Given the pervasiveness of melamine, it’s always possible that trace elements will end up in food. The F.D.A. thus sets the legal limit for melamine in food at 2.5 parts per million. This amount is indeed minuscule, a couple of sand grains in an expanse of desert that pose no real threat to public health. Moreover, the 2.5 p.p.m. figure is calculated for a person weighing 132 pounds — a cautious benchmark given that the average adult weighs 150 to 180 pounds.

But these figures obscure more than they reveal. First, while adults eat about one-fortieth of their weight every day, toddlers consume closer to one-tenth. Although scientists haven’t measured the differential impact of melamine on infants versus adults, it’s likely that this intensified ratio would at least double (if not quadruple) the impact of legal levels of melamine on toddlers.

This doubled exposure might not land a child in the hospital, but it could certainly contribute to the long-term kidney and liver problems that we know are caused by chronic exposure to melamine.

On a more concrete note, melamine not only has widespread industrial applications, but is also used to buttress the foundation of American agriculture.

Fertilizer companies commonly add melamine to their products because it helps control the rate at which nitrogen seeps into soil, thereby allowing the farmer to get more nutrient bang for the fertilizer buck. But the government doesn’t regulate how much melamine is applied to the soil. This melamine accumulates as salt crystals in the ground, tainting the soil through which American food sucks up American nutrients.

A related area of agricultural concern is animal feed. Chinese eggs seized last month in Hong Kong, for instance, contained elevated levels of melamine because of the melamine-laden wheat gluten used in the feed for the chickens that produced the eggs.

To think American consumers are immune to this unscrupulous behavior is to ignore the Byzantine reality of the global gluten trade. Tracking the flow of wheat gluten around the world, much less evaluating its quality, is like trying to contain a drop of dye in a churning whirlpool.

More ominous, the United States imports most of its wheat gluten. Last year, for instance, the F.D.A. reported that millions of Americans had eaten chicken fattened on feed with melamine-tainted gluten imported from China. Around the same time, Tyson Foods slaughtered and processed hogs that had eaten melamine-contaminated feed. The government decided not to recall the meat.

Only a week earlier, however, the F.D.A. had announced that thousands of cats and dogs had died from melamine-laden pet food. This high-profile pet scandal did not prove to be a spur to reform so much as a red herring. Our attention was diverted to Fido and away from the animals we happen to kill and eat rather than spoil.

Frightening as this all sounds, the concerned consumer is not completely helpless. We can seek out organic foods, which are grown with fertilizer without melamine — unless that fertilizer was composted with manure from animals fed melamine-laden feed (always possible, as the Tyson example suggests).

We could further protect ourselves by choosing meat from grass-fed or truly free-range animals, assuming the grass was not fertilized with a conventional product (something that’s also very hard to know).

But as all the caveats above indicate, these precautions will only go so far. Melamine, after all, points to the much larger relationship between industrial waste and American food production. Regulations might be lax when it comes to animal feed and fertilizer in China, but take a closer look at similar regulations in the United States and it becomes clear that they’re vague enough to allow industries to “recycle” much of their waste into fertilizer and other products that form the basis of our domestic food supply.

As a result, toxic chemicals routinely enter our agricultural system through the back channels of this under-explored but insidious relationship.

So, sure, let’s keep the heat on China. And, yes, let’s take with a big dose of skepticism the Chinese government’s assurances that they’re improving the food supply.

At the same time, though, instead of delivering righteous condemnation, the United States should seize upon the melamine scandal as an opportunity to pass federal fertilizer standards backed by consistent testing for this compound, which could very well be hidden in plain sight.

James E. McWilliams, a history professor at Texas State University at San Marcos, is the author of “American Pests: The Losing War on Insects From Colonial Times to DDT.