WASHINGTON (AP) – The Environmental Protection Agency is warning that two nonstick and stain-resistant compounds found in drinking water are more dangerous than previously thought and pose health risks even at levels so low they cannot currently be detected.

The two compounds, known as PFOA and PFOS, have been voluntarily phased out by U.S. manufacturers, but there are a limited number of ongoing uses and the chemicals remain in the environment because they do not degrade over time. The compounds are part of a larger cluster of “forever chemicals” known as PFAS that have been used in consumer products and industry since the 1940s.

PFAS is short for per- and polyfluoroalkyl substances, which are used in nonstick frying pans, water-repellent sports gear, stain-resistant rugs, firefighting foam, cosmetics, and countless other consumer products. The chemical bonds are so strong that they do not degrade or do so only slowly in the environment and remain in a person’s bloodstream indefinitely.

The EPA on Wednesday issued nonbinding health advisories that set health risk thresholds for PFOA and PFOS to near zero, replacing 2016 guidelines that had set them at 70 parts per trillion.

The revised health guidelines are based on new science and consider lifetime exposure to the chemicals, the EPA said. Officials are no longer confident that PFAS levels allowed under the 2016 guidelines “do not have adverse health impacts,” an EPA spokesman said.

While the new guidelines set acceptable risk below levels that can currently be measured, as a practical matter the EPA recommends that utilities take action against the chemicals when they reach levels that can be measured—currently about four parts per trillion, a senior administration official told reporters Tuesday night.

The EPA said it expects to propose national drinking water regulations for PFOA and PFOS later this year, with a final rule expected in 2023.