An analysis found that most of the potentially toxic dust particles collected in the week after the attacks were too large to lodge deep in people's lungs. Only 1 percent of the dust samples was composed of finer particles, researchers said.
In addition, the chemical composition of the dust appears less toxic than originally feared.
Scientists said the dust particles were large enough that people probably coughed them out of their upper airways. But the study did not directly test whether people had deeply inhaled any of the dust.
"In terms of potential lifetime exposures, we're probably going to be very lucky in that these may not be exposures of significant health risk," said Paul Lioy, one of the authors of the report.
Lioy is associate director of the Environmental and Occupational Health Sciences Institute, jointly run by Rutgers University and the University of Medicine & Dentistry of New Jersey.
Scientists examined the dust for Persistent Organic Pollutants, highly stable compounds that endure in the environment and can be toxic to humans and wildlife.
They said they found no evidence of high levels of two particular POPs: pesticides and polychlorinated biphenyls, which were used in hundreds of industrial and commercial applications until their production was banned in 1977.
However, the team estimated that the dust contained 100 to 1,000 tons of polycyclic aromatic hydrocarbons, a group of compounds including some that are classified by the Environmental Protection Agency as probable human carcinogens.
The report will be published in the February issue of Environmental Science & Technology. A summary of the article appears on the Web site of the American Chemical Society, which publishes the journal.
The report is unlikely to resolve all the air quality issues still swirling more than 15 months after the attacks.
The Environmental Protection Agency has come under fire from some critics who say it failed to warn rescue workers — or the general public — about the risks of breathing air near the Trade Center site in the aftermath of the attacks.
For example, on Sept. 13, 2001, the EPA said that air samples showed that lead, asbestos and volatile organic compounds " were not detectable or not of concern." CDC samples taken in October 2001 backed this up.
But since then, a report by the National Library of Medicine states, "there has been an ongoing debate about the accuracy and relevancy of the air monitoring being conducted and the overall danger of the air." One charge is that the standards employed by the EPA for their air testing were set rather arbitrarily.
In some apartments and offices the dust was three inches thick. The EPA has pledged to clean up residences throughout lower Manhattan.
Many lower Manhattan residents and rescue workers at ground zero have reported continuing respiratory problems. The dust from the collapsed World Trade Center towers was a largely a combination of pulverized glass and concrete, among other materials, that can be extremely alkaline and irritating.
A Centers for Disease Control and Prevention study released on the first anniversary of the attacks found that among firefighters, "the number of respiratory medical leave incidents increased five-fold during the 11 months after the attacks."
A recent report by Mount Sinai Medical Center found that half of those Ground Zero rescue and salvage workers screened required treatment for respiratory ailments.
Lioy's team took dust samples from 13 locations around the site during Sept. 12-17 last year. They did not collect any samples in Brooklyn and other areas farther from ground zero where the dust plume spread.
The smaller particles have been the focus of air quality regulations nationwide. Even in normal city air, high levels of the particles can accumulate from vehicle exhaust, pulverized sand and dust from tires and brakes.
The director of the Environmental Research Foundation, a New Jersey-based public-interest group, said there were still enough toxic particles to cause concern.
"One percent of a million tons is 10,000 tons," Peter Montague told The New York Times. "So you've got 10,000 tons of fine particles. That's a lot."