Kong has restrictions on nine kinds of pesticide residue, but not on any of the ones found in the five Hong Kong samples. Lin said the city's law is outdated as the nine pesticides it names are no longer used.

Greenpeace is urging the government to update the law to cover more pesticides and Chinese herbs, and to step up sampling at the border and in markets. It called on sellers of Chinese herbs to improve quality control on the herbs they purchase.

It is difficult for consumers to identify pesticide residues in herbs, as most herbs have a strong scent that covers any chemical odour. Washing and soaking herbs may help remove some of the residue, Lin said.

Other herbs found to contain pesticide residue that are on sale in Hong Kong are honeysuckle and wolfberry. Both were tainted by 12 kinds of pesticides, including the banned substance carbofuran, which was found in both. Wang Jing, food and agriculture senior campaigner with Greenpeace East Asia, said mainland herb growers were heavily dependent on pesticides to prevent diseases and insect infestations. The use of chemical pesticides has become more common on the mainland as plantation sizes increase in response to rising global demand for Chinese herbs.

Greenpeace visited mainland plantations and found that many farmers were unaware of the harmful effects of pesticides on their own and consumers' health, and on the environment, Lin said. In Shandong's Pingyi county, where honeysuckle is grown, the farmers are mostly over the age of 50 and do not know how to use pesticides scientifically, Wang said.

"Neither the government nor the manufacturer has any guidance for them," she said. "They often take the advice of local shops, which tend to recommend highly toxic pesticides."

The Health Department said it was concerned about the results and was investigating. It will inspect traders to ensure they are complying with the law.

Beijing Tongrentang did not respond to the 's requests for comment by press time.