NEW YORK – Two major medical studies have failed to find significant increases in deaths or cancer among people exposed to dust from the World Trade Center. In one, researchers who studied cancer rates among nearly 9,000 firefighters who spent time at ground zero found four more cases of the disease than might normally be expected in a group of American men of the same age and ethnicity, an increase independent experts said was small enough to be caused by chance alone. Fire Department doctors also compared exposed firefighters to a group of 926 colleagues who were never at the trade center, and had no contact with the dust. There, they found that cancer rates were about 19 percent higher for in the exposed group, but, again, experts uninvolved in the study said the difference was not statistically significant. Advocates for the firefighters said the trend was still worrisome, however, and doctors said they could not rule out the possibility that more cancer cases will develop among the firefighters as time goes on. In the other study, researchers with New York City's health department who studied death rates among 42,000 people potentially exposed to trade center dust found no evidence of a spike in fatalities. In fact, they found that the 790 deaths among people in the study group was about 43 percent lower than the mortality rate for New Yorkers in general. They were also less likely to have suffered fatal respiratory ailments. Those findings, however, was also written off by scientists as too premature to mean much. Because the attacks happened in a business district and presumably involved people who were fit enough to be reporting to work, the study group was probably healthier than the general public to begin with, said New York City's health commissioner, Dr. Thomas Farley. "I wouldn't interpret it as that the World Trade Center has somehow helped people live longer," he said. Also, the types of toxins released in the trade center disaster usually take decades to result in deaths, not the few years covered in the study, he and other experts said. Donald Berry, a professor of biostatistics at the University of Texas M.D. Anderson Cancer Center, said the two studies "provide no evidence that living or working in the former shadow of the World Trade Center increases one's risk of anything." "Occupational hazards are real," he said. "An extreme example is the plight of asbestos workers. But occupational risks accrue over years of exposure. With the exception of a nuclear explosion or meltdown, it's difficult for any single event to cause an increase in cancer or in mortality." Both studies were being published Friday in The Lancet, a British medical journal. Also set for publication in The Lancet on Friday is a study of nearly 27,500 people enrolled in a World Trade Center health monitoring program that found that nearly 28 percent had asthma, 42 percent had sinus problems and 39 percent had acid reflux disease, a condition related to heartburn. The study also found large numbers of rescue and recovery workers suffering from depression or panic disorders. Those findings echo the results of several other studies. Dr. David Prezant, the fire department's chief medical officer, said he believed the firefighters study indicated "a moderately strong correlation" between World Trade Center dust and cancer. He said he did not agree with other experts who said the study failed certain key tests of statistical significance. The inquiry found that 242 of the nearly 9,000 firefighters exposed to the attacks had developed cancer within the study period, compared to the 238 that researchers would have expected in the general public. Researchers found less lung cancer than expected — only 9 cases instead of the 21 they expected to see. That's reassuring because people are concerned about inhaled dust particles. All 9 of the cases involved smokers. Conversely, they found 12 cases of thyroid cancer in the study group, compared to the 6 they might have expected based on rates in the general public. Dr. James M. Melius, director of the New York State Laborers' Health Fund and one of the leading advocates for ground zero workers suffering health problems, said that even though the cancer research on firefighters was inconclusive, it showed enough possibility of a risk that U.S. officials should consider adding cancers to a list of conditions covered by a multi-billion dollar health aid bill passed by Congress last year. Doing so would qualify exposed people for sizeable compensation payments. "Are we going to wait until we have definitive evidence, which could be 20 or 30 years? Are we going to say, decades from now, `Yeah, you did get cancer because of the World Trade Center, and we should have helped you out back then?'" he said. "It's limited information. It isn't a perfect study ... It still provides compelling evidence that we should be providing at least health care for these people." Experts said both the mortality study and the cancer study are limited, in part because of the difficulty of finding a proper comparison group. Drawing conclusions can also be difficult because researchers don't know the full medical history of the subjects. Dr. Michael Thun, vice president emeritus of epidemiological research for the American Cancer Society, said it isn't surprising that the study would fail to detect any major trends so soon after attacks. Typically, the types of cell mutations caused by toxic and carcinogenic exposures take decades to develop into a diagnosable cancer, he said. Outside of cancers in children, he said, "You can't really go from the earliest stage to lethal in just a few years." But it is possible that a cancer that already existed might have been accelerated by something in the dust, and on that point, "the results are neither conclusively negative, or conclusively positive," Thun said. He called the Fire Department research "a solid first study on the issue," but said it will likely be another decade before scientists can really see whether people exposed to trade center toxins have an increased risk of getting cancer. Follow Yahoo! News on Twitter, become a fan on Facebook