Researchers from nonprofit organizations and government agencies collaborated to test wrappers from 27 different fast food chains and local restaurants across the country. Their findings, published in the journal Environmental Science and Technology Letters, reveal that 40% of overall samples tested positive for fluorine, which suggests the presence of perfluorinated chemicals (PFCs).

Some types of food packaging were more likely to contain fluorine than others. For example, dessert and bread wrappers contained fluorine 56% of the time, while sandwich and burger wrappers contained the chemical 38% of the time. None of the paper cup samples tested positive for fluorine.

Producers have voluntarily eliminated many fluorinated chemicals in response to growing awareness around the potential threat to human health, but the chemicals have been replaced with similar ones that scientists now believe may have the same health effects.

“Up and down the line there needs to be a greater level of concern,” said Bill Walker of the Environmental Working Group (EWG), one of the groups behind the study, in an emailed press release. “We’re not pointing the finger at the fast food chains or saying it’s safer to eat at one chain compared to another. It’s possible they’re not getting the straight story from their suppliers.”

The EWG called for food companies to evaluate their supply chain more closely and for the Food and Drug Administration (FDA) to change its approach on the issue.

Fast food wrappers are among a slew of products—including car seats, receipts, and canned food—containing toxic chemicals that humans are exposed to on a regular basis.