Thirty-eight years after DDT was banned, Americans still consume trace amounts of the infamous insecticide every day, along with more than 20 other banned chemicals.

In a photograph from a 1947 newspaper advertisement, a smiling mother leans over her baby's crib. The wall behind her is decorated with rows of flowers and Disney characters. Above the photo, a headline reads "Protect Your Children From Disease Carrying Insects."

The ad, for wallpaper impregnated with DDT, captures a moment of historical ignorance, before the infamous insecticide nearly wiped out many birds and turned up inside the bodies of virtually everyone on Earth.

The story of DDT teaches a lesson about the past. But experts say it also provides a glimpse into the future.

Thirty-eight years after it was banned, Americans still consume traces of DDT and its metabolites every day, along with more than 20 other banned chemicals. Residues of these legacy contaminants are ubiquitous in U.S. food, particularly dairy products, meat and fish.

Their decades-long presence in the food supply underscores the dangers of a new and widely used generation of chemicals with similar properties and health risks. "They're manmade, and they're toxic, and they bio-accumulate," said Arnold Schecter, a professor at the University of Texas School of Public Health who has been studying human exposure to chemicals for more than 25 years. "So the fact that they're still around a long time after they've been banned isn't surprising."

Recent studies sketch a complex profile of legacy contaminants in U.S. food - a profusion of chemicals in trace amounts, pervasive but uneven across the food supply, occurring sometimes by themselves, but more often in combination with others. Included are DDT and several lesser-known organochlorine pesticides as well as industrial chemicals such as polychlorinated biphenyls, or PCBs, which were used until the late 1970s in electrical equipment.

This picture raises a host of equally complicated questions: Are small amounts of these chemicals dangerous, by themselves or in mixtures? Why are they still around and how are they getting into our food?

Think of these chemicals like sand in your shoes after a trip to the beach. Despite our efforts to rid ourselves of it, we discover more later - sometimes that evening, sometimes years later - when we put on the same pair of summer shoes and feel the grains between our toes.

Like those grains of sand, many chemicals stick around. They belong to a class called "persistent organic pollutants" or POPs - which take decades to break down in sediment and soil and can travel globally on wind and water, ending up in regions as remote as the Arctic. These migratory POPs, when ingested, take up semi-permanent residence in the fat tissue of living organisms. In animals, and sometimes in humans, many of them can raise the risk of cancer or other diseases, alter hormones, reduce fertility or disrupt brain development.

The good news is that DDT and other organochlorine pesticides, PCBs and industrial byproducts called dioxins have declined significantly in food and the environment since they were banned or restricted decades ago. A few have dipped below detectable levels. "We don't expect the levels in food or people to go down abruptly, we expect them to go down over time. And that's what we're seeing," Schecter said.

Precise trends of chemicals in food are hard to identify because both government and independent studies have focused on different foods in different places at different times. However, levels in human breast milk indicate that, by 1990, DDT had dropped to one-tenth of 1970 levels, according to a 1999 report in the International Journal of Epidemiology. Similar trends exist for PCBs and dioxins. In most places, POPs are a mere fraction of what they were.

Last year, as part of an ongoing study of POPs in the food supply, Schecter and his colleagues collected and analyzed more than 300 samples from supermarkets around Dallas, Texas. The samples were combined into 31 food types, such as yogurt, chicken and peanut butter, and tested for old contaminants as well as newer ones. "Every food within this study contained multiple pesticides," the authors wrote in a paper published in February in the journal Environmental Health Perspectives. The DDT metabolite DDE was the most prevalent, occurring in 23 of the 31 foods sampled.

People consume more DDT than any other persistent organic pollutant, the researchers found. Its relative abundance in food today is due to its widespread historical use. In the United States alone, an estimated 1.35 billion pounds were sprayed to wipe out mosquitoes and agricultural pests over a period of about 30 years, according to the Environmental Protection Agency.

Other banned pesticides that have lingered in food for decades include dieldrin, toxaphene, chlordane, hexachlorocyclohexane and hexachlorobenzene. Although lesser known, they pose risks similar to the infamous DDT and PCBs.

Salmon was the most contaminated food, with traces of six types of PCBs, two flame retardants and 25 pesticides, including DDT, dieldrin and toxaphene. Samples of canned sardines and catfish also contained numerous banned chemicals.

In general, the higher the food's fat content, the more chemicals it contained. Peanut butter, ice cream, cheese, butter, oil, fish and high-fat meats were all more contaminated than low-fat milk and vegetables.

This high-fat, high-chemical connection is no coincidence. POPs are lipophilic, or "fat-loving" chemicals - they take up residence in the fat of animals and take many years to break down. This fat moves through food chains, and the contaminants become progressively more concentrated in a process called bioaccumulation. In Lake Michigan, for example, PCBs cling to inorganic sediments and are then absorbed by microscopic, free-floating plankton. A mollusk feeds on the plankton by filtering water through its digestive tract, and over its short life pollutants build up in the mollusk's fat tissue. A small perch eats hundreds of mollusks before a larger predator, the lake trout, eats the perch - and all the chemicals it carries. Then an eagle or a fisherman comes along and eats the trout.

