THE DISCOVERY in Hong Kong of eggs contaminated with melamine has confirmed what experts have long feared; that the chemical is deeply embedded in the food chain.

Here are some questions and answers on China's latest food scandal and the people and companies it is affecting:

WHAT IS MELAMINE?

-- Melamine 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?

-- Melamine 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?

-- Melamine 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 crystalize in small kidney tubes resulting in kidney damage or even renal failure, health experts say.

WHICH CHINESE COMPANIES ARE IMPLICATED?

-- 22 Chinese companies have been listed as producing the tainted milk powder.

WHICH JOINT VENTURES ARE INVOLVED?

-- New Zealand dairy giant Fonterra, the business partner of Sanlu Group, China's top seller of infant milk powder and the first to go public with melamine contamination, has cut the value of its 43-percent stake in Sanlu to about $42 million. It said the writedown reflected the damage done to Sanlu by the scandal.

-- Danish-Swedish dairy cooperative Arla's Chinese joint venture Mengniu Arla's baby formula is also implicated.