A new study shows that you might want to think about more than calories, fat, and sodium when you’re grabbing a burger and fries from the drive-thru.

The study shows that fast-food packaging, such as wrappers and boxes, might contain chemicals that have negative health effects.

Researchers from the Silent Spring Institute; the University of California, Berkeley; the University of Notre Dame; and other institutions collected more than 400 samples of packaging from fast-food restaurants across the country—including Chipotle, McDonald’s, and Subway—and found that 33 percent of them contained some form of the chemical fluorine.

“We’ve all heard that eating more fresh foods is better for our health for a wide range of reasons,” says Laurel Schaider, Ph.D., research scientist with the Silent Spring Institute and the lead study author. “This study provides another reason why.”

The good news, however, is that most fast-food packaging did not contain any fluorine, Shaider says. This shows that some manufacturers might be using fluorine compound-free chemicals to get the water- and grease-resistant effects they want without using compounds that carry a health risk, she says.

Not all of the chemicals in the fluorine family are harmful. However, experts are most concerned with a group of fluorinated chemicals called per- and polyfluoroalkyl substances, or PFASs. PFASs are often used to coat food wrappers and containers so that grease and water don’t leak through them.

But they’ve been linked to health problems, such as some cancers, decreased fertility, hypertension in pregnancy, low birth weight, thyroid disease, and a weakened immune system. These chemicals stay in the body (and in the environment) for long periods of time.

Although this study didn’t test food items for PFASs, other studies have shown that these chemicals can leach from the packaging to the food, especially when the food is hot.

When the researchers analyzed a subset of 20 pieces of fast-food packaging, they found PFASs in all of them, and six contained perfluorooctanoic acid (PFOA). PFOA is one of the fluorine compounds that have been most strongly linked to health problems.

Schaider says this study shows that fluorine can be a useful marker for the presence of harmful PFASs.

“The fact that they’re so prevalent is worrisome,” Schaider says. “Millions of Americans, including children, eat fast food every day.”

It’s especially concerning that the researchers found PFOA, says Consumer Reports’ senior scientist Michael Hansen, Ph.D., because in 2011, many manufacturers agreed to voluntarily stop using the chemical in the production of food packaging.

And even when PFOA isn’t used, Hansen says, manufacturers often substitute similar fluorinated chemicals in the PFAS family that aren’t as thoroughly studied as PFOA. Preliminary research suggests that these substitutes also could be linked to some of the same dangerous health effects.

Lynn Dyer, president of the Foodservice Packaging Institute, the trade association for the North American food packaging industry, says that all chemicals used in food packaging “go through rigorous testing to ensure that they meet stringent U.S. Food and Drug Administration regulations, providing the safe delivery of foods and beverages to consumers.”

She also points out that since the study’s authors tested the food packaging in 2014 and 2015, new fluorine compound-free products have been approved by the FDA for waterproofing and greaseproofing.

Reducing Your Exposure

Aside from cutting down on fast food altogether, Hansen says, one thing consumers can do is to limit the amount of time they leave your food in its packaging. If you can, once you arrive home or at the office, take food out of wrappers and use your own plates and bowls instead.

You might also want to consider what type of packaging your food is delivered in. The researchers found that more than half of dessert and bread wrappers and 38 percent of burger and sandwich wrappers they tested in their study contained fluorine. Just 20 percent of paperboard, which might hold French fries, was found to have fluorine.

And if you have leftovers: “You shouldn’t be storing food or reheating it in those packaging materials,” Hansen says.