When planes flew into the World Trade Center on September 11, 2001, New York City's emergency workers raced to lower Manhattan.
Upon arrival they were confronted by New Yorkers running in the opposite direction, away from the danger.
Those trained to confront such horrific scenes rushed inside the Twin Towers and hundreds never came out. Officially, 411 emergency workers lost their lives that day, including almost 350 firefighters.
Those who survived are the lucky ones, but they didn't escape unharmed.
A new report, released less than two weeks before the 15th anniversary of the worst terror attack on American soil, examined the long term health impacts of first responders.
While previous studies have examined the physical impacts - respiratory problems and cancer diagnoses - this study focused specifically on mental impairment.
Researchers from several universities in America and London spoke with 818 rescuers. The findings, published in the journal Alzheimer's & Dementia: Diagnosis, Assessment & Disease Monitoring, showed approximately 12 per cent of those who entered Ground Zero developed post-traumatic stress disorder and some cognitive impairment.
DIGGING DEEPER INTO THE NUMBERS
More than 10 per cent of those who took part in the survey showed signs of experiencing dementia. The lead author says his findings are a first. He says they are "staggering".
The researchers found that of 818 responders who took part in the study, 104 suffered a cognitive impairment and 10 had possible dementia.
Extrapolated to include all 33,000 responders, they predicted that more than 5000 could suffer cognitive impairments and as many 800 could have dementia.
"Thousands of responders who helped in search, rescue, and clean-up efforts after the World Trade Center were exposed to an extraordinary array of psychological traumas and toxic exposures," the report reads.
"Although few were physically injured by their efforts, many responders witnessed the disaster or death and dismemberment of others, helped civilians flee, lost colleagues in the tower collapse, and dug through debris to search for survivors."
The impact, the study found, was that rescuers consistently showed signs of dementia. Of most concern was the age at which sufferers were being diagnosed.
Cognitive impairment and dementia are most commonly diagnosed in sufferers over the age of 70, but the responders taking part in the study were on average just 53 years old.
"These numbers are staggering," researcher Sean Clouston, an assistant professor of public health at Stony Brook University, said.
FLASHBACKS AND NIGHTMARES BEGAN IN 2002
Dr Clouston said rescuers suffering PTSD were being treated for the condition. More than 70 per cent are receiving medications to help them cope. It's an ongoing battle.
He said "flashbacks and nightmares" started for "most of them in 2002" and that "we know the PTSD is World Trade Center-related.
Others live with the spectre of cancer. As one rescuer put it last year: "The events of 9/11 don't haunt me, it's the fear now of having cancer."
A report from 2014 showed the cancer rate among Ground Zero workers was skyrocketing. More than 2500 responders were suffering from one form of cancer or another, the New York Post reported.
The figures were tallied by staff at Mount Sinai Hospital's World Trade Center Health Program. The Post reported that 2518 firefighters, police officers, sanitation workers and volunteers had succumbed to the disease.
A retired FDNY captain, who did not wish to be named, left the force in 2008 after struggling to breathe. He reportedly weighed more than 100kg in 2001 and, when interviewed in July, 2014, was down to 72kg following chemotherapy and radiation.
At the time, cancer claims related to the fallout from September 11 had totalled more than $50 million ($A66 million).
In December last year, President Barack Obama made several important changes to the September 11th Victim Compensation Fund. He signed into law a bill extending the deadline for claims from October, 2016, to December, 2020.