A wave of new patients is registering for care at local clinics that are part of the World Trade Center Health Program and doctors said they tend to be sicker, signaling more serious health impacts to come from the Sept. 11 terrorist attacks.

Some advocates and experts said the number of people who die from 9/11 illnesses eventually will surpass the 2,977 killed in the attacks.

As of last month, 1,140 members of the WTC Health Program had died, although it is not known whether all of those were from a 9/11 health condition. The documented cases of cancer linked to the chemical and dust exposure caused by the fallen towers have tripled in the past two years to 5,441.

Many more cancer cases are expected, as are diseases of the immune system and nervous system that take time to present, medical researchers said.

By comparison, 2,753 people died in the World Trade Center attacks 15 years ago, with 224 more killed in the jet crash at the Pentagon and the downing of United Airlines Flight 93 in Shanksville, Pennsylvania.

“They told us back then in 2001 that if there were going to be problems, it was going to happen 10 to 15 years after the fact,” said health program participant Dennis Nourry, a retired construction foreman who worked 12-hour night shifts, seven days a week, for nearly nine months searching for remains in the pile at Ground Zero. “Now it is happening. These are the lingering effects of 9/11.”

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More than 2,500 new people enrolled in the WTC health program during the 12 months ending June 30, bringing the total being monitored for 9/11 illnesses nationally to nearly 75,000, according to data by the Centers for Disease Control and Prevention.

The vast majority — more than 56,580 — includes emergency responders, recovery and cleanup workers, and volunteers who spent days, weeks and months in air filled with toxic dust from pulverized building debris.

Thousands more who came to the aid from states across the country also are eligible for care, as well as New Yorkers who may have moved away since the attacks and those from the lower Manhattan neighborhood near the trade center.

A network of doctors paid by the WTC health program screen and treat 9,507 people who live outside of the greater metropolitan area and see a network of doctors who participate in the WTC health program. Another 8,881 are people who worked and lived near Ground Zero.

The program, a key component of the James Zadroga 9/11 Health and Compensation Act of 2010, provides health care, medical monitoring and treatment to the people directly affected by 9/11. Those seeking to join must complete paperwork with the CDC before they are evaluated for enrollment in the program.

The medical screenings and treatment are delivered via five main providers: Icahn School of Medicine at The Mount Sinai Hospital, the New York University School of Medicine, Northwell Health system, Stony Brook University Hospital and the Robert Wood Johnson Medical School at Rutgers University.

On Long Island, the main Stony Brook University WTC Wellness Program is in Islandia. Winthrop-University Hospital in Mineola runs the Nassau County Satellite of the World Trade Center Medical Monitoring and Treatment Program, a subcontracted clinic for Stony Brook University.

“What seems to be so surprising is that, even though it’s been 15 years, we continue to have new enrollees into the program,” said Dr. Benjamin Luft, director of Stony Brook’s clinic. “We feel that the new people who are enrolling tend to be sicker than the old enrollees.”

The new enrollments, doctors and advocates said, give credence to last year’s fight for reauthorization of the Zadroga Act. The bill, signed by President Barack Obama in December 2015, funds the health program until 2090, ensuring uninterrupted medical care and monitoring of the responders and other survivors of the attacks.

Doctors at the Stony Brook-run center have seen 400 new patients in the past nine months in the clinic in Islandia.

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The clinic, which includes a reception area, exam rooms and cubicles, is expanding so rapidly that it is outgrowing its space in an office building off Motor Parkway.

Luft points to two main reasons for the rise in enrollment.

The first is that many 9/11 responders, putting off their registration in the program, now are approaching retirement age with less structure and more time to focus on monitoring their health.

The second reason is that their symptoms may have become more difficult to manage and they are experiencing more discomfort earlier in life than those of the general aging population.

“They have been putting 9/11 in a box for years and now, all of a sudden, it becomes overwhelming,” Luft said.

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Doctors said the latency period — time between initial exposure to a toxin and the time a cancer is diagnosed — for many serious 9/11 conditions is about 15 to 20 years.

“So 15 years later, we are only at the very beginning of the curve for things like lung and asbestos-related cancers. Thankfully, the health program will outlive all of us,” said Dr. Jacqueline Moline, director of the Queens World Trade Center Health Program in Rego Park, part of the Northwell Health system.

“As the memory of that day fades away for so many people, it was the defining moment for our generation,” she said. “These health programs allow us never to forget 9/11.”

John Feal, president of the FealGood Foundation, a Long Island-based 9/11 first-responders advocacy group, said there is more work to be done on the Zadroga legislation, such as getting more conditions added to the list of those treated under the health program.

“The weight of the world was on those men’s and women’s shoulders who worked on the pile,” said Feal, 49, who lost half of his left foot digging in the rescue and recovery effort days after the attacks. “We wake up in the morning and we are reminded by our injuries. It’s easy to forget that there are still tens of thousands of people sick and dying because of that day.”

Retired NYPD detective Kenny Anderson, 46, of Levittown, said it wasn’t until he retired and the cost of his medication rose that he decided to make full use of the program.

“When you retire, it is at the point when you are at your sickest,” said Anderson, a first responder later diagnosed with a list of medical problems, including lung disease and nerve damage. He was at Ground Zero on Sept. 11 and watched as the north tower fell.

Dr. Marc Wilkenfeld, chief of Winthrop’s Division of Occupational and Environmental Medicine, said about 150 new patients have come to the clinic at the Mineola hospital over the past nine months.

Wilkenfeld said he sees a mix of new patients and longtime participants with new ailments such as nerve damage or neuropathy. The retired construction foreman, Nourry, 71, of Little Neck, is one of Wilkenfeld’s new patients — diagnosed with an incurable autoimmune condition after first noticing symptoms, including loss of balance, just a couple of years ago.

“We are seeing too much of those conditions now in this population and believe that it must be linked,” Wilkenfeld said. “The thing is that people are coming in with conditions that are not yet covered.”

Some people, confident that the screening program has become more streamlined, are motivated to enroll after seeing their peers suffering with a possible 9/11-related cancer, he said.

“I don’t ask ‘Why did you wait so long?’ It doesn’t really matter. I just want to get them into the program and get them the care they need,” Wilkenfeld said.

In addition to new patients, longtime enrollees are returning with new symptoms, concerns and sometimes side effects from medication.

On a recent afternoon, Dorothy Curran sat on an examination table at the Islandia clinic, explaining how attending a simple summer barbecue a month ago triggered a serious cardiorespiratory event.

Curran, 65, of Manorville, worked across the street from Ground Zero as a data manager at Barclays Bank. For nine months after the attack she reported to work in an office that overlooked the pile.

In 2003, she came down with a cold that wouldn’t go away and eventually turned into bronchial pneumonia. She spent a while addressing health problems, including tuberculosis and two cancers, but for the last few years she had been without any ailments, she said.

“I went to a barbecue and there were about 15 people smoking,” she said. “I thought my heart was going to pop out of my chest.”

In an exam room down the hall, Kristin Gelzinis, 36, a former nursing student and emergency medical technician with Medford Volunteer Ambulance, said she stays in the program because “they’re fantastic with taking care of us.”

Gelzinis, of Lake Grove, who volunteered on search and recovery efforts at Ground Zero, said she takes several medications for respiratory problems and needs to be monitored. The medications have taken a toll on her bone density and she still suffers from post-traumatic stress disorder and anxiety, she said.

“I am absolutely terrified of getting cancer. That is one of my biggest, biggest fears, especially because I have a 5-year-old,” she said. “I don’t want to leave him.”