Pennsylvania officials are telling anglers not to eat fish caught along the Neshaminy Creek basin in Bucks and Montgomery Counties due to “extremely high levels” of a PFOS, a so-called forever chemical that’s also contaminated the drinking water of local residents living near military bases.

The Neshaminy Creek, which runs though Tyler and Neshaminy State Parks, is the largest length of waterway in the state to carry a do-not-eat advisory. Though the 40-mile-long creek lies in Bucks County, its watershed spills into Montgomery County.

The joint announcement by the state’s Departments of Agriculture, Environmental Protection, and Health, as well as the Fish and Boat Commission extends to all species of fish.

“An advisory like this is not something that we recommend lightly,” DEP Secretary Patrick McDonnell said in a statement. “The possible PFOS levels found in fish tissue is greatly concerning. DEP will continue to sample fish species and revisit this recommendation in the future.”

While fishing is still allowed, officials urged anglers to practice catch-and-release. All stocking of the waterway has been stopped. Tim Schaeffer, Fish and Boat Commission executive director, said his agency will stock fish in nearby uncontaminated waters in time for opening day of trout season in the spring.

The state’s Fish Consumption Advisory Program tests sport-caught fish for PCB, pesticide, or heavy metal contamination. In 2019, PFOS was added to the list.

Neil Shader, a spokesperson for the DEP, said the state recently reexamined tissue samples collected prior to 2019. It found a concerning level of 0.2 parts per million of PFOS.

“Because PFOS is bio-accumulative,” Shader said, “there is reason to believe that it would be present in fish in the creek today, and we want to take every precaution to protect people that may be eating fish from the watershed.”

Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are human-made chemicals that are resistant to heat, water and oil, and persist in the environment and the human body. Perfluorooctane sulfonic acid (PFOS) is a member of that family of compounds. Because the compounds do not break down, they are often referred to as forever chemicals.

PFOS are known as emerging contaminants because of their potential risks to human health, which aren’t completely understood. Some studies suggest PFOS can affect cholesterol and uric acid levels and may be associated with an increased risk of high blood pressure. Exposure may also cause liver damage.

They have been used for decades to make cookware, carpets, clothing, and fabrics because they resist water, grease, or stains. They are also used in firefighting foams.

The drinking water of residents living near the Naval Air Warfare Center in Warminster and Naval Air Station in Willow Grove was tainted by the “forever chemicals” that leached into the water basin from firefighting foam.

An Inquirer analysis earlier this year found that PFAS “forever chemicals” were detected in 33 of 46 public water locations in Philadelphia’s suburban counties, or 72% of samples, although none exceeded totals of federally suggested limits of 70 parts per trillion, though some scientists and advocates contend there are no safe levels.

» READ MORE: PFAS found in 72% of drinking-water samples in Philly’s suburbs

In The Inquirer analysis, the data indicated most PFAS samples were found in Bucks and Montgomery Counties, most likely because of their proximity to the military bases.

Shader said the state is looking for possible sources of contamination other than the bases.