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© The New York TimesRoy Chelsen
A new study says firefighters who toiled in the wreckage of the World Trade Center in 2001 were 19 percent more likely to develop cancer than those who were not there, the strongest evidence to date of a possible link between work at ground zero and cancer.

The study, published Thursday in the British medical journal The Lancet, included almost 10,000 New York City firefighters, most of whom were exposed to the caustic dust and smoke created by the fall of the twin towers. The findings indicate an "increased likelihood for the development of any type of cancer," said Dr. David J. Prezant, the chief medical officer for the New York Fire Department, who led the study. But he said the results were far from conclusive. "This is not an epidemic," he said.

Cancer is not on the list of illnesses covered by the James Zadroga 9/11 Health and Compensation Act, which has set aside $4.3 billion to treat, compensate and monitor those suffering from health problems associated with the attacks and their aftermath, like asthma and other respiratory ailments. But the law requires officials at the National Institute for Occupational Safety and Health to conduct periodic reviews of studies to assess whether to add illnesses to the list.

The first review, released on July 26, said that available research had not yet confirmed a link between exposure to substances released after the attack and cancer. But the study published Thursday is the largest assessment of cancer to date in firefighters who worked at ground zero.

The report studied cancer occurrence in nearly 10,000 male Fire Department personnel in the seven years after Sept. 11, 2001. (There were too few women to create a meaningful sample size.) Firefighters were eligible to participate if they were active on Jan. 1, 1996, and if their exposure to substances at the World Trade Center site was known. Of those in the study, 8,927 were classified as exposed, meaning they spent at least one day at the site in the 10 months after Sept. 11. Almost all of those were exposed in the first two weeks after the attack.

There were 263 cancer cases in the exposed population, reflecting a cancer rate 19 percent higher than that of the group not exposed. The cancer rate of the exposed group was only 10 percent higher than that of American men over all. And the group of firefighters who were not exposed had a lower rate than the general population, which the researchers said may be a result of their overall physical fitness and low smoking rates.

The research team said the findings should be interpreted cautiously because many cancers caused by exposure to substances found at the site, including asbestos and jet fuel, can take decades to manifest.

It can be difficult to reach conclusions in studies like this one because it is hard for investigators to account for factors, other than exposure to toxic ground zero materials, that might contribute to the difference in cancer rates.

The study established no link between exposure to the trade center site and particular cancers, though there were indications that certain cancers including melanoma, non-Hodgkin's lymphoma, and thyroid and prostate cancer occurred more frequently among exposed firefighters than in the general population. Occurrences of lung cancer did not increase.

"Is it a definitive study? No," said Dr. James Melius, the administrator of the New York State Laborers' Health and Safety Trust Fund and one of the peer reviewers of the study. "But it provides information that there may be a significant cancer risk for these people."

Dr. Melius said the study alone would probably not be enough to persuade federal officials to include cancer as one of the diseases covered under the Zadroga Act, because cancer is widespread in the general population. The study was limited to firefighters, who were present in large numbers at ground zero, especially in the first weeks after Sept. 11. He added that these results hinted at a correlation, but did not yet prove a direct link.

Still, the study does provide a reassuring jumping-off point for further assessment, said Representative Carolyn B. Maloney, who was the lead sponsor of the Zadroga Act in the House. "It's a scientific study; it is building the case," she said. "You have to get a consensus in the medical community that supports this. I believe we will."

Fred Blosser, a spokesman for the National Institute for Occupational Safety and Health, said the institute was encouraged by the appearance of additional peer-reviewed studies. The second review will take place in early to mid-2012. "We look forward to giving this paper, plus any others that appear, serious consideration," Mr. Blosser said.

Trish Chelsen, whose husband, Roy, was a New York City firefighter, said she believed his death in January of multiple myeloma was caused by the chemicals at ground zero. "In time, research will prove that, but in medicine, everything is evidence-based," she said. "He wouldn't have died at 51, that's for sure."

Before chemotherapy and two stem-cell transplants sapped his energy, Mr. Chelsen was strong and determined, his wife said, "like a typical Viking." She said she hoped finding a link between cancer and toxic ground zero materials would save other families from financial difficulties.

The Lancet also published other reports related to the Sept. 11 attacks on Thursday, including one by researchers at Mount Sinai Medical Center showing the persistence of respiratory, gastrointestinal and mental illness among rescue and recovery workers. Another report, by the New York City Department of Health and Mental Hygiene, however, indicated a lower mortality rate among recovery and rescue workers compared to the general population of New York City. Its authors attributed the lower rate to the "healthy-worker effect" - those present at the site were more likely to be employed, a group typically healthier than the general population.

Dr. Philip J. Landrigan, head of a Sept. 11 treatment, monitoring and research program at Mount Sinai Medical Center and the principal investigator of the Mount Sinai report, said he had no doubt that time would reveal a connection between certain cancers and exposure to the trade center site. "We know full well that the responders were exposed to a whole soup of carcinogens," he said. "We're all looking for a signal today, but we're all anticipating that the signal will get stronger in the years ahead."