Many grease-resistant fast-food wrappers and boxes contain potentially harmful chemicals that can leach into food, a new study contends.

Testing on more than 400 samples from restaurants nationwide revealed that nearly half of fast-food wrappers and one out of five paperboard food boxes contained detectable levels of fluorine, said lead researcher Laurel Schaider. She’s an environmental chemist at the Silent Spring Institute in Newton, Mass.

Previous studies have linked some fluorinated chemicals such as perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) to kidney and testicular cancer, low birth weight, thyroid disease, decreased sperm quality, pregnancy-induced high blood pressure, and immune system problems in children, the study authors said in background notes.