This effect is well-documented and helps explain why declining populations of birds of prey are often the first sign of pollution that may threaten people's health. Birds have relatively small bodies compared to people, and if they feed exclusively on contaminated fish, their body burden quickly can reach toxic levels that can kill chicks, destroy eggs or cause deformities.

Farmed fish are even more contaminated. A 2004 study found that farmed salmon contained 10 times higher levels of POPs than wild salmon. The source of the pollutants, said Dr. David Carpenter, director of the Institute for Environmental Health at the University of Albany, New York, and one of the paper's authors, is the contaminated mixture of fish fats and proteins in fishmeal.

The same problem applies to meat and dairy products, said Carpenter. A 2003 report, published by National Academies Press, noted that feed containing animal fat was a major source of people's continued exposure to dioxins, which are carcinogenic.

"We recycle waste animal fats back into the food supply," Carpenter explained. "We feed the cow fat to the pigs and the chickens, and we feed the pig and chicken fat to the cows." These waste animal products make up the vast majority of animal feed.

Scientists are uncertain about the human health risks posed by trace levels of most POPs. Some, such as dioxins, carry risks even at very low levels. According to animal studies, as well as some human studies, exposure to these chemicals might raise the risk of cancer or other diseases, reduce fertility, alter hormones, damage brain development and interfere with immune function.

For some chemicals, the EPA has set reference doses-amounts deemed safe for daily consumption, based mostly on animal tests. None of the foods in Schecter's 2009 study contained concentrations over these guidelines. Americans' daily dietary intake of DDT, for example, is about half the reference dose. But health officials don't know enough about the effects of many of the chemicals to set reference doses, said Schecter. And they know even less about what happens when people are exposed to multiple contaminants.

"Where reference doses do exist, they're for individual chemicals. And we have no reference doses for a combination of several chemicals," he said. Determining "safe" doses for combinations of chemicals would be difficult and expensive, and so far few studies have been conducted.

Dr. Alex Stewart, a public health doctor in the United Kingdom who published a paper on chemical mixtures in 2009, said pollutants may be more harmful when combined in our foods and bodies than they are individually. Some chemicals, such as dioxins, are likely to add up to cause effects because they affect the same body systems in teh same ways, he said.

The combined potential of some substances has been well-documented. "Everyone knows that smoking causes lung cancer, and if you breathe radon, that causes lung cancer. Both smoking and radon cause lung cancer, but if you are exposed to both, your risk is greater," Carpenter said. While long-banned contaminants carry on their legacy, a younger generation of pollutants is joining them.

One of these is a group of flame retardants called polybrominated diphenyl ethers or PBDEs. PBDEs have been used in a variety of consumer goods since the 1970s. In the decades since, human exposure has risen sharply, and levels in human breast milk and blood are orders of magnitude higher in North America than in other parts of the world.

PBDEs in food are highly variable. One sample of beef, for example, might have twice as much as another. But despite this variability, the average American diet includes some daily consumption of PBDEs, primarily from eating dairy and meat, according to Schecter's study.

Animal studies have linked PBDEs to compromised thyroid and liver function and impaired brain development. Concern over their health effects prompted a U.S. ban of several PBDEs in 2004. Last year, they were added to the Stockholm Convention's list of targeted pollutants.

The Stockholm Convention - an international effort of nearly 100 countries to eliminate or restrict POP production and use - was signed in 1991. The original list, called the "dirty dozen," included several pesticides and PCBs still present in food. The list was expanded in 2009 to include nine new chemicals, many of which showed up in Schecter's food study.

The study also identified trace amounts of perfluorooctanoic acid, or PFOA, a chemical that had not previously been identified in the food supply. Perfluorinated compounds are commonly used for manufacturing grease repellants and water-repellant consumer goods, including Teflon. Some of these compounds have been banned or industry has reduced their use. But because they are still in consumer goods, people are exposed through both dust and diet.

It isn't clear how perfluorinated compounds are getting into food, said Dr. Tom Webster, an epidemiologist at the Boston School of Public Health. It's possible that they are bioaccumulating like other pollutants, but they may also be leaching directly into foods during the packaging process, he said. Because consumer goods containing these chemicals will eventually be sent to landfills, they are "an indoor problem that's going to increasingly become an outdoor problem," said Dr. Mike McClean, an epidemiologist also at Boston's School of Public Health. Rain leaches contaminants from landfills to groundwater, where they travel through the environment and eventually enter food chains.

Perfluorinated compounds break down faster than other pollutants, but flame retardants behave more like other POPs by building up in fat tissue and taking many years to break down.

"We expect they're going to be in the environment for many decades to come," Schecter said.

Schecter also said that, in light of the large number of pollutants in the U.S. food supply, more government testing, for a broader range of contaminants, is called for. "This would be a major effort, but since they're still around, it seems reasonable to have more inspection," he said.

And people should try to eat less animal fat to avoid consuming high levels of POPs, Schecter said. Carpenter put the situation more bluntly. "We've got to get these chemicals out of our food," he said.