This is because the chemical was not considered a substance found in food before a scandal broke out in China where milk products were tainted with the substance, Peter Ben Embarek, WHO food safety expert told AFP late Thursday.
"To my knowledge, the EU, Hong Kong and New Zealand have only in recent weeks fixed a transitory limit of 2.5 milligrams per kilogramme for food products, with the limit lowered to 1 milligram per kg for food consumed by children," Embarek said.
He added that "melamine has nothing to do with the food chain and therefore no standard had been fixed on a global level.
"There are billions of chemical products that normally should not be found in food -- it is therefore impossible to fix a limit for all the chemicals. That's the case of melamine," he said.
Standards are only fixed for chemicals or elements that are known to be present in food products as they were used during the production process, such as pesticides, explained Embarek.
Melamine, besides internal contamination, could also be found in traces due to accidental contamination through product packaging or through contact with certain surfaces during the manufacturing process.
Some fertilizers, which are rarely used, also contain melamine which could then be the source of traces found in food products, he added.
However, at low levels, the substance posed no health threat.
"Melamine has a rather low toxicity -- in traces or in low doses, it is eliminated by the organism without harm," he said.
But with at least four children killed and 53,000 sickened in China by milk products tainted with melamine, governments around the world have moved to test food products containing milk from China.
Authorities, including those in Hong Kong and Taiwan, have banned products that were found to contain melamine.