HONG KONG—Melamine, a chemical that has tainted milk formula and made thousands of Chinese children ill, is used as an agricultural pesticide in China and may have been part of our food chain for a long time, according to experts.

Chan King-ming, associate professor of biochemistry at the Chinese University, said cyromazine, a derivative of melamine, was commonly used in China as a pesticide.

“It is absorbed into plants as melamine ... of course it is already in our food chain and animal feed,” Chan said in an interview on Tuesday.

“So, it is not just in milk products, but also in farm products and animal feed, fish diet,” he said.

An Internet search for Chinese suppliers of cyromazine pesticides yielded many entries.

Harm to humans

But experts were uncertain as to what this means for human health, or for people who may have been exposed to the chemical over the long term, albeit in very small amounts each time.

Of the thousands of Chinese children who have fallen ill with kidney stones, 80 percent are 2 years old, and they would have relied almost entirely on the tainted formula milk for food.

No studies have so far been done on the harm melamine can cause human subjects.

“What we know is that melamine gives kidney stones and problems in the kidney,” said Peter Yu, associate professor in applied biology and chemical technology at Hong Kong Polytechnic University.

“But we don’t know if there are other ill effects in the long term. These are ingredients that shouldn’t be in food,” he said.

Other experts have said that, while kidney stones can be removed easily, it is far more worrying that kidney damage and even failure can result when melamine starts crystallizing in and blocking small kidney tubes that filter blood.

‘Unlikely to pose risk’

Melamine first turned up last year in Chinese pet food exported to the United States, where many cats and dogs developed acute kidney failure and died.

But in a report released in May 2007, the US Food and Drug Administration said its investigations found that consuming “pork, chicken, fish and eggs from animals that had inadvertently been fed animal feed contaminated with melamine ... was very unlikely to pose a human health risk.”

Citing other studies, Daniel Chan, nephrology professor at the University of Hong Kong, said: “Results from the investigations that followed the pet food incident in 2007 suggested the level of contamination in our food chain was low and thus unlikely to cause significant adverse effect in humans.”

Cap on melamine

Hong Kong placed a cap on melamine in food on Tuesday, restricting it to no more than 2.5 milligrams per kilogram. Melamine found in food for children under 3 and pregnant and lactating mothers should be no higher than 1 mg per kg.

Offenders could be jailed for up to six months and fined up to HK$50,000 (US$6,410).

But experts criticized the limits.

“That 1 mg limit is arbitrary. Like a lot of carcinogens, the limits are constantly being brought lower. 1 mg may be harmless, but over the long term, it is not good. Ideally, it should be banned, it shouldn’t be allowed,” Yu said.