An apartment in Battery Park after 11 September

This, they say, could explain why many local residents and workers have been complaining of coughing, headaches, nosebleeds and chest infections.

But the tests' findings have been disputed by the US Government's Environmental Protection Agency (EPA) and the Environmental Department of the New York City council.

Anecdotal evidence

According to the International Herald Tribune, about a quarter of the city's firefighters who have worked at the devastated twin towers' site - dubbed Ground Zero - have complained of severe coughing.

Last week four police officers were taken off the site after testing positive for abnormally-high levels of mercury in their blood.

Dozens of students at Stuyvesant High School nearby have developed rashes, breathing difficulties, nosebleeds and headaches.

Dr Stephen Levin, medical director of Mount Sinai I.J. Selikoff Center for Occupational and Environmental medicine, says there are "cases of new-onset reactive airway disease for people who were in excellent physical condition prior to 11 September".

But other doctors stress that some of the symptoms could be caused or worsened by stress and will dissipate over time.

This is proving cold comfort for residents.

Long term fears

George Tabb, who lives in Tribeca, one of the newest neighbourhoods in lower Manhattan, says his and his wife's symptoms - asthma attacks and pounding headaches - disappear within 48 hours of leaving the area.

Properties around the twin towers were once sought-after

The EPA initially reassured New Yorkers that the air was safe, although tests carried out by the agency shortly afterwards found elevated levels of dangerous chemicals such as lead and benzene.

More recent tests commissioned by residents in apartments north of Ground Zero have also found asbestos, which can cause cancer, at levels 500 times higher than the authorities say is acceptable.

Now the EPA is giving a mixed message - it says the area is safe but it is also advising landlords to employ professional asbestos cleaners, and says workers at the twin towers site should wear respirators.

Even if the coughs and breathing problems subside, local residents are worried some of the substances released into the air by the collapse of the twin towers may lead to long-term health problems.

"We're going to have kids," Mr Tabb told the Herald Tribune, "and I don't know what's going to happen."