Esther Regelson lived two blocks south of the World Trade Center on Sept. 11, 2001.

She was there when the towers collapsed, spewing dust that was filled with hundreds of carcinogenic substances, including jet fuel, asbestos, lead, mercury, and fibrous gas.

Several months later, she noticed that she was having a harder time breathing. Regelson had lived with asthma since she was a child, but this felt different.

She went to a clinic, and found out that she had only 49 percent lung capacity.

"They were saying it was unheard of that I was as functional as I was," she told Business Insider in a 2016 interview.

Regelson, who still lived in the same apartment over 15 years later, also dealt with acid reflux and had bouts of chronic bronchitis for a while.

She's not the only one with these conditions — those responding to the attack and living in the area were exposed to a lot of carcinogenic substances, and the effects are still being felt 16 years later. As time has passed, more lasting health conditions, including cancer, seem to be linked to the 2001 attack.

For example, Marcy Borders, the subject of an iconic photo in which she was shown covered in dust after the attack, died of stomach cancer in August 2015. She was one of thousands who have reported cancer cases linked with the aftermath of the terror attacks.

The World Trade Center Health Program, a federal program designed to treat those living with conditions that have a connection to 9/11, was put into effect in 2011. It covers trauma-related injuries, disorders related to breathing and digestion, mental health conditions, and a long list of more than 50 cancers that have been connected to the dust and rubble of 9/11. The program was renewed in 2015 as part of the James Zadroga 9/11 Health and Compensation Act.

Who was affected

The health program covers two groups: the survivors who lived, studied or worked in the area, and the responders — firefighters, police officers, construction workers, and others who came to help and clear out the dust-coated area.

As of June 2017, almost 80,000 people are registered through the health program. Of that, more than 10,000 were residents living around the area at the time of the attack.

Because of the program, those patients don't have to pay anything out of pocket for their medical care. Before the law was put in place, however, people incurred massive amounts of medical debt as they confronted 9/11-related illnesses. But not all survivors — in particular, residents — have sought out the proper care.

"Unfortunately a lot of people who live and work in this neighborhood don't put two and two together," Regelson said. "They don't realize the program is for them — it's not just for first responders not getting help."

Kimberly Flynn, director of the 9/11 Environmental Action group, which works to connect residents to the health program, put the problem this way: "There are a lot of people who should be in this program who are not. That's because they moved on."

Finding the medical evidence

Pinning down the link between health problems and 9/11 is no small feat, either. Researchers are still trying to pin down the association between the attacks and conditions like cancer, respiratory issues, and other ailments. That can be hard to prove because the events are just one factor of many when it comes to conditions like cancer.

A 2012 study in the Journal of the American Medical Association looked at 55,000 New Yorkers that had enrolled in the World Trade Center Health Registry. That study found that for cancer, "no significant associations were observed with intensity of World Trade Center exposures," though the researchers noted that more follow up for cancers that take a longer time to show up are needed. They noted that "the presence of carcinogenic agents raises the possibility that exposure to the WTC environment could eventually lead to cancers."

The World Trade Center Health Program has different criteria for what can be linked to 9/11, but it still requires some proof or a pattern. And those with conditions that could possibly relate to 9/11 but haven't had a proven link might not get the care they need. Flynn said there can be a major time gap between when doctors first see a patient with a certain problem and the time at which that condition gets added to the health program's coverage.

"It's essential that it exists. It provides expert care for a wide range of conditions," she said of the program. "But people are still getting sick with conditions that may not be added for some time because there's not sufficient evidence."

The coming years

The World Trade Center Health Program will be in place until 2090.

A lot is still unknown about how the lasting health effects of 9/11 will affect people in the coming decades. For example, cancers might start to become more frequent a few more years down the line. The health program's enrollment has steadily been rising since it opened, with a few hundred more responders and survivors joining each month.

Ideally, by monitoring this group, doctors and health officials can get a better idea of what's to come for those who were exposed to toxic dust during and after 9/11.

"Hopefully the science can direct us to what these things do to people," Regelson said.

This article first appeared in Business Insider. Read the original here. Follow Business Insider on Twitter.