When you grab a burger, a cookie, or a cup of coffee from a local fast food establishment, you know that it’s not necessarily the healthiest nutritional choice, but you don’t assume that the companies behind the meals use packaging that may leach harmful chemicals into your food. Yet a recent study of fast food wrappers shows that a surprising number contain a potentially harmful chemical.

These substances are usually referred to as perfluroinated chemicals (PFCs), though the scientists behind the food wrapper study called them per- and polyfluoroalkyl substances (PFASs). They gathered wrappers from fast food and quick serve restaurants in Boston, San Francisco, Seattle, Washington, DC, and Grand Rapids, MI, using spectroscopy to measure levels of fluorine, a substance that signals that PFCs are present.

Why would you want to avoid food wrappers that use these chemicals? According to the Environmental Working Group, PFC exposure has been linked to higher levels of kidney and testicular cancer, high blood cholesterol levels, obesity, thyroid problems, and pregnancy complications.

“There is very little public information on how much leaching occurs, as there are lots of different types of coatings made with this family of chemicals,” David Andrews, Ph.D., a scientist with EWG and one of the wrapper study’s co-authors, said in a statement. “Our tests show they are not necessary, because there are PFC-free food wrappers readily available.” You know, like the other 66% of packaging items tested that didn’t have detectable levels of fluorine.

The results showed that 33% of all packaging items tested positive for fluorine. No paper cups had fluorine, but items like wrappers for items like sandwiches or burgers (38%) or dessert wrappers (57%) had much higher levels.

The percentage also varied by fast food brand, with some chains’ packaging showing no detectable fluorine. One doesn’t normally think of Carl’s Jr., Checkers, Culver’s, Domino’s, or Round Table Pizza as shining examples of quick-serve healthfulness, but there you have it.

What did the brands have to say? The researchers contacted the chains, only two of which really responded. The study doesn’t specify which brands were the ones to respond.

“One stated that they believed none of their food packaging contained fluorinated chemicals, and the other stated that they verified with their suppliers that their food packaging did not contain PFASs,” the researchers explain, even though both of these companies had PFCs in their food packaging.

Keep in mind that these samples were collected in 2014 and 2015, and fast food chains have become much more conscious of “clean” food products since then. Has that extended to their packaging supply chain? That’s possible, but there are also ways to grease-proof packaging other than through the use of PFCs.

Among the EWG’s other recommendations for avoiding PFCs include avoiding waterproof and stain-resistant clothing, using cast iron or stainless steel cookware instead of pans with nonstick coating, and cooking with those metal pans and utensils at home as much as possible rather than eating fast food.

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