The terrorist attacks that brought down New York's World Trade Center on 11 September 2001 are continuing to trigger health problems for tens of thousands of people.
More than 400,000 officer workers, residents, rescuers and passers-by were in lower Manhattan at the time of, or shortly after, the attacks.
The Journal of the American Medical Association says an estimated 25,000 people who were exposed to the dust have since developed asthma.
Some 61,000 suffer from post-traumatic stress.
The research was carried out by the New York City Health Department, Columbia University and the US Centres for Disease Control and Prevention.
It was based on two surveys performed in 2003-2004 and 2006-2007.
The work is part of The World Trade Centre Health Registry, the largest post-disaster exposure registry in US history.
