FACTBOX - Who's affected by China's milk scandal, and why?

(Reuters) - Nearly 10 percent of milk samples from three top Chinese dairy companies have been found to be tainted with toxic melamine, China's quality watchdog has said after tests for the banned chemical that has killed four children.

Here are some questions and answers on China's widening toxic milk scandal and the people and companies it is affecting:

WHAT IS MELAMINE?

-- Melamine is a toxic industrial chemical used in plastic-making. It was first synthesised by a German scientist in the 1830s.

WHAT IS IT USED FOR?

-- Its most common form, melamine resin, a mix of melamine and formaldehyde, is used in the manufacture of formica, floor tiles, whiteboards and kitchenware.

WHY ADD MELAMINE TO MILK POWDER?

-- Melamine is rich in nitrogen, and relatively cheap. Adding it to milk makes watered-down milk's protein level appear higher. Standard quality tests estimate protein levels by measuring nitrogen content.

IS THIS WHY IT WAS ADDED TO PET FOOD?

-- Yes. 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 about 13,000 more have been hospitalised after drinking the contaminated milk.

-- More than 80 percent of the sick are under two years old. Young babies that 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. Medical experts warn however, that even if victims who have developed kidney stones due to tainted milk consumption have these removed, melamine may crystallise in small kidney tubes and block connecting ducts, resulting in kidney damage or even renal failure.

WHICH COUNTRIES ARE AFFECTED?