The Environmental Protection Agency stunned scientists and local officials across the country on Wednesday by releasing new health advisories for toxic "forever chemicals" known to be in thousands of U.S. drinking water systems, impacting potentially millions of people.

The new advisories cut the safe level of chemical PFOA by more than 17,000 times what the agency had previously said was protective of public health, to now just four "parts per quadrillion." The safe level of a sister chemical, PFOS, was reduced by a factor of 3,500. The chemicals are part of a class of chemicals called per- and polyfluoroalkyl substances (PFAS), also known as forever chemicals due to their extreme resistance to disintegration. They have been linked to different types of cancer, low birthweights, thyroid disease and other health ailments.