Doctors at Mount Sinai Medical Center, which conducted the study, said the results prove that working in the toxic gray dust at ground zero made many people sick, and some will likely suffer the effects for the rest of their lives.

"There should no longer be any doubt about the health effects of the World Trade Center. Our patients are sick," said Dr. Robin Herbert, co-director of the group that investigated the long-term effects from exposure to dust at the site.

Almost 70 percent of World Trade Center responders had new or worsened lung symptoms after the attacks.

Among responders who had no health symptoms before the attacks, 61 percent developed lung symptoms while working on the toxic pile.

One-third of those tested had abnormal lung function tests.

"Mainstream drugs are ineffective against multiple drug-resistant TB," said Nunn. "However, there are half a dozen second-line medicines that can be used to tackle it. Now this new extreme resistant strain has appeared. It is not only resistant to our principal anti-TB drugs, but to many of our second-line defences. In short, we are now on the last line of our defences against tuberculosis."

Among the areas found to have been affected by extreme drug-resistant TB are Latvia and South Africa. Scientists discovered the strain last month among HIV-infected patients in the Kwazulu-Natal region. "Fifty two of the 53 infected people are already dead, and the last may well have died by now," added Nunn.

An estimated 4.5 million people in South Africa have HIV. Extreme drug-resistance TB could devastate the population. "If countries don't have the diagnostic capacity to find these patients, they will die without proper treatment," said Nunn.

As a result, WHO is to hold its emergency meeting in Johannesburg to help establish measures that will lead to the rapid diagnosis of the new strain.

Only one responder's death-that of New York City police detective James Zadroga, who succumbed to respiratory failure in January-has been directly linked by a medical examiner to his exposure to environmental toxins at Ground Zero. But at least six other deaths (from causes ranging from heart failure to lung cancer) have been reported among responders in their 30s and 40s who worked at the World Trade Center site. And thousands more are struggling with health problems far worse than officials initially anticipated. "People think that it's just a few guys from 9/11 suffering," says Feal, "but there are literally thousands of us."

It's too early to know with certainty how many deaths may result from the cocktail of asbestos, mercury, silica, fiberglass and other potentially hazardous materials released when the twin towers collapsed. Nor is it possible to say with certainty which health problems are related to the responders' work in the rubble of the trade center. But it's clear that many of the estimated 40,000 police, firefighters and other workers who came to the site to assist in rescue and recovery efforts have begun suffering from similar and sometimes serious ailments during the past four and a half years. "You can't witness and be exposed to what these people were exposed to without it taking a toll," says Dr. Stephen M. Levin, codirector of the World Trade Center Worker and Volunteer Medical Screening Program at Mount Sinai Hospital in New York. "To listen to how life has become for some patients, it’s absolutely horrifying."

Dr. John Howard, who was appointed in February as federal coordinator of 9/11 health issues by the Bush administration, has said that the government will likely need to monitor the health of those who were at or near Ground Zero for 20 to 30 years. How much money—and manpower-that will require is unclear. But it’s likely more than the $125 million initially allocated by the federal government for 9/11 responders as part of a $20 billion federal aid package for areas affected by the terrorist attacks. "I don't think we're prepared to come up with a number [yet]," said Howard earlier this spring.

The government's point man on Sept. 11 health programs said he is worried that an autopsy linking a retired detective's death to recovery work at ground zero may be a warning sign of other life-threatening cases.

Dr. John Howard also said it will take time to determine whether there is a scientific link between deaths and exposure to toxic dust. Some epidemiologists have said it will take 20 years or more to prove such a link.

Howard, who is to meet in New York this week with congressional leaders about ground zero health issues, was tapped by the Bush administration in February to coordinate the federal response to ongoing Sept. 11 health programs.

Global annual deaths from smoking are expected to double to 10 million by 2020 but researchers said on Friday the real figure could be much higher.

The projection may be too low because an international survey of 13-15 year-olds has uncovered an unexpected rise in young female smokers, widespread exposure to second-hand smoke and the use of other tobacco products.

"The effect of tobacco use on worldwide deaths could be even greater than expected," said Dr Charles Warren, of the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia.

But Robin Herbert, who directs a medical-monitoring program at Mount Sinai Medical Center for more than 14,000 ground zero workers, said "certainly it is not inconceivable" that a person could die of respiratory disease related to Sept. 11.

Karin DeShore said she does not need scientists to tell her what caused the death of her friend Keller, 41. DeShore was a Fire Department captain who took Keller to the trade center on Sept. 11, and barely escaped the south tower's collapse.

"He came back coughing" two days later, she said. Faeth said that Keller told him that he coughed up debris so violently he could barely breathe on Sept. 11, and later developed emphysema.

Keller went home to Levittown on medical leave in March. He died on June 23 of heart disease complicated by bronchitis and emphysema, the Nassau County medical examiner's office said.

David Worby, an attorney representing more than 5,000 plaintiffs suing those who supervised the cleanup over their illnesses, said 21 of his clients have died of Sept. 11-related diseases since mid-2004. He said he was not authorized to release their names, but represented people who toiled at ground zero, at the Fresh Kills landfill in Staten Island where trade center debris was moved, and at the city morgue.

"This is just the tip of the iceberg," Worby said. "Many, many more people are going to die from the aftermath of the toxicity."

Dr Bishop said: "Although it will be some years before we are able to build actual human lungs for transplantation, this is a major step towards deriving cells that could be used to repair damaged lungs."

Professor Stephen Spiro, professor of respiratory medicine at University College London Hospitals NHS Foundation Trust and spokesman for the British Lung Foundation, said: "This is very exciting, but there is a lot more work to do."

He said there were many other cell types that make up the lung that would be needed to make new organs.

But he said the cells that the Imperial team had made were crucial for lung function.

"It's always been a huge challenge to replace the damaged air sacks in ARDS. Maybe these cells will be the beginning of something," he said.

Dr Alexander Gourine from UCL's Department of Physiology, who worked on the research, said: "Whether you are sprinting for an Olympic medal or simply running for the bus, your body has to increase your breathing to ensure you are absorbing enough oxygen to fuel your muscles and exhale extra CO2 for the duration of your sprint.

"While the broad control mechanism for breathing has been known for many years, we have discovered how this works at the molecular level.

It could help people with sleep apnoea, sudden infant death syndrome, and those affected by Ondine's curse, a rare breathing disorder where patients who fall asleep stop breathing and die, the researchers say.