PFAS (per- and polyfluoroalkyl substances) are water-, stain-, and heat-resistant chemicals added to products like nonstick pans, nail polish, food wrappers, and firefighting foam. They have been linked to health problems, including thyroid disease, liver damage, and kidney cancer. Increasingly, PFAS have become a public health concern because they don't break down, instead accumulating in the environment. These so-called "forever chemicals" are now found virtually everywhere, from human blood to the top of Mount Everest. Researchers are urgently trying to understand how PFAS move through the environment to get a better grasp of the extent of the problem and how to control it.