HONG KONG - Scientists in China and Hong Kong have established for the first time in a study that consuming the plastic-making chemical melamine can cause kidney stones in people.

At least six children died and 290,000 fell ill in China last year after consuming milk formula tainted with melamine, which was added to cheat protein tests. But the causal link between melamine and kidney problems the children suffered was never scientifically established until now.

The experts studied urine samples of 15 mainland Chinese toddlers with kidney stones and compared those taken from 20 children in Hong Kong who also consumed tainted milk but who did not develop stones.

"We proved that melamine alone can cause stones ... Our conclusions are that the higher the concentration of melamine in the urine, the bigger the stones," said Lawrence Lan, associate consultant at the paediatrics surgery department in Hong Kong's Queen Mary Hospital.

The experts also found what they considered to be a "safe level" of melamine in urine, above which the person may be prone to develop kidney stones.

"Everyone with stones had melamine levels above that level, and those without stones had melamine concentrations below that level," Lan said.

The study was published in the International Journal of Clinical Chemistry and Diagnostic Laboratory Medicine.

The milk scandal battered already dented faith in China-made goods and prompted massive recalls of dairy and other food products around the world.

Melamine is used to make fertilisers, plastics and other industrial goods. Rich in nitrogen, it later found its way into food products to fool tests for protein.

It has also been detected in eggs, chocolates, ice creams, yoghurts and other foods.

China has suffered other food additive scandals in the past, including the use of carcinogenic chemicals as food colourings.

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