In a statement to the Guardian, the EPA said its latest definition was more “expansive” than the previous. Its approach was designed “to focus on substances most likely to be persistent in the environment, including some chemical substances whose structures or sub-structures resemble, at least in part” more thoroughly studied PFAS compounds, like PFOS and PFOA.

But public health advocates warn that all PFAS are persistent in the environment and all that have been studied are toxic, and for those reasons many are calling on the government to largely restrict the entire chemical class.

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. The employee said they first learned of the latest change in public comments made by Michal Freedhoff, a Biden-appointed toxics office administrator.