The US Environmental Protection Agency (EPA) office responsible for protecting the public from toxic substances has changed how it defines PFAS for a second time since 2021, a move critics say they fear will exclude thousands of "forever chemicals" from regulation and largely benefit industry. Instead of using a clear definition of what constitutes a PFAS, the agency's Office of Pollution Prevention and Toxics plans to take a "case-by-case" approach that allows it to be more flexible in determining which chemicals should be subjected to regulations. Among other uses for the compounds, the EPA appears to be excluding some chemicals in pharmaceuticals and pesticides that are generally defined as PFAS, current and former EPA officials say, and the shift comes amid fierce industry opposition to proposed limits on the chemicals. PFAS, or per- and polyfluoroalkyl substances, are a class of about 15,000 compounds most frequently used to make products water-, stain- and grease-resistant. They have been linked to cancer, birth defects, decreased immunity, high cholesterol, kidney disease and a range of other serious health problems. They are dubbed "forever chemicals" because they do not naturally break down in the environment. Speaking on condition of anonymity for fear of reprisal, a current EPA employee in the toxics office said the chemical's definition has been evolving for several years. "EPA can't get its act together on what PFAS are," they added.