It's in the news. Babies are being hurt, people are becoming worried about tainted foods. That made me curious: What exactly is melamine? What is a safe dose? Does it eventually get cleared of the body? The last question is important because if a toxic substance never leaves after ingestion, then the previous question of what a safe dose is becomes a red herring.
Here is the wikipedia article describing Melamine.
Turns out melamine by itself isn't as big a problem as melamine cyanurate, which is what is formed by the combination of melamine and cyanuric acid. Also, turns out:
A survey commissioned by the American Association of Veterinary Laboratory Diagnosticians suggested that crystals formed in the kidneys melamine combined with cyanuric acid, "don't dissolve easily. They go away slowly, if at all, so there is the potential for chronic toxicity."
Scary, means we really should avoid melamine so we avoid the risk of interaction with cyanuric acid where possible. Finally, this is the extent of regulation of melamine where the US Food and Drug Administration is concerned:
The Food Safety and Inspection Service (FSIS) of the United States Department of Agriculture (USDA) provides a test method for analyzing cyromazine and melamine in animal tissues in its Chemistry Laboratory Guidebook which "contains test methods used by FSIS Laboratories to support the Agency's inspection program, ensuring that meat, poultry, dairy and egg products are safe, wholesome and accurately labeled." In 1999, in a proposed rule published in the Federal Register regarding cyromazine residue, the United States Environmental Protection Agency (EPA) proposed "removing melamine, a metabolite of cyromazine from the tolerance expression since it is no longer considered a residue of concern." Melamine, classified a controlled substance in China, has been illegally used in the high profile 2008 baby milk scandal case which led to the death of at least 4 infants.
On October 3, 2008, the U.S. Food and Drug Administration said that up to 2.5 parts per million of melamine was safe for adults, but declined to set a standard for children. The FDA also implied it would not permit the sale of food deliberately adulterated (rather than accidentally contaminated) with melamine.
So, really, the safe ppm rating is for background or accidental contamination of foodstuffs by melamine. No one's supposed to be doping any food products deliberately with melamine. I'm also guessing the 2.5 ppm number may be derived partially from expected consumption ... that you're fine because the accumulated content of melamine cyanurate as well as pure melamine during your lifetime will be below their individual toxicity thresholds.