The dust that covered large parts of lower Manhattan following the September 11 attack was less toxic than had been feared, research suggests.

Experts have concluded that the dust is unlikely to have caused serious long-term health problems.

We're probably going to be very lucky Dr Paul Lioy

This is because most of the individual particles were large enough to be expelled from the lungs, rather than penetrating deep into the delicate tissues of the respiratory system.

A team of environment scientists collected and analysed millions of tons of the dust in the aftermath of the destruction of the World Trade Center towers.

They found that only 1% was composed of finer particles, which are far more likely to trigger illness and disease after being inhaled.

Researcher Dr Paul Lioy, associate director of the Environmental and Occupational Health Sciences Institute in New York, said: "That means, 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."

Symptoms reported

Many lower Manhattan residents and rescue workers have reported respiratory problems since the attacks.

Breathing problems were so common among on-duty firefighters that they were dubbed World Trade Center cough.

A study published in the New England Journal of Medicine estimated that 332 firefighters reported symptoms, including a severe cough and shortness of breath.

Peter Montague, director of the Environmental Research Foundation, warned that there were still enough potentially dangerous particles to cause concern.

He said: "One percent of a million tons is 10,000 tons.

"So you've got 10,000 tons of fine particles. That's a lot."

The research, reported by the New York Times, will be published in the journal Environmental Science & Technology.