When you make the conscious decision to eat fast food, you know you're choosing to eat food with ingredients you normally wouldn't want to consume. You may not realize, though, that the packaging may be adding unwanted things to your food, too.

In order to keep the water, oil and other liquids that can soak through food packaging at bay, synthetic chemicals that resist heat and grease are added to the wrappers and boxes. But these chemicals, known as per- and polyfluoroalkyl substances (PFASs), can leak into food, a new study shows. And they have been found in almost every American who has been tested for them, as well as in animals like polar bears, which should never get near fast food or its packaging, a previous study showed.

Citing these studies and more, Senator Chuck Schumer, the Senate minority leader, recently issued a letter to the Food and Drug Administration (FDA) asking the agency to study the consequences of using phthalates in food packaging. “To think that we have all this data on phthalate chemicals from doctors, scientists, health experts and other industries just sitting around, frozen like a beef patty and begging for the FDA to take it to the next appropriate level of scrutiny is worrisome for the consumer,” Schumer said in a statement.

These PFASs, which are the same chemicals used in nonstick cookware, are believed to cause a variety of health issues including thyroid disease, delayed puberty and infertility. Studies have shown that the PFASs in food packaging are much more likely to leach into food when the food is hot or oily, just like the the chemicals on the nonstick cookware can leach into food when it is hot or has oil in it.

For the new study, researchers collected more than 400 samples of paper and cardboard food wrappers and packaging at U.S. fast-food restaurants. Researchers made sure the samples never touched so PFASs in one wrapper could not rub off onto another. When tested, they found that 46 percent of the "food contact papers" and 20 percent of the "paperboard samples" contained PFASs. The packaging most likely to contain the synthetic chemicals were from Tex-Mex food and dessert and bread wrappers, says Eureka Alert. (Although not part of this study, PFASs are also very prevalent in microwave popcorn bags.)

These chemicals were supposed to be phased out of American food packaging by 2015; but in China, where much of the food packaging comes from, it's still allowed, reports The Verge. The chemicals they are being replaced with — many of which were detected during the study — "have not been shown to be safe for human health," according to Arlene Blum, founder of the Green Science Policy Institute and co-author of the study.

Why kids are more at risk

family-fast-food Kids are particularly at risk when the chemicals in fast-food packaging leach into food. (Photo: Kanashkin Evgeniy/Shutterstock)

Children's bodies in particular are susceptible to the PFASs found in fast-food packaging, microwave popcorn, nonstick cookware and more. "Children are especially at risk for health effects because their developing bodies are more vulnerable to toxic chemicals," says Laurel Schaider, an environmental chemist at Silent Spring Institute and the study's lead author. A 2015 study found that more than one-third of kids and teens eat fast food every day.

In addition to being bad for human health, these chemicals are bad for the environment. They end up in landfills and may even end up in compost piles because they are allowed in compostable containers.

If you really needed another reason to stop eating fast food or at least make it a rare occurrence, toxic packaging could be just the reason you've been looking for.

Editor's note: This story has been updated since it was originally published in February 2017.

Robin Shreeves ( @rshreeves ) focuses on food from a family perspective from her home base in New Jersey.