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Two decades after , people are still coming forward to report illnesses that might be related to dust that billowed over the city after the terror attack.

To date, ��has spent $11.7 billion on care and compensation for those exposed to potential toxins in the dust and fires that burned at for weeks after the attacks. More than 40,000 people have gotten payments from a government fund for people with illnesses potentially linked to the attacks.

More than 111,000 people have signed up for the World Trade Center Health Program, which gives free medical care to people with medical problems potentially linked to the dust.

Enrollees include people like Barbara Burnette, a retired police detective still suffering from severe respiratory problems two decades after she spat soot from her mouth for weeks as she worked on the burning rubble pile without a protective mask. She credits the intensive health monitoring she got through the health program with helping to spot lung cancer.

“Had I not been in the program... I don’t know that they would have found it,” Burnette says. Since then she has had two rounds of chemotherapy to keep the cancer at bay.

Scientists still can’t say for certain how many people developed health problems as a result of exposure to the tons of pulverized concrete, glass, asbestos, gypsum released when the towers fell.

Many people enrolled in the health program have conditions common in the general public, like skin cancer, acid reflux or sleep apnea. In most situations, there is no test that can tell whether someone’s illness is related to the Trade Center dust, or a result of other factors, like smoking, genetics or obesity.

Over the years, that has led to some friction between patients who are absolutely sure they have an illness , and doctors who have doubts.

Mariama James, who had to clean up copious amounts of the gray powder that billowed through the open windows of her Manhattan apartment, says she initially had a hard time persuading doctors that the chronic ear infections, sinus issues and asthma afflicting her children, or her own shortness of breath, had anything to do with 9/11.

“Most people thought I was crazy back then,” she says.

Years of research have produced partial answers about 9/11 health problems like hers. The largest number of people enrolled in the federal health program suffer from chronic inflammation of their sinus or nasal cavities or from reflux disease, a condition that can cause symptoms including heartburn, sore throat and a chronic cough.

Post-traumatic stress disorder has emerged as one of the most common, persistent health conditions. Nearly 19,000 enrollees have a mental health problem believed to be linked to the attacks, according to health program statistics. More than 4,000 patients have some type of chronic obstructive pulmonary disease, a family of potentially debilitating breathing problems.

Many first responders who developed a chronic cough later had it fade, or disappear entirely, but others have shown little improvement.

About 9% of firefighters exposed to the dust still report a persistent cough, according to Fire Department research. About 22% report experiencing shortness of breath. About 40% still have chronic sinus problems or acid reflux.

On the encouraging side, doctors say their worst fears about a possible wave of deadly 9/11 cancers haven’t come true. Not yet, at least.

Nearly 24,000 people exposed to trade center dust have gotten cancer, but for the most part it has been at rates in line with the general public. Rates of a few specific types of cancer have been found to be modestly elevated, but researchers say that could be due to more cases being caught in medical monitoring programs.

“We really don’t have the tremendous elevations in cancer I was afraid of,” says Dr. Michael Crane, director of the World Trade Center health clinic at Mount Sinai.

One study showed that cancer mortality rates have actually been lower among city firefighters and paramedics exposed to Trade Center dust than for most Americans, possibly because frequent medical screenings.

In the federal health program's early years, many people enrolling were police officers, firefighters and others who worked on the debris pile. More recently, though, a majority of applications have been from people who worked or lived in Lower Manhattan -��folks like Carl Sadler, who was enveloped by the dust cloud after escaping from his office at the World Trade Center.

Initially, Sadler’s health seemed fine. But years later he started to get winded while exercising.

“I just had breathing problems," he says, “but I never knew what they were."

Now 80, he has been diagnosed over the years with acid reflux disease, asthma, and also thyroid cancer and skin melanoma, for which he was successfully treated.

Last year another 6,800 people joined the health program, which has grown so much it might run out of money. Members of Congress have introduced a bill that would provide an additional $2.6 billion over 10 years to cover an expected funding gap starting in 2025.

The average age of enrollees in the federal health program is now around 60, and Dr. Jacqueline Moline, director of the World Trade Center health clinic at the Northwell Health medical system, is concerned that people's health problems will worsen as they age. Cancer caused by asbestos, she noted, can take as long as 40 years to develop after exposure.

“We are just getting to the point where we might start seeing stuff,” Moline says.

Crane, who has been treating ground zero responders since the beginning, says one thing is clear based on the continuing stream of new patients: The issue isn’t going away.

“They keep on coming," he says. “They keep on coming in the door.”