The New York City's Department of Health has estimated that some 70,000 people may have developed post-traumatic stress disorder following the 9/11 terrorist attacks.
The data for the study was taken from the World Trade Center Health Registry, which has 71,437 people enrolled in it.
"The consensus among physicians is that when it comes to physical health, the vast majority of people felt symptoms in the first year," said Lorna Thorpe, the deputy commissioner for epidemiology at the New York City Health Department. "A small proportion of people, however, developed symptoms years later. And in some cases, it's hard to tell whether they're World Trade Center-related or a result of allergies or existing conditions."