Hong Kong mother Shirley Lo stocked her refrigerator with soymilk and switched to buying imported chocolates for her son after melamine was found in baby formula and milk products in China.

But when eggs from China tested positive for melamine in Hong Kong late last month, Lo threw up her hands in despair. ?It?s horrifying,? she said. ?It?s clear it has gone into basic foods and into our food chain. My son has been trying to comfort me, saying he must be very strong because his body must be full of this stuff and yet he is not sick?.

The discovery of melamine in eggs as well as in baby formula, milk products, biscuits, chocolates and other foodstuffs containing milk derivatives confirms what experts have long suspected; that the chemical is deeply embedded in the human food chain.

And it?s not just melamine; heavy metals such as lead and mercury which can cause brain damage, as well as cadmium, a compound used in batteries, pesticides and antibiotics are all present in the human food chain.

China is a major transgressor as carcinogenic chemicals are regularly used as food colouring agents or as preservatives, experts say.

?In China, food safety is not a concern and all sorts of things like Sudan red, Malachite green are added in food, so food contamination is widespread,? said Peter Yu, a professor of biology and chemical technology at the Hong Kong Polytechnic University.

?We also have environmental contamination from pesticides, formaldehyde (to kill bacteria),? Yu said, citing the use of Malachite green, a carcinogenic agent, that in 2006 was found in fish from China. It had been added to eradicate fungal disease in the fish.

?We know they would kill people?

Leading food manufacturers regularly test their ingredients and final products for many of these contaminants, but experts say it?s impossible to keep up with all the foreign compounds that land up on the dinner table, especially in China where regulation is lax and difficult to enforce.

In the wake of the melamine scandal, China is reviewing a tougher draft food safety law following criticism from the United Nations for its sluggish response to the tainted milk scandal.

The melamine saga has surprised even some food producers, who say they find it hard to keep up with strange additives that are added to food. Melamine, for example, was added to baby formula to cheat protein level tests.

?How did we come up with cadmium or heavy metals? Because we know they would kill people. That?s why we test for them. But we didn?t know melamine would even be in food,? said a manager who works for a major foreign food producer with factories in China.

?We never had melamine in our specifications (contaminants to look out for). If it is melamine today, it will be something else tomorrow. We can?t possibly test for every toxin in the world,? said the manager, who declined to be named because he was not authorised to speak to reporters.

Risks of long-term consumption

Tens of thousands of children in China have fallen ill with kidney problems in recent months, and at least four have died, after being fed infant formula that was later found to have been mixed with melamine.

Subsequent tests found melamine in a variety of Chinese-made products from milk and chocolate bars, to yoghurt and other products exported around the world, leading to items being pulled from shop shelves and massive recalls. But with the discovery of melamine in eggs, apparently due to contaminated feed given to chickens, the chemical appears to be far more entrenched in the human food chain than first thought.

Melamine and its derivatives are widely used in animal feed and pesticides in China but no one knows how harmful they can be to people after prolonged exposure.

Hong Kong imposed a cap on melamine in September to no more than 2.5 milligrams per kilogram, while food meant for children under 3 and lactating mothers should be no more than 1 mg per kg.

Experts say the limits are arbitrary and called for more tests and science when imposing safety limits. ?The limits are derived from animal studies but we don?t know what our exposure is. What if we are accumulating more than is safe?,? said Chan King-ming, biochemistry professor at the Chinese University. ?There should be surveys to find out what foods have melamine and their concentrations. Then we know how serious it is.?

A World Health Organisation official said this week some of the affected children in China, most of whom are believed to be under the age of 3, have ?crystals? in their kidneys. Some might need surgery to avoid potentially deadly kidney failure.

In Hong Kong, parents have thronged public clinics to get their children tested for melamine by laboratories that analyse urine samples with sophisticated spectrometers. ?Melamine is not soluble. But if it is very concentrated as in the case of these Chinese kids (whose diet was mostly formula), it forms into crystals,? said Allen Chan, associate professor of chemcal pathology at the Chinese University.

Permanent liver damage can be caused when crystals suddenly form into large numbers of tubules in the kidneys of children that have consumed melamine, causing chronic kidney failure and requiring dialysis and even kidney transplants later on in life.

The WHO plans to make a detailed assessment of the risks of long-term consumption of melamine. It has asked China to provide information for a meeting of experts in December.

WHAT?S MELAMINE AND HOW DID IT GET INTO EGGS?

● WHAT IS MELAMINE?

It is a white powder used in plastic-making. It was first synthesised by a German scientist in the 1830s.

● WHAT IS IT USED FOR?

Melamine resin, a mix of melamine and formaldehyde, is used in the manufacture of Formica, floor tiles, whiteboards and kitchenware.

● HOW DID IT FIND ITS WAY INTO CHICKEN EGGS?

It was added to animal feed in China and such feed was apparently given to chickens that produced eggs tainted with melamine.

● WHY ADD MELAMINE TO MILK POWDER, ANIMAL FEED?

The product is rich in nitrogen, and relatively cheap. Adding it to sub-standard animal feed or watered-down milk makes their protein levels appear higher. Standard quality tests estimate protein levels by measuring nitrogen content.

● WHAT OTHER PRODUCT SCANDALS HAVE INVOLVED MELAMINE?

Melamine was linked to the deaths of cats and dogs in the United States last year after it was added to wheat gluten and other pet food ingredients exported from China, in another attempt to boost the products? apparent protein content.

● WHO HAS BEEN AFFECTED BY THE TOXIC MILK?

So far four infants have died in China, and at the peak of the crisis in late September, up to 22,000 infants were in hospital on any given day, Chinese health officials say. More than 99 percent of the sick are under 3 years old. Infants who depend solely on milk are most vulnerable.

● WHAT ARE THE LONG-TERM HEALTH EFFECTS?

Little scientific information exists about the compound?s effects on humans. Melamine can cause kidney stones and the chemical can crystallise in small kidney tubes resulting in kidney damage or even renal failure, health experts say.