It’s no secret that fast food isn’t healthy, but could the wrapper that holds your greasy sandwich together also pose a threat to your health?

A new study revealed that some harmful substances, called perfluorinated chemicals (PFCs), were found in fast food wrappings. Typically, these chemicals are use in stain-resistant products, firefighting materials, and even nonstick cookware. The real problem, however, is that these PFCs can leach into your food, and eventually build up to toxic levels in your body, according to a press release from the University of Notre Dame.

Read: Fast Food, Candy, And Soda May Not Be Causing Obesity In America; So What's Really Driving Big Waistlines?

Researchers tested over 400 samples of packaging materials and, according to the press release, found PFCs in 56 percent of dessert and bread wrappers, 38 percent of sandwich and burger wrappers, and 20 percent of paperboard.

Samples were collected from a total of 27 fast food restaurant chains, including McDonald’s, Burger King, Chipotle, Starbucks, Jimmy Johns, Panera and Chick-Fil-A. The release reported that takeout containers — such as Chinese food boxes or pizza boxes — were not tested.

PFCs tend to take a long time to break down in the body, and as a result, their extended stay can cause serious health issues, Medical Daily previously reported. Previous studies have also linked the chemicals to kidney and testicular cancers, thyroid disease, low birth weight and immunotoxicity in children.

Read: 9 Breakfast Foods To Eat For Optimal Weight Loss, And What To Avoid

“This is a really persistent chemical. It gets in the bloodstream, it stays there and accumulates. There are diseases that correlate to it, so we really don’t want this class of chemicals out there,” said researcher Graham Peaslee, in the press release.

Source: Schaider LA, Balan SA, Blum A, Andrews DQ, Strynar MJ, Dickinson ME, et al. Fluorinated Compounds in US Fast Food Packaging. Environmental Science & Technology. 2017.

See Also:

If I Have No Choice But To Eat Fast Food, How Do I Eat Healthier?

Healthy Fast Food Is A Myth: Panera Bread Meals Have More Calories Than McDonald’s And Burger King's