About 200 residents plan to file a lawsuit against Suffolk County and the state, alleging they were exposed to a hazardous chemical used for decades by firefighters at Gabreski Air National Guard Base in Westhampton Beach, their attorneys said Monday.

New York State added the air base to its Superfund list in September after sampling of water supplies detected perfluorooctanesulfonic acid, or PFOS, which potentially can cause blood, immune system, thyroid and fetal growth issues. The chemical was part of a liquid foam that firefighters used at the air base to practice putting out airplane fires.

Marie Napoli, an attorney with offices in Melville and Manhattan, said her firm has filed a notice of claim against Suffolk County and the state Department of Environmental Conservation on behalf of some 200 residents who say they have been affected by the chemical.

Attorneys have not determined an exact amount of damages they will seek, but it will be in the tens of millions of dollars, Napoli said.

Suffolk County officials said they could not comment because of the pending litigation. DEC officials did not comment.

Al Green, 44, a longtime resident who grew up in the area, said he and two of his brothers have a rare disease, sarcoidosis, that they believe was caused by the chemical. His parents were named in the notice of claim.

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Green, who has two other brothers, said Monday he was “upset, knowing that all these years we’ve been poisoned.”

The DEC last year said elevated levels of the fluorinated organic chemical first were detected in public-supply wells south of Gabreski airport in 2014.

It was found in private wells last year, and in July county workers went door-to-door south of the airport, offering residents whose homes rely on private wells bottled water for cooking and drinking, and offering free testing for the presence of PFOS.

The Suffolk County health commissioner said last summer that foam containing PFOS had been used at the air base since the 1970s; it was phased out for training purposes in 2014.

Another longtime resident, Jerome Liggon, 58, said Monday he recalled the foam that firefighters routinely used at the air base.

“When you’re a kid, that’s cool,” he said. “But you don’t know the effect that’s going to have on your neighborhood later on.”

When he was a boy, Liggon said, “everyone lived to go to the air shows” that were put on at the base.

“You could step out on my block and see the smoke plume from training,” he said. “Anytime we saw that smoke plume, we knew we were going to see the foam right behind it.”

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Napoli and another attorney from her firm, Patrick Lanciotti, said the problem is not limited to Westhampton Beach. Air bases around the country used the same foam for years in drills, they said.

“This is something in our backyard, but it is an ongoing problem nationally,” Napoli said. “Westhampton was not the only ones doing these drills. We have a big problem, the extent of which we don’t know yet.”

Two years ago, she said, authorities were not even testing for PFOS and now they are discovering traces of it across the country. Her law firm is pursuing lawsuits at other locations, including Newburgh in Orange County.

The notice of claim seeks compensation for the residents because of “the negligence, recklessness and carelessness” of Suffolk County and state officials.

It notes that Suffolk County issued a news release on July 22 stating that PFOS was detected in public and private water supply wells near Gabreski. The Suffolk County Water Authority tested the water in 2015 and 2016.

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Suffolk County has owned the air base since the end of World War II.

The DEC previously said it will pay for Suffolk County to connect 66 homes with private wells in the Westhampton Beach area to municipal drinking water supplies.

With Emily C. Dooley