Emergency crews exposed to dust after the collapse of the World Trade Center had lung problems equal to 12 years of age-related respiratory damage.
Firefighters inhaled high amounts of 'dust' after 9/11
Lung tests of 12,000 rescue workers in the year after the disaster showed those present in the very early stages of rescue suffered the most damage.
Montefiore Medical Center in New York researchers compared lung function with tests carried out before the attacks.
An ongoing study showed some still had breathing problems, they said.
Dr Gisela Banauch and colleagues at Montefiore Medical Center in New York were able to look at the respiratory function of New York City Fire Department workers from 1997 onwards because routine lung tests were carried out every 18 months as part of routine medical screening.
They compared previous results with tests they carried out in 12,079 rescue workers in the 12 months after the World Trade Center collapse.
There was a substantial reduction in forced expiratory volume - the amount that can be breathed out in one second - in the year after September 11 2001, they report in the American Journal of Respiratory and Critical Care Medicine.
A total of 13.7% of the rescue workers were exposed to the most amount of dust by arriving on the morning of the disaster and being present when the towers collapsed, they reported.
A further 67.8% arrived on the scene two days after the collapse, and 16% arrived after the third day.
Those exposed in the first couple of days after the collapse had significantly more frequent and more severe respiratory symptoms than workers who arrived later.
Only 22% of workers who arrived early reported wearing a mask on the first day, which had increased to 50% by the third day. But the researchers did not find using a mask had a protective effect.
Previous research has shown exposure to airborne pollutants was greatest during the collapse, but then gradually dissipated.
Dr Banauch, associate professor of medicine at Albert Einstein College in New York said: "We were in a very unique position because these workers had had routine monitoring of their lung function before and that's never happened before in a group this large."
She said that some of the effects were the result of acute irritation of the airways but that smaller follow-up studies showed some rescue workers were suffering from more long-term effects.
"My personal opinion is that when the towers collapsed a lot of building materials were pulverised so there was a lot of cement in the dust, which is very alkaline and which we know that causes burns."
She added that it would have been impossible to properly protect the firefighters against the effects of the dust.
Dr Paul Cullinan, an occupational and environmental respiratory disease specialist at Imperial College, said the study was "impressive".
He added: "They have done a really good job to maintain contact with all these people.
"For example, after the King's Cross fire there was no follow-up of firefighters and there should have been."
He added that the extent of respiratory problems identified by the researchers was surprising.
"Firefighters are pretty healthy people so it's a bit surprising that in fit youngish men that it would have an effect.
"Normally they would have to be exposed for years and years."