Sharon Lerner has been reporting on “forever chemicals” for the better part of a decade. These manmade compounds — known as PFAS for short — resist oil, water and heat, take an incredibly long time to break down in nature, and have been used widely in products like Scotchgard, Teflon and firefighting foam. Lerner has focused part of her work on understanding the flow of information inside manufacturers like 3M. By the 1970s, Lerner says, Minnesota-based 3M had established that they were toxic in animals and were accumulating in humans’ bodies. But who inside 3M knew? And what did they know? Reporting for ProPublica, a nonprofit investigative news site, Lerner got a complicated answer after coming across a former 3M scientist named Kris Hansen. Marketplace’s Lily Jamali spoke with Lerner about her recent investigation.