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 lower 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 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)

However, the New York surgeon emphasised the risk of exposure to asbestos didn't necessarily correlate to the risk of asbestos-related disease.

Dr Flores 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.

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)

"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 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.

Lower Manhatten was covered in asbestos-filled dust after 9/11 which blew across the entire city and over to Brooklyn. (Getty)

"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.

However, Dr Flores' prediction is not without its detractors.

A 2012 New York City health study published in the Journal of American Medical Association couldn't draw a conclusive link between the noxious dust released from the Twin Towers' collapse and the development of cancers.

The study found that while an elevated risk for some cancers was demonstrated, there was insufficient evidence to draw a firm conclusion that the cancers were related to exposure.

People take cover as a dust cloud from the collapse of the World Trade Center envelops lower Manhattan. (AFP)

"Among persons enrolled in the World Trade Center Health Registry, there was an excess risk for prostate cancer, thyroid cancer, and myeloma in 2007-2008 compared with that for New York State residents; however, these findings were based on a small number of events and multiple comparisons. No significant associations were observed with intensity of World Trade Center exposures. Longer follow-up for typically long-latency cancers and attention to specific cancer sites are needed," the study read.

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.

The research assessment concluded that while "the presence of carcinogenic agents raises the possibility that exposure to the WTC environment could eventually lead to cancers," the link between the two could not be substantiated.

Former New York City health commissioner Dr Thomas Farley said it was still too early to establish a link between 9/11 and cancer during a 2012 interview .

"Cancers take 20 years to develop and we might see something different 20 years down the line," he said.

Dr Flores warned children exposed to the dust in the days following the terror attack had a much greater risk of developing cancers and severe pulmonary diseases.

"The main option you have if you've been exposed is to get screened. A low dose CAT (CT) scan once a year to see if you have asbestos changes in your lungs," Dr Flores said.

Dust blanketing downtown Manhatten and moving across New York after the twin towers collapsed. (AFP)

However, the surgeon warned that even with a screening there is no treatment.

Dr Flores has accused property owners and bodies with vested economic interests in New York of "sowing doubt" about the dangers of asbestos and how widespread the September 11 exposure really was.

"My concern is people – property owners, asbestos companies and people that are worried about getting sued - are playing down the harms of asbestos. Asbestos kills, we know that," he said.

"They try to influence the population by sowing doubt, saying it's 'not that bad' or that there are 'safe levels' you can be exposed to. That's all BS and lies because they know it will take 20-30 years for it to manifest itself and kill people."

Firefighters work beneath the destroyed mullions, the vertical struts which once faced the soaring outer walls of the World Trade Center towers. (AAP)

In a 2012 Tweet, current US president Donald Trump claimed the World Trade Centre wouldn't have collapsed if there had been more asbestos in the buildings.

"If we didn't remove incredibly powerful fire retardant asbestos and replace it with junk that doesn't work, the World Trade Center would never have burned down," he wrote.

In 2010 the Zadroga Bill was signed into law by former US president Barack Obama to provide free medical treatment and compensation to first responders of the 2001 terrorist attack.

While the bill has provided invaluable medical assistance to the community population who lived, worked, or went to school in lower Manhattan during the WTC disaster, it does not cover the rest of New York's general population, who according to Dr Flores have already started to develop asbestosis and lung cancers.

First responders fleeing a dust cloud after the collapse of the World Trade Centre. (AFP)

The New York surgeon said an immediate change to the Zadroga Bill is needed to cope with the still-developing aftermath of 9/11.

"We need legislative change to start saving some lives and I think we can save more lives with the pen than with my knife," he said.

The September 11th Victim Compensation Fund (VCF) was created in 2001 to provide compensation for any anyone killed or injured in the attack or during the lengthy clean up that followed.

The program – which covers pulmonary diseases and 58 types of cancers - expired in 2004 but was extended until 2020 by Obama.

It has already paid out more than $9.4 billion in exchange for a legal agreement that victims won't sue the airlines for negligence.

Dozens of law firms in New York have been inundated with applications since the program started, with many devoting staff and entire sections of their websites to the 9/11 victims.