"It's cancer on steroids." The death toll for September 11 continues to rise.

“It’s the longest day in the history of days,” first responder John Feal says about September 11, 2001.

For him, and tens of thousands of others, it’s a day that never ended.

It was a clear, still Tuesday morning in New York City, when civilians looked up and saw an American Airlines plane flying unusually low.

At 8:46am, a terrible sound pierced one of the biggest cities in the world, as the domestic passenger plane flew through the North Tower of the World Trade Centre, instantly killing all 92 people on board.

Why the September 11 death toll continues to rise. Post continues after podcast. 

Seventeen minutes later, the world watched a terror attack live on television for the first time in human history, as United Airlines Flight 175 crashed into the South Tower, taking the lives of 65 passengers with it.

Once the tallest buildings on the planet, standing proud in Lower Manhattan, it only took an hour and a half for both towers to collapse to the ground.

The terror attack killed 2,996 people and injured at least 6,000 more.

John Feal wasn't there that day. His work began on September 12.

Feal was one of thousands of first responders who arrived at the scene - a carpenter and demolition supervisor who was needed to clear through the debris.

He can block out what he saw there, Feal told NPR in 2017. But he can't block out the smell.

"When I close my eyes, I can smell ground zero.

"There's not a word invented yet that describes the smell of ground zero... It's a smell that I've never smelled before or [since]. It's a smell of destruction, devastation, carnage. It was everything combined in one that created the smell..."

For five days, Feal worked tirelessly alongside thousands of first responders who were entirely aware of the risk involved in the rescue effort.

And then, on September 17, 2001, 8000 pounds, or 3,600 kilos, fell on Feal's left foot.

"I spent 11 weeks in the hospital with gangrene," Feal told Mamamia. "I wound up losing half of my left foot."

"At the time it was the worst thing that ever happened to me," Feal said. But for the New Yorker, things were about to get worse.

Twice he applied for compensation from the September 11th Relief Fund, and twice he was denied.

Even though the injury was classified as "life-threatening and catastrophic," it happened 96 hours after the terror attack, meaning he didn't qualify for assistance.

That's when Feal realised something.

The sun had never really set on September 11, 2001. The suffering of survivors wasn't getting better, in fact, it was getting worse.

And the death toll continued to grow.

Of the NYPD officers called to the scene in the wake of September 11, 23 lost their lives. But in the 17 years since, the department has lost another 156 as a result of illnesses they contracted while on 'the pile' - the term for the "mountain of toxic debris from the fallen towers".

"We're probably about two or three years away from outnumbering those who died on that day," Feal told Mamamia. 

Of the 90,000 people who are registered with the World Trade Center Health Program, almost 10,000 have been diagnosed with at least one cancer believed to be related to September 11.

Rob Serra graduated from the Fire Academy on September 10, 2001. He had not yet started his job as a New York City firefighter when he arrived on scene at the World Trade Center's the following day. Serra was 21 years old.

"I do remember thinking," he told NBC news, "this is probably going to kill me."

The glass. The steel. The asbestos. Serra says "you could taste it". Everyone around him, he says, "had a half inch of white paste on their face".

He lives in perpetual fear that he will develop cancer. "I hope I have a lot of years left," he told NBC, "but common sense and reason tells me I don't."

Marcy Borders featured in one of the most memorable photographs to emerge from September 11; the 28-year-old standing inside a nearby office, in shock, covered head to toe in dust, disguising the clothes she'd worn to work that morning.

In 2015, Borders died of stomach cancer. She told The Jersey Journal that she always believed her cancer was a result of what happened to her that day.

"It's cancer on steroids," is how Feal puts it. The latest research suggests those involved in the recovery effort on ground zero are five to 30 per cent more likely to develop cancer.

But Feal believes there's a bigger threat to the 9/11 community than cancer.

"I believe Post Traumatic Stress Disorder (PTSD) will be the number one killer," he told Mamamia. "While most people will die from cancer or something else, post-traumatic is the leading contributor to these deaths because it's a physical injury... it's stress mentally and physically on the body.

"It breaks down the immune system and doesn't let the body fight the other illnesses you have."

PTSD is understood to be the most common illness suffered by those who attended ground zero. Feal himself has been diagnosed with it, and has watched multiple friends take their own lives over the last 20 years.

For researchers, the first responders who arrived in Lower Manhattan following the most deadly terror attack in US history, are guinea pigs.

"They'll be better able to understand the health impacts of those that lived through it, and responded to the act," Feal said.

These men and women have comprised a critical database, that will go a significant way to ensuring the mistakes of the past are not repeated in the future.

Of the tens of thousands who worked on ground zero, it's believed only half wore protective gear. Of those in the surrounding areas, hardly anyone did.

The site was a nightmare no one was truly prepared for.

"I was in the Army," Feal said. "I'd seen some pretty bad things, but I never saw that kind of destruction, devastation, carnage. I was in awe."

He likely didn't expect that years later he'd still have nightmares - or that the worst was yet to come.

And perhaps it is that knowledge that is Feal's greatest legacy.

The impact of September 11 cannot be reduced to a day. In fact, it cannot even be reduced to a generation.

It is, as Feal put it, "the longest day in the history of days."

And it won't be over for a long while yet.

Visit the Feal Good Foundation: No Responders Left Behind.