While the dust was highly toxic, Lioy said it's impossible to say which specific substances contributed to the health problems since it was a mixture of things and the effects of gases couldn't be taken into account.
"The gases were released very quickly and then went away, so it's impossible to determine what could have been there."
Normally, the body is able to filter out certain substances, but the dust that fell from the towers overcame the body's defenses.
"The concentration of toxic materials was so high that when those people took a breath, the junk in the air overwhelmed the normal mechanisms we have in our nose and throat," said Landrigan.
In January, President Obama signed into law the James Zadroga 9/11 Health and Compensation Act, named in honor of a New York City police officer whose death was linked to dust exposure. The bill set aside money to provide health monitoring, treatment and compensation to affected workers.