Retired New York City police officer John Aris sat in my office recently waiting for the results of his latest CT scan.

Seven years earlier, I had removed part of his right lung because of cancer. Since then, every follow-up appointment has carried the same anxiety. Before I had even closed the examination room door behind me, I offered the words he had hoped to hear: “No cancer.”

He let out a long breath and settled back into his chair. As always, we lingered and talked about life, politics, and family.

Then I asked him what he thought about something different.

On Friday, during the 25th anniversary ceremony at the World Trade Center, organizers will add a new moment of silence. It will honor those who died not on Sept. 11, 2001, but in the years that followed from illnesses linked to their exposure at ground zero.

His smile faded. He stared ahead for a moment before speaking. His voice cracked as he began talking about the people he had lost. Not the people killed when the towers fell, but those who died years later.

He remembered his friends. Construction workers, firefighters, and police officers, who stood shoulder to shoulder with him for months amid choking dust, burning debris, and twisted steel. He remembered the smell that lingered as they worked.

Many of his friends went home. At least at first. Then, years later, the funerals began.

One friend developed esophageal cancer. Another died from leukemia. Another succumbed to a respiratory illness. The phone calls never seemed to stop. Every year brought another diagnosis, another funeral.

Beside him sat his wife of 44 years, Kathleen. She had lived through every one of those losses with him.

Each obituary reopened a wound that had never completely healed. I realized we had spent most of the visit talking not about his cancer — which could have been related to his exposure at ground zero, though we can’t say for sure — but about everyone else’s.

Later that afternoon I saw another patient, retired firefighter William Corrar, whom I had operated on for esophageal cancer. Like many responders, he spent months working at ground zero after the attacks. For years he dismissed worsening reflux as simply getting older before learning it was cancer.

Sept. 11 is personal for me, too.

On the morning of the attacks, I was already a thoracic surgeon at Memorial Sloan Kettering in New York. I was in the hospital operating on a patient suffering from cancer caused by asbestos exposure when I heard what had happened. All of our remaining cases were canceled, so I got on my bike and rode downtown to see if I could help.

There was surprisingly little for a surgeon to do. The people who could escape the towers largely walked away. So many who could not were simply gone. I had ridden downtown expecting to help take care of the injured. Instead, like so many New Yorkers that day, I was left trying to comprehend what had just happened.

My connection to the World Trade Center went back much further. I’m a native New Yorker and skateboarded through the plaza between the Twin Towers as a kid. Later I worked in the South Tower making deliveries for UPS as I was going to college. The towers had been part of the landscape of my life.

I could not have imagined that they would eventually become part of my life as a surgeon as well.

In the years after the attacks, the patients began appearing. First responders, construction workers, and others who had spent months breathing the dust and smoke came to us with diseases that surgeons treat. What had begun as a catastrophe I had witnessed was becoming something I encountered in my operating room.

These scenarios have become a familiar part of my practice. I have spent much of my career caring for people exposed to the toxic aftermath. I continue to treat patients with lung and esophageal cancer, mesothelioma, and other illnesses associated with their exposure to the disaster site.

But there is an important distinction between association and certainty.

I cannot look at an individual’s lung or esophageal cancer and prove that it was caused by something he inhaled at ground zero. Cancer can take years to develop, and an individual patient may have several potential risk factors.

That does not mean there is no connection. Medicine rarely establishes environmental risk by identifying a single exposure that caused a single person’s cancer. We study populations. We ask whether disease occurs more often among exposed people, whether the timing makes biological sense, and whether substances involved are capable of causing disease.

The cloud that engulfed Lower Manhattan contained pulverized concrete, glass fibers, asbestos, heavy metals, and combustion products. Thousands rushed toward danger without knowing exactly what they were breathing.

Others never chose to be there. Residents, office workers, and students were exposed simply because they happened to live, work or study nearby.

They believed the air was safe. Then came asthma, chronic sinus disease, and persistent cough.

Today more than 140,000 responders and survivors are enrolled in the federally administered World Trade Center Health Program. Tens of thousands have developed cancers, respiratory diseases, digestive disorders, and mental health conditions linked to their exposure. More than 8,000 enrolled members have died, and that number continues to grow.

Disease rarely captures the nation’s attention the way catastrophe does. It unfolds one diagnosis at a time, one funeral at a time. But for thousands of families, the attack did not end when the fires were extinguished. It simply changed form.

When the crowd falls silent this September, John will not only remember those who have died. He will think of friends still living with illnesses, like himself, and the uncertainty they carry into each new year.

Because for them, Sept. 11 was not just one day. It has been 25 years.

And it is still not over.

Raja Flores is the chair of the Department of Thoracic Surgery at Mount Sinai School of Medicine, where he treats patients with mesothelioma, lung, and esophageal cancer.