The death toll from the September 11 terror attack could soar into the millions as a result of the toxic dust that blanketed New York City when the Twin Towers collapsed.
Residents have already started to develop cancers caused by the asbestos-filled ash that erupted from the World Trade Centre skyscrapers, and a leading health expert has warned it will only get worse.
"In 15-20 years we are going to see a serious health issue here in New York. We haven't even seen the tip of this iceberg yet," Dr Raja Flores, Chief of Thoracic Surgery at the Mount Sinai Hospital, NY, told nine.com.au.
"There's almost nine million people in New York… a lot more people may have been exposed (to asbestos fibres) than we first thought. It contaminated a huge portion of the city and that entire population was exposed," Dr Flores said.
The most infamous terror attack in modern history killed more than 2700 people when two aircraft hit the World Trade Centre's north and south towers on September 11, 2001.
That death toll also included the innocent lives of those who died when another plane hit the Pentagon, and a hijacked plane crashed into a field near Shanksville in Pennsylvania.
However, the true scope of the attack may only start to be understood due to the dust and debris that scattered across Manhattan and Brooklyn.
Pedestrians flee the dust-filled area surrounding the World Trade Center. (AAP)
Hidden in the dust was more than 400,000 kilograms of asbestos, mercury, fibreglass and benzene.
It was inhaled – in varying doses – by almost everyone in the city.
Last weekend, hero US firefighter Ray Pfeifer died from brain and lung cancer that was linked to his work at Ground Zero.
Pfeifer was one of many emergency servicemen and women who spent more than eight months cleaning up the wreckage of the World Trade Centre.
Dr Flores - considered one of the world's leading experts in pleural mesothelioma and asbestosis – said while downtown Manhattan copped most of the dust, anyone in the city at the time of the attack and the days that followed were potentially exposed to deadly asbestos fibres.
"There are a lot of people now who are having respiratory issues and we are starting to see more cancers," he said.
"People are already getting sick from the asbestos and they have died from it. If I had to bet my house, in 15-20 years we are going to see a serious health issue here.
"I think you will probably see double or tripling of the number of lung cancers in people who were in New York City on 9/11 and mesothelioma and people dying of pleural fibrosis from asbestosis."
Dust-covered 9/11 survivor Marcy Borders died of stomach cancer in 2015. (AFP)
The New York surgeon objected to reports that only first responders and people in the vicinity of the World Trade Centre were at risk of developing lung cancer, warning the US may be a few years away from a catastrophic national health crisis.
During the construction of the World Trade Centre, the Port Authority of New York and New Jersey originally planned to use more than five million kilograms of asbestos in the buildings.
Its controversial installation was halted in the 1970s after a link was proven between asbestos fibre exposure and lung cancer by Dr Irving Selikoff from Mt Sinai Hospital.
However, by that time almost half a million kilograms of the deadly insulation had already been sprayed in the first 40 storeys of the buildings.
"It's one of the most dangerous substances around. Every day I operate (I see) patients that have been affected by asbestosis. The safe amount of asbestos in the air for someone to inhale is zero. There's no safe amount," Dr Flores said.
"The entire city was blanketed, especially downtown and Brooklyn. You went down there months later and you still smelt it. That dust was still around for a very long time."
Dr Raja Flores warned New York could face a health epidemic with the number of cases of asbestos related lung diseases expected to soar. (YouTube)
Dr Flores' grim prognosis for the people of New York comes from a comparative study he undertook in the town of Libby, Montana, where the asbestos used in the World Trade Centre was originally mined and manufactured.
His study investigated the contaminated vermiculite mines in the region and the local population's exposure to tremolite asbestos.
More than 400 people in the town of 2,656 have died from asbestos-related illnesses since the 1990s.
"It's common sense that what happened in Libby will start happening to the people in New York. That's frightening and it's a big deal," he said.
Lower Manhatten was covered in asbestos-filled dust after 9/11 which blew across the entire city and over to Brooklyn. (Getty)
However, Dr Flores' prediction is not without its detractors.
A 2012 New York City health study published in the Journal of American Medical Association dismissed links between the noxious dust released from the Twin Towers' collapse and the development of cancers.
The study examined 55,700 people who were in the vicinity of the World Trade Centre during 9/11 and in the weeks that followed.
The paper revealed 1,187 people (439 rescue/recovery workers and 748 general public) had been diagnosed with three different types of cancers between 2007 and 2008.